Aftermarket iMiEV battery upgrade project

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nuggetgalore
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Re: Aftermarket iMiEV battery upgrade project

Post by nuggetgalore »

coulomb wrote: Sun, 04 Apr 2021, 17:07
nuggetgalore wrote: Sun, 04 Apr 2021, 16:46 I doubt that the shock from connecting two packs with 2 or 3 volt difference could kill a cell, but maybe I am wrong again?
I agree, paralleling the packs with such a small voltage difference would not have caused much current to flow, much less killed a cell.

I thought you measured the 25 V on the bench, with a power supply limited to 25 V. If it's still buried in the pack, by guess is that a link has gone open circuit or high resistance.
Still buried in the pack when I measured the high volt across that cell. I have removed the cell from the module and it is definitely dead.
coulomb wrote: Sun, 04 Apr 2021, 17:07
I can imagine about a 25 V drop (7.5%) of the main pack under load; if there was an open circuit link, most of that 25 V would appear to be that cell's voltage. Actually, I would expect it to appear to reverse in voltage.
Edit: Not quite correct.
Thanks for pointing this out, yes it must have been reverse, as the good cell volts did not change but the pack volt went down as I drove a few km up hill with the main pack behaving normal.Yes it must have been reverse volt while driving, but it was actually while charging that I was able to measure module and cell voltages where I found the discrepancy of pack voltage v cell volts x 88. I did not measure module or cell volts while driving, I can only observe (relatively safely) what the EvBatMon app displays with large digits and the millivolts measured across the current shunt between the two packs.
coulomb wrote: Sun, 04 Apr 2021, 17:07 Luckily the main pack was there to limit the reversal.

So maybe the cell is OK after all.
Dead as a dodo. Does not even light a tiny torch light globe.
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Re: Aftermarket iMiEV battery upgrade project

Post by nuggetgalore »

nuggetgalore wrote: Sun, 04 Apr 2021, 18:29

Dead as a dodo. Does not even light a tiny torch light globe.
I still have no idea why this cell died and coincidentally showed up as I paralleled the two packs the first time (with practicably no voltage difference ie no current shock).
I replaced that cell with the one spare I had left and charged and driven the car successfully now for several hundred km.
I get a realistic 120 km range. I think I can detect a very small increase in energy use per km due to the added weight of approx 100kg by comparing trips I recorded prior (CaniOn) with similar trips now. In the big scheme of things I consider it negligible. The bigger question is how long until the next cell dies for no apparent reason. So far they all appear to be ok.

Here a few observations re the current draw while charging and driving.
Scanners and apps show the amps exactly as before what goes in from charger to battery resp out to inverter. I have a current shunt showing what goes in and out of the additional pack.
Standard charging at 12.5 A from 240 VAC indicates 7 or 8 A DC (depending on app or scanner).
Strangely only about 2.5 to 3 amps go to the additional pack.
But the reduced current into the main battery reduces the difference of the voltages of the lowest to the highest cell considerably (from >0.04 to <0.02 V) during charging (for other reasons charging stops just short of the pack getting to 361V, resp cells to 4.11V).* The same is true for discharge under load, the voltage differences shown on EvBatMon are considerately smaller than before with either of the packs.

I had a mate recording videos of the EvBatMon app next to the DMM showing the current in mV that is drawn out or charged back to the additional pack while I was driving.
The amps to and from the additional pack read as much as 60 to 70% of the total current drawn or regenerated (opposite to when charging of the wall).
Quite often after stopping, there is a current of a fraction of an A up to 2 or 3 A flowing either one way or the other for a few minutes until the packs are equal.

* francisco.shi probably knows the answer to this with his knowledge of SoC and coulomb counting.
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Re: Aftermarket iMiEV battery upgrade project

Post by francisco.shi »

I assume you have two packs connected in parallel.
So if they had different impedance then one would put out more energy than the other.
Let's say we have 2 packs of the same capacity and one has 1Ohm and the other 0.5Ohm internal resistance. We put a load of say 10A. So the voltage will sag to 3.3V (1 Ohm in parallel with 0.5Ohm gives you 0.33ohms)
So now we get that 3.3v into 1 Ohm is 3.3A and 3.3v into 0.5Ohm is 6.6A.
Let's say we do this for 1min. So the pack with 0.5Ohm has supplied 6.6Amin and the other pack has supplied only 3.3Amin. So one pack has discharged more than the other. When you remove the load the pack that has more charge will now try to charge the pack that has less charge hence thehe current flow. If both packs were made of the same cells (and same age) the difference will be so small you wouldn't notice it. Also note that the length of cable connecting the packs is part of the internal resistance. So if one pack is in the front the other in the back and the inverter is at the front the pack at the back will appear to have higher internal resistance.
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Re: Aftermarket iMiEV battery upgrade project

Post by nuggetgalore »

francisco.shi wrote: Fri, 16 Apr 2021, 15:40 I assume you have two packs connected in parallel.
So if they had different impedance then one would put out more energy than the other.
Let's say we have 2 packs of the same capacity and one has 1Ohm and the other 0.5Ohm internal resistance. We put a load of say 10A. So the voltage will sag to 3.3V (1 Ohm in parallel with 0.5Ohm gives you 0.33ohms)
So now we get that 3.3v into 1 Ohm is 3.3A and 3.3v into 0.5Ohm is 6.6A.
Let's say we do this for 1min. So the pack with 0.5Ohm has supplied 6.6Amin and the other pack has supplied only 3.3Amin. So one pack has discharged more than the other. When you remove the load the pack that has more charge will now try to charge the pack that has less charge hence thehe current flow. If both packs were made of the same cells (and same age) the difference will be so small you wouldn't notice it. Also note that the length of cable connecting the packs is part of the internal resistance. So if one pack is in the front the other in the back and the inverter is at the front the pack at the back will appear to have higher internal resistance.
Thanks for that explanation. There is a further complication in that the two packs are different chemistries.
One pack,the original iMiEV LMO LEV50s are degraded to about 65%.
The other pack with the so called "60 Ah" NMC LIYUAN cells that gave me about the same range as the degraded old cells at first but then dropped of (maybe because of the failed cell although the BMS did not show any bad cells while they were installed as the sole traction pack).
The discharge curves show that this is not a good marriage, but like in real life one has to make compromises....and so far it is working as good as I could hope, the accidental reset of the battery health a definite bonus now.
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brunohill
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Re: Aftermarket iMiEV battery upgrade project

Post by brunohill »

Hey Andreas,
Does your iMiEV have Chademo? Are you brave enough?
Last edited by brunohill on Fri, 16 Apr 2021, 20:58, edited 1 time in total.
francisco.shi
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Re: Aftermarket iMiEV battery upgrade project

Post by francisco.shi »

The different chemistries shouldn't be a problem as long as you don't exceed either of the voltage limits. If the voltages are not the same then one will be underutilized.
And each pack should have its own BMS.
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Re: Aftermarket iMiEV battery upgrade project

Post by nuggetgalore »

brunohill wrote: Fri, 16 Apr 2021, 20:45 Hey Andreas,
Does your iMiEV have Chademo?
yes.
brunohill wrote: Fri, 16 Apr 2021, 20:45 Are you brave enough?
No, not yet anyway!
I have fast charged the original pack only once, but the previous owner did it all the time.
The aftermarket pack I charged twice at Monash Uni Chademo to check if it is working, but since the battery was about 3/4 full it only charged for for a few minutes at more than about 8 A which is the same rate as the OBC pumps in and I stopped at 80%.
I cannot see any real problem once I have a better BMS on the additional pack. Currently I only have cell balancers that start balance if they detect 100mV difference, so far the worst is only
40 mV while charging and I do keep a keen eye on it.
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Re: Aftermarket iMiEV battery upgrade project

Post by nuggetgalore »

francisco.shi wrote: Fri, 16 Apr 2021, 20:49 The different chemistries shouldn't be a problem as long as you don't exceed either of the voltage limits. If the voltages are not the same then one will be underutilized.
For better or worse, the two packs are combined by a relay designed for EVs that energises when the main pack relays are on. Additionally I have a low amp fused manual switch connecting the two packs that allow me to observe the voltage difference and/or current flow between the packs when the car is switched off. That way I make sure the two packs are always exactly the same volts when the car is switched on.
francisco.shi wrote: Fri, 16 Apr 2021, 20:49 And each pack should have its own BMS.
Yes I know, I am working on that but shortly the car gets mothballed for 3 months and if it has not died by then I get cracking.
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Re: Aftermarket iMiEV battery upgrade project

Post by jonescg »

Might pay to make that fuse a bit more than 'low'. Perhaps rated to 50 amps or something? It wouldn't need to be a high voltage fuse as the potential across it should only be in the tens of volts at worst, but it would need to be a fast acting one.
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nuggetgalore
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Re: Aftermarket iMiEV battery upgrade project

Post by nuggetgalore »

jonescg wrote: Sat, 17 Apr 2021, 11:05 Might pay to make that fuse a bit more than 'low'. Perhaps rated to 50 amps or something? It wouldn't need to be a high voltage fuse as the potential across it should only be in the tens of volts at worst, but it would need to be a fast acting one.
The potential between the two packs has never been more than about 3 V when I connected them after a few hours disconnect. When they are not connected, the LEV50 cell pack loses a tiny bit more of the top than the aftermarket one .
The spare cell I had was still 4.11 V after sitting on the shelf for months, a feature I found was common with these cells when I charged them up originally. But as soon as a small load is attached, they drop the voltage at a great rate and a lot different from one to the others.
I have high voltage fuses but maybe I should get a fast acting one.
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