Honda Insight Forum banner

101 - 106 of 106 Posts

·
Premium Member
Joined
·
10,104 Posts
"Balanced" or "equal"?
(Med time)
 

·
Registered
Joined
·
6,313 Posts
Discussion Starter #102 (Edited)
That shows the importance of cell level work which is even more time consuming and tricky than stick level!! I def need to build my semi automated stick tester. It's been discussed lots of times. I even have a schematic somewhere...
Indeed. Yet, I was also thinking that most of the stick voltages are so similar that a value about 0.1V lower could be a tip-off... Problem is, I don't think the typical pack has such similar voltages, plus, it's really only obvious when you see all the cell voltages, plus, if anything, we usually only look at tap voltages. So, yeah, really hard to get anywhere just looking at aggregates.

I too start thinking about some sort of automated cell-level tester when doing this stuff. Yet I'm also always thinking 'this is the last time I mess with NiMH'... I'm really having a hard time 'this year', as I have all these sticks with great cells - except for these seemingly aberrant single cell HSD ones. I'm still mulling over whether it's worth building a cell zapper, such as using one or more of the MDM caps. I've been leaning away from that, I've been trying to peel myself away from the NiMH all together and get to building an LTO pack. But, it's hard to just say "No" to Insight NiMH...

....You may have touched on the reason folks have such poor success with rebuilding efforts. I have done a lot of cell level qualification based of heavy test currents and high indicated internal resistance(IR), but even that did not insure good packs. Looks like I may have overlooked the importance of self discharge(SD). One has to be impressed with how uniformly "good" cells are in their SD rates.
Yeah, I think single high self discharge cells is probably one of the main reasons for failed rebuilds. Unless you look at the cell level, it'd be hard to spot. I didn't start looking at cell level with respect to self discharge until late in the game (a few years ago), and even then I still gave it short shrift. I think the main reason we didn't pay enough attention is because we didn't fully appreciate that voltage alone can indicate HSD; you don't need to discharge the cells, but you do need to look at the voltages...

Of the three major parameters, IR, self discharge, and capacity what is your ranking of defect likelihood?
I haven't worked with enough packs to really know. I think HSD is probably the kiss of death for most packs. For high IR and capacity issues the car seems to be able to work with those, where failure will only be gradual, or where you can actually alter your driving behavior or putz with the OEM management to make things work. But with high self discharge - there's a threshold beyond which you'll never be able to use the pack.

I think I'm starting to grasp how these failure modes work or don't work with the OEM management. For example, with a really fast self discharge cell, you can have a totally depleted cell in a matter of days. If you left the pack after a pos recal, say 75% charge state, and let the car sit for a few days, you could come back and have an imbalance of just about 75%. You'll get a neg recal, the car will try to charge the pack and it will allow more charging than usual, but if your other cells are at 75% charge state, you only have a max 25% headroom. In effect, your working, usable capacity will always be under 25%, and probably quite a bit less. And then, you only have ~15% wiggle room until you get a P1449-78 code, when the car can't charge the pack more than 10% (and I've actually seen this triggered at 20%). In other words, if most cells are at 75% and one is empty, the car will struggle to charge the empty cell more than 10% while keeping the other cells from overcharging.

So basically, HSD is what I think must push packs over the edge. IR and capacity are usually at least partially intermingled. In the first instance a deteriorated pack will have low capacity and high IR -- low capacity due to degraded active material and low capacity because high IR makes voltage sag and triggers end of discharge prematurely (and also triggers premature end of charge, due to voltage spike). But a lot of this is reversible via discharge, deep/ultra-deep discharge, etc. After that, though, you might be left with cells that seem damaged in some more material way, that still exhibit high IR. I just can't say how common that is.

But I don't think even these high IR cells will cause a rebuild to fail. I think it's probably pretty cut and dried, that it depends on just how bad the IR is, and also on how you use the car. High IR cells won't charge as much as the others, and I think the amount is probably at least in part a strictly 'ohmic' calculation: if you have a 2 cell stick with one cell at 1.40V during charge and one at 1.45V during charge, where the voltage difference is due to higher IR in the 1.45V cell, then the 1.45V cell will charge less at a given current; I think it'd be simply ~3.6% less, or perhaps you could say it would require 3.6% more current to charge the same amount as the 1.40V cell. So, for every 100mAh of charging you 'lose' 3.6mAh in the 1.45V cell. That's a relatively tiny amount. I don't think that would add up or compound in any debilitating way over time and usage, vis-a-vis the mild balancing that you can get under OEM management. But, I don't know, I'm not too sure about this stuff, it gets confusing and complicated.

To answer your question, I think most high IR and capacity issues are the more common, perennial issue in our packs and have to do with 'crud'. (Ultra-)Deep discharging and grid charging can get rid of a lot of the crud and can lower IR and increase capacity. So, I think I'd rank these as number 1. HSD cells probably 2 but a more serious failure. Aberrant high IR cells 3... I could be wrong though. Also, perhaps at the core of all issues, it seems like my middle cells are more often the failures, so I'm thinking that differential heating and cooling could be the or a initial source of failure, the thing that kicks it all off. Middle cells get hotter and stay hotter longer than stick end cells, all else being equal. Their position in the pack would also make a difference...

Also, overall, any degradation is competing with that mild balancing I mention above. Off the top of my head, I'm thinking OEM management might be able to cope with something like 5-10% imbalance under some generic driving circumstances, the typical person's usage... I think it's possible or likely that a 'typical drive' can restore at least a few percentage-points to any cell-to-cell imbalance.

I'm a bit confused by the term "equilibrium voltage." How do you define this term?
As I recall it's based on thermodynamic calculations of the active materials of the cell - the potential difference (i.e. voltage) you calculate based on negative electrode stuff and positive electrode stuff. This 1.318V value though is something I saw in I think that 'Civic' battery research paper that's been around IC for ages*... What's fascinating is just how close a lot of my cell voltage readings are to this value... "Equilibrium" conditions in this context is what the battery folks call anything other than charging or discharging, it's similar to "resting voltage," only a resting voltage that'd be measured under ideal circumstances, such as a new cell, half charged, 25C temp, etc. I think it's also an antonym of "transient" conditions, where charging and discharging are considered transient...

*Page 32 of 'Civic Battery Paper.pdf' among docs in the downloadable manual bundle. Given the half reaction potential at the positive electrode, of +0.49V, and the half reaction potential at the neg electrode, of -0.823V, the total reaction is 1.318V (though combining these I only get 1.313V, must be some rounding or something). Here's a snip:

83653
 

·
Registered
Joined
·
6,313 Posts
Discussion Starter #103
I took a closer look at the sticks with 'red-highlighted' cells, the high self discharge cells. I discharged those cells individually to see what capacity they had left given their relatively lower voltages. Although it's clear that all the red cells have high self discharge, given the remaining capacity they wouldn't all cause problems in a rebuild, at least in the near term. If self discharge didn't get worse, a couple of them would probably be OK. All the non-HSD cells discharged almost exactly the same amount, between 4600mAh and 4700mAh, so they probably self discharged very very little, remaining at about 71% SoC.

Here are the data, starting with the stick with the lowest voltage HSD cell:
S50: cell 2 at 1.225V vs. other cells averaging 1.324V, -0815mAh, about -115mAh SD/day
S37: cell 5 at 1.249V vs. other cells averaging 1.324V, -1494mAh, about -096mAh SD/day
S41: cells 3 & 5 at 1.268V vs. other cells averaging 1.326V, -2408,2343mAh, about -70mAh SD/day
S41: cell 1 at 1.319V discharged -4735 mAh
S45: cell 5 at 1.282V vs. other cells averaging 1.324V, -3233mAh, about -041mAh SD/day

So, a couple thoughts: You can see that S45 cell 5, at 1.282V, still held quite a decent amount of charge, losing on average only 41 mAh per day. If this cell didn't deteriorate in terms of SD, it'd probably work.

S41 cell 1, a "questionable" cell at 1.319V, put out as much as the good cells...

The middling value HSD cells, 1.249V and 1.268V, lost about 96 mAh/day and 70 mAh/day, respectively. After a bit over a month sitting, they held about 32% to 51% of the good cells. My guess is that these would eventually cause problems in a normally driven/used car, though one could probably be vigilant, take steps to help ensure that they're more likely to stay charged.

S50 cell 5 at 1.225V, losing -115 mAh/day, holding only about 18% after a month sitting? Same thing, it'd cause problems without 'vigilance'.

....All the sticks were more or less conditioned, balanced, and left at about 73% charge state 33 days ago. Most of the cells still should be at around 70% charge state. The red-ish highlighted cells are ones with high self discharge (HSD) - they likely have very little charge left in them. Most of the sticks with these cells won't work, they'll cause any pack they're in to fail. Stick 50 for sure, probably stick 37, and 45 and 41 will cause problems eventually. The yellow highlighted cells are questionable. Sticks 71 to 74 at the bottom come from a different pack, which is why voltages are slightly different. All the sticks are 2007 Civics, though.
 

·
Registered
Joined
·
6,313 Posts
Discussion Starter #104
Here's a link to a post that goes into the battery management implications of some things I saw working with another fast self discharge cell/stick: https://www.insightcentral.net/threads/oem-pack-management-efficiency-not-so-good.117785/#post-1450808
I posted a graphic there illustrating how the deep discharge that a fast self discharge cell will experience actually ends up preventing voltage depression - in that particular cell - while it would end up inducing voltage depression in the other cells. I guess I'll just post the graphic here as well. If you're interested in more info you can go over to that thread.

83680
 

·
Registered
Joined
·
6,313 Posts
Discussion Starter #105
I've talked quite a bit about self discharge in this thread lately, and also in a couple related threads. And, in general, I've talked about 'battery management' in related ways in one or two other threads. Since I've put the most about self discharge in this thread I want to put the following musings here as well, though it has more to do with 'battery management' than 'NiMH voltage' per se...

I started talking about self discharge some posts back, linked here: https://www.insightcentral.net/threads/the-quintessential-insight-nimh-voltage-thread.89298/page-5#post-1450644

I was thinking about self discharge, battery management, and reconditioning last night, and vaguely realized that it wouldn't be a very difficult 'calculation' to figure out just how much self discharge is or would be too much. I think there's only a handful of things to think about, to understand. Here's a back of the envelope sketch:

-In general, we have a max usable capacity of about 4000 mAh. Self discharge is a matter of X many mAh per time span, such as days, for example 100mAh per day.
-Driving, particularly certain kinds of driving and use of the IMA, produces some balancing. We can quantify that by saying we get X mAh of balancing 'per drive', or 'per day' or something like that.

-Add one difficult nuance: 120 cells at potentially different charge states with potentially different self discharge rates. I don't think we need to deal with this yet for the sake of just painting the broad strokes, but it matters...

I think that's it. The idea is simply this: Your self discharge rate has to be slower than the net balancing effect/rate you get from however much and the type of driving you do, in the context of a total usable capacity of about 4000mAh.

So, some posts up I think my worst self discharge cell was something like 115mAh per day. Assuming the pack were fully car-charged (i.e. about 4000mAh usable capacity) and balanced at start, that cell would be totally empty in 4000mAh/115mAh/day = 35 days. Obviously I'd need to drive it more than once a month.

The question is, 'How much would I need to use the car to continue using this pack without having that self discharge cell mess things up?'

This hinges on a couple things, mainly, How much balancing do/can I get from top-end pack use?

I don't have solid data for this, but I think I can make some worthwhile guesstimates. The amount of self discharge 'off the top', that is, after a true 100% full charge, is something like a few hundred milliamp-hours. If you charge a cell to full and discharge shortly after the charge you get an ~6000mAh discharge; if you charge a cell to full and discharge the next day, you get something like 5700 mAh, i.e. 300mAh self discharge off the top. Something similar plays a role in the top-end balancing that should happen in the car.

Now, I'm gonna say that this off-the-top self discharge can play out over the top ~1600mAh of capacity. So if you have a 6500mAh cell you're gonna get top-end SD only over the top 25% of the charge state range, i.e. anything above 75%. This means that if you charge the cell above 75% you'll get some 'top-end' self discharge. If you charge to 100% you'd get the full 300mAh; if you charge to 75% you get nothing. This means that the type of driving, the type of IMA usage that needs to happen to get balancing has to happen above 75% SoC...

So let's assume we get to charge to 85% SoC, or 10 points above 75%. This means we can get 10%/25% X 300mAh = 120mAh of top-end SD balancing. Though cell voltage differences play a role as well, where you get some balancing during the charge, where higher voltage cells will charge slower, I think it's a relatively small amount, so I'll ignore it and assume the only balancing we get is the top-end SD variety...

In my example case I have a 115mAh SD/day cell, and I'm saying I can get 120mAh top-end SD balancing per... day: On day 1 I drive the car, the cells are balanced, and they're all charged to 75%. On day 2 this high SD cell has lost 115 mAh. IF on day 2 I drive and charge the pack to car-full, calling this 85% SoC, I should get a balancing effect that restores this cell to the level of others. On day 3 the cell will again be -115mAh...

So, I'd have to drive every day and charge to full every day in order for top-end SD balancing to overcome a 115 mAh/day self discharge rate.

Although some of these values are guesstimates, I think they're probably ballpark. You should be able to extrapolate from this example to generate scenarios for different driving regimes and different self discharge rates. My -115mAh SD/day cell seems borderline. Having to drive every day and charging to full every day is an extreme usage scenario: People don't usually do these things on a normal, day-to-day basis. If I were driving normally, this cell would gradually self discharge and drop lower and lower relative to other cells, and cause IMA problems.
 

·
Registered
Joined
·
6,313 Posts
Discussion Starter #106 (Edited)
For a while now I've been trying to get a sense of how well or whether voltages can be used as a proxy for capacity balance or imbalance. I think the general assumption is that balanced voltages, such as at cell or tap level, should approximate balanced capacity, i.e. if the taps or cells are balanced the voltages should be similar. To some extent this is certainly true, but after doing a grid charge, driving and draining the pack, logging some tap voltages, I can more clearly see at least one pretty solid case where it doesn't hold true very well.

I've generally known that voltages can vary more after a full charge, such as cell voltages for a stick or tap voltages for a pack. But I've never really had a good sense of the amount of variation that's 'acceptable' and how that variation may map to capacity differences. Basically, one question I've had is this: Good, balanced, 'optimized' cells will have very even voltages (like within a few millivolts), but do uneven voltages necessarily mean uneven capacity and/or to what extent?

Here's some tap voltages illustrating these points.

1) The first set is at the end of a grid charge (still loaded at about 250mA) that should have pushed all cells to about full. I estimated that some cells would have been overcharged by something like 30%.

2) The second row is the next day before a drive (~12 hours after GC).

3) And the third row is a few days later after draining the pack down to about 16% true state of charge. Most of that drain happened on day 1 (about -5000mAh).

1) 17.54,41,44,44,38,31,38,55,33,40 174.3V 68F low 17.31V, high 17.55V, spread 0.24V
2) 16.73,67,69,71,61,56,62,75,58,63 166.6V 50F low 16.56V, high 16.75V, spread 0.19V
3) 14.91,90,89,89,91,91,92,94,92,91 149.1V 39F low 14.89V, high 14.94V, spread 0.05V


You can see how row 1 and 2 values are pretty widely varied. But after a drain to near empty, the voltages are quite close. This suggests that the cells are pretty well balanced capacity-wise: I'm able to drain the pack low and the voltages are close. So, despite the quite uneven voltages at end of GC and even after 12 hours rest, the cells are pretty even in terms of capacity.
 
101 - 106 of 106 Posts
Top