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Discussion Starter #141 (Edited)
I've been thinking about cobbling together an LTO-based 12V battery to replace my NiMH one, just for fun. I bought a buttload of small LTO cells a while back for an IMA pack, but haven't found time to work around some issues. So, I should have extras and thought an LTO 12V might be a relatively simple interim thing, plus it'd give me some experience with the cells to see how they hold up. The thing is, the LTO voltage is just on the cusp of not being usable in the Insight 12V context - which brought me back to this thread, revisiting the ideas around hacking the adjustability of DCDC output, which are still a bit over my head and certainly over my experience....

The cells can float at 2.7V, possibly 2.8V. That puts them at 100% full. The normal power mode 12V system voltage is something like 14V at the battery. 14V / 5 cells=2.8V. So, when the DCDC is upholding system voltage at around 14V, each LTO cell should be about 2.8V, which is really just a bit too high. Hence, my need to dig into the Arduino hack thing that Mudder mentioned some many months ago (I think I revisit this idea once a year, about this time of the year every year)... Not sure I can stomach the research to get this thing to go, but maybe. Alternatively, I might just cobble together the battery and see what happens leaving system voltage alone. The cells are supposed to be pretty robust, not prone to explode, etc., so, it might be worth just giving it a go as-is... Still deciding... I've been doing a lot of other small projects on the Insight as well, so they all kind of compete for my attention...
I'm back to this again. I have a fairly pointed question that I'm hoping someone in the know can help me with. Thought about creating a new thread with just that question/issue, but came here and see that I left off with the whole LTO12V idea - so here it will go.

This is a tough one to explain - and I think Mudder once tried to explain it, but I'm not sure where, plus, it was hard to understand. This is the core of the question: What regulates the current rate while charging the Insight 12V battery? And there's a lot of other smaller questions that follow from this.

My understanding is that the actual rate isn't regulated per se, but the voltage is - so the current going to the 12V battery will depend on the battery itself, such as its charging characteristics and its state of charge, and the power needed in the rest of the 12V system, such as ignition, lights, climate control, and so on, all within the limits of what the DCDC can actually produce (which I think is something like 700 watts)...

So to me, here's the problem I'm dealing with. First, with a lead acid 12V battery, the 'ESR' is so high that it takes very little current to push the battery voltage up to 14V. I don't know what it is exactly, but I'd guess it's below 10 amps at any charge state. That's perfectly fine for the battery and the system wiring, for instance, <10 amps...

Now, I currently have a stick-based NiMH 12V - and I can see that's a little different. Watching some OBDIIC&C parameters I can see that, when the DCDC toggles from low power mode (about 12.2V) to normal power mode (about 13.8V), and when my NiMH 12V battery is fairly discharged, the current rate can be very high. I was watching this today and saw about 150 watts charging the 12V battery, so that's about 10.9 amps (150W/13.8V). I've seen higher before, I'm sure I've seen at least 288 watts for a good 10-30 seconds or so, which would be 20.9 amps! Even this makes me nervous, makes me wonder: I know Mudder has said that the wires going from the DCDC to the 12V battery, or at least the 12V system, are skimpy. If my NiMH 12V battery is sinking 20 amps of current, doesn't that mean 20 amps must be coursing through the wires that go from the DCDC to the 12V system/battery?? This is really the critical issue I'm dealing with. I've been using the NiMH 12V for a while now with no obvious problems or troubles, but perhaps, even with the NiMH 12V, the current that the 12V system wires are seeing on occasion is higher than they were meant for, is higher than they should deal with. AND, NOW, on to the LTO cells:

The LTO cells have even lower resistance than my NiMH, are more efficient charging. Plus, a ~13.8V or 14V 'float' or system voltage pushes the per cell voltage to the very top of their range. It would take a lot of current to push a 5 cell LTO 12V battery to 13.8-14V - and that's what I'm afraid of. Is this, theoretically, or technically, how it all would work? -- If I installed the really efficient, low ESR LTO 12V battery, would the Insight DCDC toggle to normal power mode, and if the LTO 12V battery were somewhat discharged it would sink whatever current it could until its voltage hit ~14V? That could be a very large amount of current. Even at 36 amps I project that the cells wouldn't reach a voltage of even 2.7V unless they were a tad above 80% charge state... At the DCDC low power mode, ~12.2V, the cells could be as low as roughly 50% charge state; at that charge state I can't even fathom what current rate would be necessary to push their voltage up to 2.8V, such as when the DCDC toggles back into normal power mode, ~13.8-14V...

Anyway, I think that covers it. I hope someone can tell me whether I'm on the right track or not. I have concerns about the cells, but that's not even my main concern; my main concern is that the LTO12V wouldn't work in the car as-is because it could potentially induce a super high current on 12V system wiring not meant for such high current...

hmm, re-reading a bit, quick back of the envelope calc: if the DCDC can produce a max of say 700 watts, and about 150 watts are needed to run the car electronics (no accessories), then that leaves 550 watts. With my LTO12V below 80% charge state it will sink a lot of current; 550W/14V=39.2 amps -- so it seems likely that my LTO12V would max-out the DCDC, sinking that 39 amps* until it approached 100% charge state... Let's say the battery is at 50% charge state, which leaves about 1.5 amp-hours empty (it's only a 3Ah battery). How long will it take to charge to full, and thus, how long will it pose about a 40 amp current burden on the DCDC and associated wiring?: 1.5Ah/39A=0.0385hours, or about 2 minutes and 18 seconds... That seems long enough to melt some wires. It's looking like a direct swap-in of the LTO12V wouldn't work, it's looking like I really do need to hack the DCDC voltage output...

*Or rather, it would blow the 30A fuse... Or maybe that fuse doesn't factor-in to this question - I was looking at Mudder's 'unified' wiring diagram, thinking the bolded red line was the DCDC output, which goes to that 30A fuse on the IMA battery electronics board. But I think that's probably the pack high voltage DC power going into the DCDC, that's bisected by that 30A fuse, so it's meant to support 30A at pack voltages... Not sure, it's all pretty confusing...

(later...) So, as far as I can tell, there should be a white wire that goes from the DCDC all the way up to the underhood fuse box - I'm thinking that's one of the wires connected on the rear end of the DCDC (the electrical troubleshooting pdf seems to have the wrong image for this; it shows the two big cables on top of the pack that go to the motor power inverter, or whatever, over on the 'left side' of the IMA compartment, not to the DCDC). That seems like it's the main 12V system wire - the trunk that carries all the 12V system current from the DCDC to the rest of the car... Someone correct me if I'm wrong...

OK, so this doesn't make any sense. If the whole 12V system can demand say up to 50 or 60 amps, then shouldn't the wiring, like that white DCDC wire I mention above, be sized to handle up to that amount? If the DCDC can only deliver about 700 watts - well, that's it, that's the limit, and all the 'wiring' should be able to handle it. I.e. 'efficient' low esr LTO12V or not, it's only gonna maxout the system, and 'the system' should be able to handle its max, shouldn't it?? Whether my total 12V system load is say 50 amps while powering the AC, climate control, radiator fan, headlights, etc. or nothing + charging an efficient LTO 12V battery, what's the difference? Doesn't seem like there is one. So maybe my worries about the wiring are off track...
 

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I'm pretty sure the white wire that goes from the DC-DC converter to the front of the car is a 6 gauge wire. At any rate, it should definitely be able to handle whatever number of amps the DC-DC converter is able to put out.

Let's assume the DC-DC converter can output up to 14.4 volts and up to 70 amps. Let's also ignore factors like voltage drop over distance and how the DC-DC converter's output voltage varies a little based on engine temperature. Let's also say that the car is consuming 15 amps. This would give 55 amps available for other purposes.

As far as charging the battery goes, one of two things will happen.

If the battery isn't able to completely absorb a 55 amp charge rate, the voltage at the battery will rise quickly until it reaches an equilibrium point with the DC-DC converter's maximum output voltage. At this happens the numbers of amps flowing into the battery will decrease to zero. That or the battery will overheat, explode, and/or catch fire.

Now let's say the battery is able to safely absorb a charge rate of over 100 amps. The battery will act like a huge load on the system, causing the output voltage of the DC-DC to droop. This voltage droop will cause the DC-DC supply up to 70 amps in an effort to bring it's output voltage back to 14.4 volts. As the battery charges it's voltage will rise. As battery voltages approaches 14.4v the current will lessen until the battery and the DC-DC converter are at equilibrium.

The 30 amps fuse is on the high voltage side of the DC-DC converter. Even given a maximum load worst case scenario, the DC-DC converter isn't going to draw more than 10-12 amps on the high voltage side.
 

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Discussion Starter #143
^Thanks, Bull Dog. This all seems consistent with what I was thinking by the end of my post... Do you happen to know the actual max output of the DCDC? I was thinking it was only about 700W, your example puts it at about 1000W...

The idea that if the battery can absorb say 100 amps it will make the DCDC output voltage drop - that's interesting. This LTO12V just might do that. The cells are supposed to be able to charge from empty to 'over 80%' in 1 minute, or 0.0167 hours: 2.55Ah/0.0167 hours= 153 amps! Another spec is 480W input at 50% SoC: 5 cells X 480W = 2400W, 2400W / say 13.8V = 174 amps... I've only been able to test up to 20 amps.

I don't see this high current happening all the time, but rather, the worst case scenario in terms of max current would be when:

-The DCDC has been in low power mode for long-ish durations.
-This (can) drain the battery down to about 12.2V.
-Then, when the car toggles back into normal power mode, suddenly the DCDC output is at about 14V.
-The 12V battery gets drained and then has room to be charged.

It's during these intervals when the charging current to the battery would be highest...
 

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Discussion Starter #145
fyi, I more or less finished-up with the 'adjusting DCDC output voltage' aspect of my LTO 12V idea and decided to make a thread about it here: https://www.insightcentral.net/threads/mod-to-adjust-dcdc-converter-output-voltage.125303/#post-1448093

In the end, I can't adjust DCDC output voltage low enough via DVCT to make the LTO 12V perfectly acceptable. The lowest it goes is 13.85V, which would be 2.77V per LTO cell if they're all perfectly balanced. These Toshiba SCiB LTO cells should be charged to a max of 2.70V. The high total voltage risks pushing single cells well beyond 2.7V, if only for a brief duration. 2.77V in and of itself isn't a big deal; it's the higher than that risk which might matter... Not sure, I'll probably go ahead and install one anyway and see how it goes.
 

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Go for it. "You never know how far you can go till you get there". (My motto)
 

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Here is my $.02. A year ago I had a phantom drain that after a few 12V drains killed my PbS02 battery. I hated to put a $100 battery in that I knew was overkill in almost every situation.

I had already been carrying a pair of NiMH sticks in my car as an "emergency jump starter" which in fact worked like any of the low-current ones you can by for $50 or more... simply making up the difference for a "weak" main battery, especially under cold conditions, not necessarily providing the full cold-cranking current required.

So before I went out and bought a new Pb, I thought I would try using it alone. Viola! It worked like a charm... while my IMA is weak, as long as it is charged up above a certain level, the system still seems to start with the IMA, even though the 12V disconnect leaves it thinking it has a 0%SOC. At first, I thought it was the 12V starter (just because that seemed reasonable) but it wasn't most of the time, however, when it DID need to start via the 12V it fired up almost instantly... the starter (like most DC motors?) really liked the extra voltage and spun right up!

Unfortunately with 12 cells, my nominal voltage more like 16 than the DC-DC 14.5 so that in fact it didn't keep the sticks charged enough. Fortunately I carried yet another extra pair (still to be used as jumpers) that I switched in to get home the first time it failed me. I considered tapping in at 11 or 10 cells like the OP but never got around to it. Instead, confident that my NiMH "jump starter" was good enough in a pinch to start alone, I took the risk of replacing my full-sized Pb battery with the $30 utility ones from autozone. I've been running on that ever since with no problems. MY IMA is still weak but it still seems to start the car most of the time, and when it doesn't, the 12V utility battery is plenty. I do live at high altitude so winter starts can be below 0C, but not the chronic 0F of the upper midwest... so it might not work as well in those conditions.
 

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Discussion Starter #148
^ I used my 12V NiMH a bunch of times when I had the IMA bypassed. It worked pretty well, never failed to start the car... A handful of people have opted to use the small 'garden' type lead acids with success...

On your IMA starts - pretty sure whether the car starts using the IMA or 12V auxiliary starter depends on pack voltage alone, as long as you don't have a disabling IMA trouble code pending. So, for instance, if you reset by pulling a fuse or the neg 12V cable, and the dash BATT gauge goes to zero, it will still start off the IMA.
 

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Discussion Starter #149
A, what happened to the rest of this thread?? Jime recently posted the following at the end of the thread, which included several posts over the last few days, I got the email notification, yet the thread doesn't have that post nor others over the last several days... A big swath has disappeared. Here's what Jime posted:

"Thanks for the thoughts. All of it is good thought and I'm rolling over in my own mind how to implement much of it. I do have another more major concern, but for now I'm going to move ahead. The concern - I'm not quite sure that the 5 cell approach is quite right vs. using 6 cells. I've..."
 

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Sorry. I should have left a note. I move the part of the thread which applied to the LTO 12V Battery Build to a new thread. I thought is was developing into a new subject. The appropriate posts certainly needed a new title. The moved posts are here:

 
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