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Old 06-21-2012, 02:01 PM   #1 (permalink)
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Lithium cell / BMS / balancing question

So, I'm rebuilding my PHEV kit for the Prius and am also working to finish up the DIY BMS system on it. My question is kind of a balance at the top vs balance at the bottom question, but with my specific equipment and BMS. First off, my BMS doesn't do any active balancing. For the high charge cut off it monitors cell voltages and cuts the charger off once the first cell hits 3.65V. To determine the low charge cutoff point I am using a current sensor and am counting the amphours out of the pack. I have gone through and capacity tested every single cell in the pack so I know the exact capacity of the cells at a 1C discharge rage (in use it doesn't even pull 1C). So, I will be setting a conservative number to get ~70% DOD on my weakest cell and once that cell hits 70% DOD it will turn off the PHEV kit.

As of right now I have my cells fully charged. I could assemble them as is and they would be balanced at the top. The weakest cell in the pack would be charged to 95% SOC (my limit) and discharged to ~25%. The strongest cell would be charged to the same 95% SOC and discharged down to something higher than 25%.

If I discharge every cell down and balance them at the bottom I have the opposite situation. The weak cell will still be charged to 95% and down to 25%. However, the strongest cells would be charged to something less than 95% and discharged down to 25%.

So, balancing at the bottom kind of keeps the strong cells more in the 'middle' of the SOC range. Balancing at the top will keep the SOC of the strong cells more shifted towards the higher end of the SOC range.

My question is which is really going to be better?

I have arguments for both ways.

For balancing at the top:
1) Its easier & quicker. Just top off the cells and go.
2) Internal resistance increases as cell SOC approaches the low end, thus your cells heat up more at low SOC levels. So, your cells should stay a little cooler.

For balancing at the bottom:
1) May increase cycle life of stronger cells due to keeping their SOC more in the middle of their band? I'm not so sure this is actually true or not. Obviously I'm more concerned with my weaker cells and their life, and they are treated the same in both cases.
2) balancing at the bottom is a pretty great way to balance out a pack and is what I had been doing without any balancing issues before. So its kind of tried and true for me. Not that other systems all use bottom balancing, I'm just more used to it.

So, I'm really looking for some additional input on this. I know there are probably more plusses and minuses that I'm missing.

Current project: A better alternator delete
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