[retepsnikrep]

I just came back from a drive and have some real voltage readings for you.
I was able to record them on my phone:

cutoff=465
582 541 561 410 430
440 470 460 440 440

The above numbers are from a full throttle reading.

1) Was this the pack you discharged down to 60V?
Did you charge it up again before putting it back in the car and doing your full throttle event?

2) The DC-DC disables itself around 75/80V IIRC.
The MCM can disable the DC-DC whenever it feels like it if the Grn/Blk wire is intact.

3) ~120V is the minimum permitted pack voltage under assist loads > 20A or so IIRC.
It's also the voltage at which the MCM will give maximum assist current as it tries to maintain the full (amps x volts) 10kw output.

The MCM progressively throttles current below 120V which is why the voltage hack for maximum power does not allow the MCM to see < 120V, in fact it fixes it at 120V when > x assist is requested.

By progressively I mean as assist current ramps up the pack voltage falls, at just under 120V the MCM starts reducing assist power to try and stabilize current V voltage at the critical 120V point.

Depending on how bad/good/warm etc the pack is, some might put out 90A at 120V, but some might only manage 20A before the voltage starts sagging under 120V.

90 x 120 = 10.8kw
20 x 120 = 2.4kw

The car measuring systems aren't 100% accurate and MCM programming variations do give some subtle differences between cars/mcm etc. So some might allow 117V and some only a 124V minimum.

There is also a bit of time lag between sensing and action being taken so instantaneous dips below 120V can happen but it won't allow that to go on for any length of time.

 

[davismltc]

1) Was this the pack you discharged down to 60V?
Did you charge it up again before putting it back in the car and doing your full throttle event?

Yes, to both questions, and the grn/blk wire is intact.

The voltage sampling snapshot was an attempt to capture a worst case scenario on a worn out pack, and I think I happened to hit it just right. I'm sure the car quickly throttled the current, but I don't have a way to capture that. The weather here was around 70 degrees F, so that was the approximate pack temp. Routine tap samplings on this pack show it to remain well balanced on tap voltages when not in a full throttle event, within +/- a tenth of a volt at no load, or +/- two tenths with moderate acceleration. Also, after sitting overnight, the first pack reading in the morning is within a tenth. I haven't been looking for it, but I don't think I've seen pack voltage drop below 120v under normal driving conditions. The second set of voltage readings I posted in the 120s was with more throttle than I would normally incur with driving. Usually I see 130-140s under modest acceleration and 150-160s under no load. Regen can go as high as 180s depending on state of charge, but that's not typical. Usually, I see regen up to the 170s.

I think the only thing out of line here is the battery pack, which is 19 years old and in need of replacing.

What's interesting to me, or at least a curiosity, is how EQ1's tap voltages are so imbalanced and mine aren't. I don't routinely grid charge or balance my pack, maybe three times or four times, in the past five years or so.

 

[davismltc]

EQ1... Just an FYI:
I think you are exactly correct about your analysis of the BCM and its management of the pack: That the BCM only cares about the voltage drop of the lowest tap in managing assist (by limiting current) and only cares about unequal voltage taps for the purpose of monitoring for a defective cell.

I edited this to add one more: The BCM also cares about pack (or tap level) capacity.

And thanks for correcting some of my misconceptions about the BCM's management. I'm sure my ramblings about the BCM's function were intertwined with my personal thoughts and concerns about managing a secondary paralleled pack with my own tap monitoring board. Your analysis was correct about the BCM. Thanks for sharing your knowledge.

 

[eq1]

^ No worries...

What's interesting to me, or at least a curiosity, is how EQ1's tap voltages are so imbalanced and mine aren't. I don't routinely grid charge or balance my pack, maybe three times or four times, in the past five years or so.

When I've been talking about seeing imbalanced tap voltages, it's because I have purposely unbalanced them for testing purposes. When they start balanced, most of my taps stay balanced to within maybe 10-30mV up and down the charge state range...

I have one tap that's Civic sticks while rest are old Insight sticks, I've been working with that, seeing what the differences are for a few months now. And two others have some other type of imbalance, not positive what the cause is, probably either faster self discharge on 1 cell, or faster self discharge in general...

Here's my tap voltages from today, this is after a drive, pack almost 'stuffed' full with a little taken off the top prior to end of trip, measurements at auto-stop discharge load of -1.2 amps:

16.66, 16.64, 16.66, 16.65, 16.62, 16.55, 16.55, 16.63, 16.58, 16.65

In reality, most of the '16.6x' voltages are essentially identical, because I can't take the measurements in an instant, so I start at tap 1, then tap 10, then tap 2, then tap 9, etc. so by the time I get to the middle taps, time has elapsed and voltage has dropped a bit (or, in the past, I just went from 1 to 10 and by the time I got to 10 voltage dropped the most there).

So 16.62 is very likely 16.64 or 16.65; 16.55 is more like 16.58, etc. The point here is that Insight/Civic cells, when they're in good shape (and matched well), stay almost perfectly balanced, any deviation can be an indicator of 'something' being a little off... Here I know Tap 7 is Civic sticks - and the voltage is indeed a little off.** And Taps 6 and 9 - I did something different with those some months ago, haven't looked back at the info critically to figure out just what and why.

** Oh yeah, also, keep in mind that balanced voltages don't necessarily mean balanced amp-hour capacity. Pretty sure I have some not-insignificant capacity imbalances from stick to stick. So, I can either make voltages balance-out at the top, or I can make them balance-out at the bottom - but they can't be balanced at both top and bottom at the same time. If I were to discharge to empty, some of these taps would be empty before the others, simply because at this point in time they're balanced at the top, and there's capacity imbalances (probably like around a few hundred mAh at most).

 

[eq1]

I came back here to post this link to another one of my threads that has quite a lot of detailed reasoning and observations about stuff in the previous post. I re-read most of it and still find most of it true, sound. I had thought I needed to write some stuff - but most of it's already written: Contemporary musing on Insight pack and management

I'm really trying to hone-in on my "moving window theory of charge state" idea. The more I experiment, the more I believe it has to be true. But I also want to understand how it's happening, at a deeper level...

Along the lines of what I write above, I found a different 'Hysteresis' article and have sort of read through it (I can only grasp maybe 10% of what's written). I find some of the ideas reminiscent of at least parts of the 'moving window' theory, the 'cell within a cell' idea. Figured I'd post a couple things...

The first thing is a graph from the article that tends to illustrate kind of the simplest, boiler-plate example of what a 'moving window of charge state' looks like. I had been thinking about making a graph that looked a lot like this - but then I found one, already made.

The graph shows "boundary curves" - i.e. a charge to full (upper curve) and discharge to empty (lower) - of a nickel electrode (with some cobalt, a "Ni-Co film"). Two 'hysteresis loops' are superimposed, i.e. what happens when you stop the discharge and charge early and cycle within that voltage range (more or less). It's basically short cycling.

The left loop happens if you start from empty, the right loop happens if you start from full. "Z" on x-axis is state of charge.

The voltage range of each loop is the same, but the charge state range each loop traverses is different: The left loop happens between about 30% and 50% SoC, the right loop happens between about 45% and 65% SoC. So, we have usage windows about equal to 20% of the total charge state range - same size, between the same voltages, but representing actual charge state about 15% apart. I.e. the charge state window moves, even though the voltages are the same. The only thing that makes the difference is whether you approach the 'loops' from a charged state or a discharged state.

Here's a bit of text from the article, mostly part of the conclusion. The concept of "domains", as well as "hysteresis" in general, seem to parallel at least parts of what I've envisioned for 'moving window theory'...

"The close agreement of the predictions of these theorems with the experimental data on the nickel electrode leads us to conclude that the nickel hydroxide electrode’s behavior is consistent with the existence of a number of individual units or domains, each of which exhibits two or more metastable states...

We confirmed that the nickel hydroxide electrode exhibits a stable hysteresis loop, with the potential on charge being higher than that on discharge at every state-of-charge. We also showed that the charge and discharge potentials vary depending upon the number of electrons transferred (i.e., 1.67 and 1.0), the compositions (pure Ni and Ni-Co), and the defect content, but the shape of the hysteresis loop does not. Furthermore, we showed that the hysteresis loop created during a complete charge and discharge [i.e. full charge and discharge curves alone] is not sufficient to define the state of the system. Rather, internal loops within the boundary curves (i.e., scanning curves) [i.e. short cycling] can be generated that access potentials between the boundary curves. The potential obtained at any SOC, as well as how the material charges and discharges from that point, depends on the cycling history of the material.

As little information is available on electrochemical hysteresis, theories proposed in magnetism and adsorption were examined. The theory of domains, where the system is thought to be made up of many small units each of which exhibits two or more metastable states, was found to be applicable in explaining the behavior of the system. The qualitative shape of the boundary and scanning curves suggests that the domains in nickel hydroxide are characterized by two features, namely, 1) there is a distribution of critical potentials where metastability occurs over the domains and 2) the behavior of each domain is history dependent within the range of metastability. Although the actual cause for the metastability in each domain is not clear, previous research suggests possible causes as either energy changes during intercalation or phase separation. The close adherence of the system to the seven theorems outlined by Everett and Smith indicates the applicability of domain theory in explaining hysteresis in nickel hydroxide."


Anyway, I think I'll have to leave it at that. I've attached the article if interested. I think there's a lot of concepts here that can go a long way to explaining potential shortcomings of either the Insight cells or the management. Or perhaps even shed some light on what the BCM actually does. I find it interesting that this article is from 2001, right around when our cars were coming out... It's like, the authors describe problems that would pose issues for managing NiMH cells - did Honda/Panasonic have better insights and actually surmount the problems? Or maybe they didn't? Or maybe they actually implement management schemes that reflect some of the concepts/findings in this article?... It's interesting, in a geek-out sort of way...

 

[eq1]

Toward the end of a drive this morning I was thinking there's two things I wanted to note in this thread. I forgot one of them, so I'm stuck with just one...

It has to do with voltage hysteresis - related to the previous post and article. More basically, it has to do with using voltage, such as tap voltages, as a proxy or indicator for 'balance' or 'charge state balance', etc. In a nut shell, until now I haven't fully appreciated the implications of the voltage hysteresis in Insight NiMH cells on 'reading tap voltages': If you're not really careful it simply won't work.

A cell can be at virtually any voltage within the voltage range in which our cells normally operate -- at virtually any charge state. You really can't tell the state of charge using voltage. This means cells (or taps) can be really unbalanced even though the voltages are the same or similar.

I've known this, we've known this, for a long time, it's more or less common lore. But at least when it comes to my thinking, I've too often ignored it, or haven't processed that info well enough in the context of figuring out other things...

I'm not sure what to think about it. My first thought was that, yes, cells can have different voltages and be at the same charge state, or have the same voltages but be at different charge states, and that that's a problem. On the other hand, I fleetingly thought that, actually, it might be partially why our packs can work as well as they do as long as they do: Despite charge state imbalance (or capacity imbalance), for instance, the cells can nevertheless operate at the same voltages, so maybe that helps...

If you go back to that graph in the previous post, the situation here would be analogous to those two 'hysteresis loops' - one cell operating at the lower loop, at lower charge state, one at the other higher loop - but both operating within the same voltage range. Those 'loops' can be more or less anywhere along the upper and lower 'boundary curves'. When one cell's loop is toward the top, toward the far right, and another cell's loop is toward the bottom, far left - that's when you have pack-debilitating problems, like a P1449-78...

Are our cells more prone to imbalance due to hysteresis, or less? I think I'd have to say "more." It's like the fact that different voltages can happen at different charge states masks the usable-capacity imbalance. The BCM will think the cells (taps) are balanced, but they might not be. If different voltages reflected different capacities (or rather, if voltage were correlated with charge state) then you'd know the cells were imbalanced - and I guess you might be able to do something about it. As it stands, that information can't readily be gleaned from data the BCM collects during 'the typical drive'...

There's two parts to this: On the one hand, the BCM will use an overall fixed voltage range, more or less, to manage - so cells operating at the same voltage=good, at least in near-term. On the other hand, cells can be at different charge states despite those similar or same voltages, so cells operating at different charge states can at least eventually, or ultimately, be bad.

I think this interpretation, these ideas, dovetail with what we see in the BCM's general management scheme -- where usage toward the top or bottom of the absolute capacity range can reveal...problems, such as 'premature neg recal' or 'premature pos-recal'... Basically, I gather that the BCM is indeed very concerned about being able to match tap voltages with the 'proper' charge state via 'counting current'. The only thing is, it seems like it never actually forces you to use the top or the bottom, that, depending on how you drive, you could very well end up wallowing around in the middle indefinitely. That 'middle' is very wide, so it seems possible or likely that major imbalance could set in without the BCM ever knowing it, without us ever knowing it... It's not until driving circumstances force usage probably mainly at the bottom that a problem becomes apparent...

 

[eq1]

For a while now I've been trying to conceptualize the appropriate diagram or something like that to illustrate one major way I think Insight cells tend to operate in the short term. I think it applies to new and old cells, but probably becomes more obvious with old. This too is related to voltage hysteresis, I guess it's probably the same thing. In lieu of that proper diagram, I just thought of a vivid analogy...

Have you ever seen 'one of those' cartoons, where there's some protagonist trying to run away from some antagonist, running up a flight of never ending stairs - and the stairs that the protagonist has just traversed on his way up have just disappeared, and keep disappearing? He keeps climbing up the stairs, trying to stay ahead of the stairs behind him that keep falling away, chasing him up... I'm not sure what 'cartoon' I've seen this in, maybe it's the "Magic Flute," like a Disney rendition with Mickey Mouse. Or maybe it's Pink Floyd's "The Wall"...

But, Insight cell 'voltage hysteresis', perhaps coupled with one other phenomenon, seems a lot like that.

In these 'research papers', such as the one posted earlier, authors point out that difference in voltage and charge state depending on whether you approach the voltage/charge state from a discharge or approach it from a charge, for example. Others have described this as 'the charge mode' or the 'discharge mode'.

If you approach a given voltage 'from below', by charging, the charge state will be lower. The oxidation state will be lower, a lower charge state (I think that's right...). If you approach a given voltage from above, by discharging, the charge state will end up higher. That's what's illustrated in that graph I posted some posts above, with two loops: the right loop was approached from above, during discharge, while the left loop was approached from below, during charge. Same voltage ranges but different charge states.

The corollary to this, thinking of those disappearing stairs, is that, at some point, some threshold, when you keep charging high, the bottom end starts shifting 'downward', possibly to 'right'. If you picture a charge curve, starting from near empty, so down around 1.2V for a single cell, voltage ramps up quickly to say 1.37V and stays around there, only gradually increasing. Now picture a charge that's taking place at a higher charge state, say around 75%. The charge curve will increasingly become steeper 'to the right' as you continue the charge. IF you could somehow have a graph that simultaneously shows what's happening 'at the bottom end' during this charge at the top end, it'd show the lower part of the curve 'curling-up', eroding - those 'disappearing stairs'. What was a voltage within the range of 1.2V to 1.37V, over say a few minutes of charge, is becoming a range of say 1.1V to 1.25V over a longer time range... I know, this is really hard to picture... The lower part of the charge 'boundary curve', which was lofty, is now collapsing. The higher you keep charging, the more the lower charge state collapses...

In my low charge state usage, with forays to high charge state every so often, I see this all the time. It now strikes me that one effect of using low charge state, particularly because I usually start at near zero charge state, is that I'm almost always approaching higher voltages 'from below': starting at zero charge state, every voltage, every higher charge state I reach, will be reached by charging, at least initially. This is akin to always approaching the lower/left loop in that diagram, rather than the higher/right one. And as long as I don't go 'too high', the 'stairs don't disappear'...

I'm thinking that new cells probably have a downward shift to the charge boundary curve (toward the discharge boundary curve), at the bottom end; whereas older cells probably have a downward + rightward shift. As you charge higher, voltages at lower charge states will drop with new cells; for old cells, voltages would drop + the absolute usable capacity will shrink, so the leftmost point on the charge boundary curve shifts to the right.

What's interesting, I think, is that I think I would have called this 'voltage depression' up to now, but this is normal operation of our cells. Voltages do get depressed, but that depression is associated not with more pernicious, longer-term degradation, but rather, simply the natural NiMH voltage hysteresis plus how the cells are used/managed.

 

[eq1]

I've got some new ideas floating around in my head, about the implications of... cell capacity and/or charge state imbalances on pack failure, management, longevity, somewhat related to whether one should or should not use the pack at high charge state, or low charge state.

In a nut shell, my 'old thinking' is that, when cells (or taps) become imbalanced for whatever reason, the 'low charged cells' (or taps) disable assist early, and the car tries to charge the pack. Then it's the 'high charged cells' that disable the charging, once a tap hits the 'full' voltage threshold. BUT, given some things I've seen, read, etc., I'm starting to wonder whether it could actually be the 'low charged cells' that disable assist (cause the 'neg recal') AND disable the charge, too?

The assumption or whatever is that, once a tap reaches a particular voltage threshold, the charging is stopped - so it's the tap with the highest voltage that stops the charge, determines whether the pack is 'full' or not. And, we assume it's the most charged tap that has the highest voltage - that's the 'old thinking'.

The new thinking is this: The least charged tap, or the one with the least charged cell, might actually end up with the highest voltage soonest. There's kind of two things in play here, the normal voltage hysteresis of the NiMH chemistry, and something akin to 'reconditioning' that happens when cells are allowed to fully discharge. If you fully discharge cells on a semi-routine basis, they seem to uphold voltage much better than if you short cycle them at intermediate or high charge states.

Consider this graph I posted in this thread a while back, and in another one. Here's a link to the original context: OEM pack management efficiency: Not so good(?)...

I had a stick with a fast self-discharge cell, so I pulled the stick and discharged that cell and the others. The yellow curve in the graph above is the fast self-discharge cell (FSD), the black curve is one of the others. Due to the FSD, this cell was perpetually the least charged cell, it was draining first and triggering 'empty (neg recal). The cell represented by the yellow curve is only charged by about 700mAh, but its curve is lofty, the voltages are high. The other cell is charged higher/more, about 3500mAh, but its voltage curve is shallow and saggy.

The cell that had been fully discharging ends up with a much loftier curve, higher voltages. I see higher voltages, of course, continue up the charge state range when charging is continued, i.e. if charged more the yellow curve continues on its higher trajectory.

So, it seems plausible to me that the lowest charged cells in a given tap actually end up with higher voltages than taps that aren't allowed to discharge fully - and they can?, do?, end up triggering the 'pos recal', i.e. 'full'. I'm thinking it's likely they end up with higher voltages before the other taps, even though the other taps are the ones that are technically(?), or in some sense, charged to the higher charge state...

I think I'll leave it at that. The next step would be to hash-out those management, etc. implications... I think one thing that nags at me about using low charge state like I do and have been, is that I wonder if there's a possibility low charge state usage can induce faster self discharge. Part of that has to do with what I wrote some months ago about the 'cobalt conductive matrix' and maybe more generally about our cells' 'low end structure'... I could see how something bad could happen 'down there'. And then, I could see how voltage behavior between cells that are dropped low, discharged near completely, and those that aren't will start to deviate, where you end up with some cells with one voltage-capacity-charge state relationship and other cells with a totally different voltage-capacity-charge state relationship. Think about that yellow curve above, only spread across the entire pack, half the cells behaving like the yellow curve and half behaving like the black one. The two sets of cells end up experiencing totally different 'lives'...

 

[Jonny69]

The new thinking is this: The least charged tap, or the one with the least charged cell, might actually end up with the highest voltage soonest. There's kind of two things in play here, the normal voltage hysteresis of the NiMH chemistry, and something akin to 'reconditioning' that happens when cells are allowed to fully discharge. If you fully discharge cells on a semi-routine basis, they seem to uphold voltage much better than if you short cycle them at intermediate or high charge states.

I have some evidence which might support this from my recent discharge log. Graph attached. See my first few cells dropping out.

I was interested to see how much charge was in the cells that dropped out early in that discharge. The calculation is a bit down and dirty because I'm missing the first 1-2 hours of the log, but I can make an approximation. The first couple of cells that drop out were holding between 4-4.3 Ah, or 61-66% of their 6.5 Ah capacity. The second set that dropped out were holding between 5.1-5.5 Ah, or 79-84% of their 6.5 Ah capacity. I know from previous discharges that the 'strongest' of the cells in the pack will hold on until the 18-19 hour point, which relates to around 6-6.3 Ah, or 94-96% of their 6.5 Ah capacity.

Before the discharge, I had grid charged the battery the previous weekend and the car had been used for a week. The gauge was showing around 90%. We know that the car uses a charge window of 20-80%. So the car thought my pack was charged to around 70%, when in fact it contained a spread of cells between 60% and 95%.

No idea where those cells are in the pack, but let's say there's a single stick with a mix of 95% and 60%. On regen, the average voltage across this stick is going to look strong, because the voltage will already be high in the highly charged cells and the regen will top them out as they reach overcharge (it's going to quickly push them to 100%), pushing the average voltage in the stick artificially high. All of a sudden, the voltage will catch up in the 60% cell and push the voltage across the stick way higher than expected. So let's say that stick is the one with the cell holding the least charge, that effect is going to be the greatest and that is the tap which ends up with the highest voltage soonest. My pack is relatively healthy. In a weaker pack, this delta is likely to be a lot higher.

 

[eq1]

^ hmm, I'm not sure I see how your scenario relates to what I was describing. What I was describing I think depends on at least two critical things:

1) at least one cell that's charged to a lower absolute capacity for whatever reason, such as at 1000mAh vs. others at say 3000mAh. Could have gotten there because it has faster self-discharge, or maybe by some other kind of mismatch/deterioration vs. other cells. The important thing is that it's charged to a lower capacity level.

And 2) the pack needs to be taken to neg recal. In the real-world this is a pack that's exhibiting 'pre-mature neg recal' - the BAT gauge de-populating by half the bars for instance and then plummeting to the bottom. This means the lowest cell or cells are getting near-fully discharged in the car. That seems to be the critical factor that leads to different charge voltage curve/behavior -- the voltage will be higher much sooner, it tends to stay high, and I think in some cases, maybe most?, it will reach the BCM's 'pos recal' voltage sooner (~17.0V at rest).

Basically, the other cells will have depressed voltages - their discharge curves will be saggy, their charge curves will be slow to climb. Meanwhile, the cells that discharge completely will have high voltages on both charge and discharge, the latter at least until it runs out of capacity.

I'm not really seeing these 'critical' features in the scenario you describe...

The first couple of cells that drop out were holding between 4-4.3 Ah, or 61-66% of their 6.5 Ah capacity. The second set that dropped out were holding between 5.1-5.5 Ah, or 79-84% of their 6.5 Ah capacity. I know from previous discharges that the 'strongest' of the cells in the pack will hold [about 6.2Ah]... I had grid charged the battery the previous weekend and the car had been used for a week

OK, so you discharge your pack with apparently a very small load, and the first cells drop out at about -4.1Ah and others at about -5.3Ah. And I guess you're using the ~6.2Ah value more or less as a baseline for what the best cells do when fully charged... Or maybe you're saying some cells in this pack would have discharged that amount if you actually kept discharging, or something. So perhaps there's 3 sets of cells at different absolute capacities.

I think I'm following so far...

The gauge was showing around 90%. We know that the car uses a charge window of 20-80%. So the car thought my pack was charged to around 70%, when in fact it contained a spread of cells between 60% and 95%.

OK, I think I see where you're going with this, it reminds me of something I used to do a lot. [Here's a link to a post where I talk a bit about that : Ideas, facts, wild-*** guesses about background charge?]

Let me paraphrase and you can approve or disapprove of my understanding:

You grid charged to full, you use the car for a week during which time the BCM supposedly establishes what it thinks the charge level is (either your "90%" gauge level or your "around 70%" level). You then discharge the pack 'on the bench' and establish the range of actual charge levels - about 4.1Ah for some cells, 5.3Ah for others, and I guess 6.2Ah for others. The car 'thinks' the charge state is "70%," or rather you think that's what the car thinks, while your 'on the bench' work says you have a mix of roughly 63%, 82%, and 95%. Good so far?

In general I think that, conceptually speaking, you're treating the "20%-80%" range and the "70%" value too literally, these aren't or simply can't be iron-clad thresholds. On the other hand, your approximately 63% charge state for your lowest cells is just about right, just about what you'd expect... i.e. if the BCM thinks the charge state is about 70%, then it's not very far off the mark, is it?.

No idea where those cells are in the pack, but let's say there's a single stick with a mix of 95% and 60%. On regen, the average voltage across this stick is going to look strong, because the voltage will already be high in the highly charged cells and the regen will top them out as they reach overcharge (it's going to quickly push them to 100%), pushing the average voltage in the stick artificially high. All of a sudden, the voltage will catch up in the 60% cell and push the voltage across the stick way higher than expected. So let's say that stick is the one with the cell holding the least charge, that effect is going to be the greatest and that is the tap which ends up with the highest voltage soonest.

I think this is where I lose you the most. For example, I'm not sure what you mean when you say, "On regen, the average voltage across the stick is going to look strong, because the voltage will already be high in the highly charged cells and the regen will top them out as they reach overcharge..."

Not sure what "look strong" means in this context... So... You have a charge-imbalanced stick, some cells at 60%, some at 95%, you hit it with regen, voltage for near-full cells will... well, first increase the fastest, then flatten out if the charge is allowed to continue.... I think you might mean 'total voltage' or 'tap voltage' when you write "average voltage"... The BCM's looking at tap voltage...

Not sure what you mean when you say "pushing the average voltage artificially high"... And I guess the same with the rest, like "All of a sudden the voltage will catch up" and "higher than expected."

In general, I can see that what you're describing isn't the same as the idea I was getting at earlier.

I haven't done enough 'work' at the top-end to know with great certainty some of the finer management strategies... In general, lately I've been leaning toward tap resting voltage being the most critical element - what usually determines '75%' and 'too full', 17.0V for the former, 17.4V for the latter. Regen doesn't last forever, it's usually only transient - so most of the time, if the BCM sees either of these resting voltages, that's what rules (well, it's that and nominal charge state; for example, hit 81% nominal and you're done)...

On the other hand, I do think the BCM can/does use slope detection at the top as it does at the bottom, just not sure how that works, exactly. In your scenario, with the 60% and 95% cells in let's call it a tap, the 95% cells would have a very steep slope and thus the tap voltage would increase fast; I think the BCM detects that, measures that, and will disable regen, pos recal, whatever, when that happens...

There's all sorts of stages to 'determining empty' and 'determining full', like check points. On the empty side - which I'm much more familiar with - assist is throttled simply based on nominal charge state, for instance. So, assuming you can bring nominal charge state down to something like 36%, you're gonna see drastic throttling regardless of what the real-time data say (like tap voltage). On the flip side, if the BCM measures steep slope before that nominal - it's going to neg recal - call the pack empty - anyway.

There's analogues on the top end. For example, once background charge is invoked, the pack will keep charging until 70.2% nominal and then quit. That's solely based on the nominal. That's one 'check point'. And then, if you regen more, eventually tap voltage will hit 17V resting, some sliding-scale value under regen load, or perhaps that steeper slope is measured - and we call it a 'pos recal'. Another check point. And then, if you have one of the BCMs that pos recal to 75%, you can charge more, until 17.4V resting is measured or a nominal 81% is reached, whichever comes first, at which point you're really done. And that's another check point...

What does this have to do with your uneven 60% to 95% charged cells in your tap? I guess I don't see that 60% cell triggering any of the top-end management behavior -- unless it were the only cell that's been consistently drained to near empty. Draining to near empty is what elevates voltages, and if all the cells are drained to near empty then they all have the same elevated voltage profile - so no single cell will cause a steep slope or higher voltage sooner rather than later. I think your 95% charged cells would be triggering top-end management...

 

[eq1]

I just want to quickly jot down an idea I'm having as I come off a string of observations having to do with usage of low and high charge state. It's pretty profound in my mind, I doubt I can convey that to yours...

In a nut shell, I'm starting to think, to believe, that like half of what we 'see' when we've worked with Insight sticks, watched IMA parameters on the OBDIIC&C, grid charged packs, or what-not, produce a really wrong conception of how the cells behave. I don't really understand it, but I'm pretty sure it has to do with the 'hysteresis' I've talked about earlier, the varied 'stoichiometry', the existence of different oxidation states...

The simplest way to say it is that you really can have a wide range of voltages at very different charge states.* I'm thinking that like half the pack behavior we see, such as this or that voltage at this or that charge state, in the car, has way more to do with BCM conventions/programming than, for instance, 'how the cells behave' themselves. For instance, it almost seems like the '25%' low-end limit and the '75%' high end limit are nearly arbitrary, have very little to nothing to do with 'cell performance'... AND, it's almost like a self-fulfilling prophecy: IF you restrict usage to some window that seems like it's roughly between 25% and 75% - you'll actually end up with a pack that can't use the bottom ~25% and top 25%...

Hard to explain. From what I'm seeing, if you juke the system, you can use the pack just fine almost near zero, empty. AND, if you juke the system, you can use almost near 100% full just fine. If the pack were a single cell, you'd really be able to use like the full range from say 5% to 95%, i.e. get full performance, full output anywhere within that range. If the cells are matched and balanced it's the same. IF the cells aren't matched and balanced, then you have to rely on 'management' to catch the imbalance and to throttle demands: I'm thinking that the 25% top and bottom buffers are there because Honda was worried about non-matching, non-balanced cells and the potential inability of the system to identify all non-matching and imbalance... BUT, the problem is that self-fulfilling prophecy: you end up with a lot of capacity that doesn't get used and it eventually deteriorates...

The bottom line is that, as far as I can tell, you can get top performance anywhere within the full amp-hour charge state range (i.e. 50% charge state equals ~3250mAh input, 100%=~6500mAh, etc.). -- provided you actually use it. You can have a nice 30% wide usable window at say 65% absolute charge state to 95%; or between 5% and 35%; or between 35% and 65% -- doesn't matter. Whatever range you use will be the range that works (assuming generally matched and balanced cells). That's the problem with the BCM - it doesn't seem to vary the usable range much if at all, it's totally dependent on how you use the car.

* Here's some tap voltages to illustrate this point rather vividly. These are taken in auto-stop ("AS") with the load indicated, at an estimated absolute amp-hour based charge state as a percentage of nominal capacity (6500mAh). You can see that, for example, at 83.6% charge state I had tap voltages at around 16.10V, had higher voltages at 35.2% around 16.16V, and quite higher at 57.3%:

AS -0.9A, 35.2%, 16.16,19,18,17,18,16,18,18,18,18
AS -1.0A, 57.3%, 16.47,48,47,46,44,41,50,46,47,48
AS -0.9A, 83.6%, 16.08,10,10,08,08,07,17,08,10,09

 

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I use two sticks, 10 cells, for my 12V battery. I swapped sticks not too long ago, maybe a month or so. I screened for fast self discharge at cell-level when choosing these sticks - they had sat for a long while and I chose sticks (cells) that didn't have unusual, outlying voltage drops. One cell was a little lower, but not too much. Since then I noticed that the 12V battery made up of these cells wasn't performing up-to-snuff - at key-ON, about a 6 amp load, I could turn the headlights ON and voltage would plummet pretty fast (so maybe a total 16 amp load, 6 amp + 2 60W bulbs). Basically, it looked pretty clear that at least 1 cell was probably not charged much, near empty. So I pulled the battery a couple days ago to check it out.

I'm seeing what to me looks pretty unusual. I thought I should document it somewhere, just in case someone else has seen this, can confirm, or whatever.

In a nut shell, two cells look like they have fast self discharge - but probably only at the 'top' of the charge state range. I don't have time or the will to do long, drawn out testing, but looking at my notes and such, this is what it appears to be. The immediate, most direct 'evidence' is comparing the discharge amount hot off the charger and then one day later and only a few hours later. Both cells have more or less normal, complete discharges 'hot off the charger', but after sitting, for only a day, or only about 4 hours, the voltages plummet - from say 1.45V to about 1.33V. The thing is, when I did my initial screening, neither of these cells appeared to have fast self discharge; their voltages were as high as the other cells, after sitting for months. But, those voltages were also below this ~1.33V level, they were around 1.31 or so. The discharges after sitting (hot off charger vs. after sitting a day, and sitting 4 hours) weren't terribly low capacity-wise, at least from what I can tell. It's the weird voltage drop that's most puzzling. It's as though the cells have fast self discharge but only at the top, say above about '1.33V'.

IF this were true, imagine what the outcome would be for a pack of cells: you'd charge the pack to say '75%', but these weird top-end fast self dischargers would hemorrhage a lot of that charge. By day 2, these cells would be at say 50% while the others would still be at 75%. You'd drive, get some assist and regen, end the drive charging to say '75%'... Say the net charge were zero, so you'd think maybe the cells would end up back at 50% and the others back at 75%, but I doubt it works that way. The '50% cells' probably get some extra charge, but due to the weird oxidation state/hysteresis thing, I'd guess it's very little: the cells quickly 'act' like the 75% cells, as if they were charged to 75% - and then they hemorrhage that 'extra' charge once again, so they end up below 50%. It's like one step forward, two steps back...

Anyway, I'm not sure there's a practical difference between fast self discharge across the board vs. FSD at the top only. I guess the take away is that you can 'screen' for FSD cells measuring voltages on cells that have sat for a long time - but some cells might have this FSD top variety and you might miss it; you'd have to charge to full and look at voltages a day or so later. If cell voltages fall below about 1.4V, they're probably FSD top-end variety, especially if it's a lot below 1.4V, like 1.33V. Most normal or at least non-FSD top-end cells should be around 1.4V, I think, after a full charge and sitting a day or so...

Here's a couple graphs: Top is discharge hot off charger, bottom is a day later (~12 hours). The time scales aren't normalized, and the top capacity curve is in two parts, because of interrupted charge. Voltage curves are on the main cables, so extra resistance, lower voltages than actual.

Discharge cap for top is about 6000 mAh total; for bottom it's 5300mAh - so over the 12 hour period this cell lost 700mAh. As I recall that's a lot more than normal, like normal is maybe 300mAh or so...

What I find interesting is, if you look closely at these curves there's a fairly long segment in each that's perfectly flat. In the top graph that flat section is about in the middle; in the lower it's at the beginning. It's like the charge above that plateau (at about 1.22V) is totally missing in the lower discharge - it disappeared over night. The 'movement' of this plateau from 'middle' to 'beginning' doesn't represent a loss of 50% charge. And as I recall I see something similar on normal cells. But, it does seem worse here than normal... Don't know...