Lithium cell / BMS / balancing question

[Daox]

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.

[mechman600]

All the BMS systems I have looked into do top balancing. I am not sure why - I would bet because of reason #2 in your top balancing pro list.
Are you a member of diyelectriccar.com? I would join and post your question there, because those guys are ridiculously smart when it comes to stuff like this.

[Daox]

I think most BMS systems do top balancing because they also do active balancing and you can't do active balancing and bottom end balancing together.

Yeah, I'm a member over there. I'd just like to see more EV discussion and info here. There are enough people doing conversion and lithium is only going to get more popular. If I can't get any answers here I probably will post there.

[XmodAlloy]

I don't like top balancing simply because it increases the chances of over-discharging your worst cells. Bottom balancing makes more sense to me simply due to the fact that it makes over-discharging much less likely. With top balancing you also tend to turn a bunch of your charging energy into heat when it goes into balancing mode.

I could be wrong though, not enough experimentation!

-Don

[Daox]

Well, my bms doesnt do any active balancing. I'm just talking about the initial balance.

[XmodAlloy]

If you aren't using an active balancing method, I am thinking top balancing might actually be better for performance, but I don't know about cell life span. There needs to be some study on running two identical batteries at low states of charge and high states of charge and see which one has more capacity after several hundred discharge and charge cycles.

-Don

[Daox]

Yeah, thats pretty much exactly what I'm looking for.

[fb_bf]

I think you want to bottom balance because it sounds like your BMS only protects you from overcharge. The guys who run without a BMS bottom balance and use a SOC gauge and pack voltage to know when to stop driving. They seem very successful at doing this, and the packs don't drift according to their data. They stop their charge when the average voltage of the pack gets to 3.6 volts. The batteries can handle a higher voltage, so the lower capacity cells end up at 3.9 volts or so when they shut off the charge. By bottom balancing you'll be better protected on the low voltage side. Using the BMS to shut off the charge when the first cell hits 3.65 volts means that you haven't completely charged your pack, but the amount your missing is probably less than 5%. If you've tested your SOC gauge, and know the voltages of your cells are safe when the gauge says to stop that will work as well. My BMS protects on both ends, and I just plug it in at night, and drive the next day. I've had no issues with it. I haven't checked my voltages for the last 6 months. I hope this helps with your decision. It sounds like you know enough about all of this that you'll be safe either way, but I think safer with a bottom balance.

[Daox]

I am not using voltage as a low charge indicator. I'm aware that that is a horrible way to do things. If that were the case I also would recommend bottom balancing, but it is not. See above for how I am detecting the low charge.

Originally Posted by Daox

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.

[fb_bf]

Like I said at the end of my post, You know what your doing, so either way you'll be safe. The guys that bottom balance do use the pack voltage to determine when to stop driving, because they know that at the low voltage all of the cells are very close to the same voltage. So even with as poor of a way to know your at the end of your batteries thay can get away with it because of the bottom balance. I'm just saying that to show that it would provide more protection from accidently driving a cell too low. Since you know all of your cells capacities, and are counting current used, and are stoping at the 70% DOD point you'll be safe with a top balanced pack.