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.

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.

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.

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

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

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

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

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.

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.

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.

So you're saying the guys that balance at the bottom just watch their voltages to determine low SOC? When the voltages all sag very close to each other they know their SOC is low? How are they doing that with any accuracy?

From what I understand, they can simply watch the voltage of the pack and know it's time to pull over. If they have 38 cells and they want to stop when the batteries are at 2.75 volts, then they wait till their pack voltage is at 104.5 volts. Since they bottom balanced, they know that all the cells are close to that, and they haven't driven any single cell below the 2.5 volt level. I suspect that many of those people also have some form of SOC meter that watches current use as well. The people that typically are doing this are afraid that a BMS system will screw up and cost them their batteries. There are more opportunities for failure in a BMS system. I think that many BMS "failures" were opperator error. The BMS system has to be set up properly which isn't always staight forward. I choose to use a BMS and take the time to set it up, rather than have to monitor my battereies every few weeks. So far I haven't had an issue with the BMS, but I can tell you it took as much time to wire it up as to wire up the rest of the car.

Time to resurrect an ancient thread.

I have been doing a lot of reading about this whole BMS vs no-BMS thing. The main proponent of no-BMS is Jack Rickard, who has been deemed a "heretic" by many in the DIY EV community. He initially bottom balances each cell manually (using a 50W 1/2 ohm resistor) to anywhere between 2.7-3.0V and then charges the pack to an average of 3.6VPC. Some end up at 3.5V (ones with more capacity), some end up at 4.0V (ones with less capacity).

He claims to have run his car for years without the cells going out of balance, never having to rebalance. He even claims that over time, the cells "grow into each other" and become more balanced on their own, though he admits his evidence is merely anecdotal.

I liken charging/discharging lithium batteries to filling/draining a bunch of slightly different sized glasses with water, each at exactly the same rate. When starting with them all empty, some will fill to the top, some will not. The important thing is that they all drain down to the bottom at the same time, every time.

Jack argues that the bottom is where cell damage happens. Overcharging obviously isn't good, but it only damages cells slowly over time. On a top balanced pack, cells reach the knee (the quick drop in voltage just before the cell is totally discharged) at different times. So if a cell falls below 2.5V and has no jam left but there are still other cells with some jam remaining, they force current through the fully discharged cell and toast it in short order.

Bottom balancing allows every single cell to reach the knee at precisely the same time, with ample warning. No chance of strong cells destroying weak cells. Also, Jack argues that with bottom balanced lithium, a voltmeter IS an effective way to determine DOD. A good Ah counter/fuel gauge is obviously a recommended tool to use as well, and they are available for $75-150.

It is amazing how heated the BMS/no-BMS discussions are.
Bottom Balancing - DIY Electric Car Forums

I do and don't agree. Let me elaborate.

I definitely think bottom balancing is the way to go. However, I do not believe that it allows all the cells to reach the knee at the same point. My experience has shown otherwise, and I had a hole in the back of my Prius' windshield due to overcharging cells, so it definitely IS a problem.

Nearly all the problems with lithium cells has to do with heat build up, and it happens at both ends of the charge. You can drain a 100% full battery down at a high C rating, and it won't REALLY start warming up until the last bit. Same with charging. Jack has proven this in his testing. You can charge a cell to 95% capacity, and it will get warm. But, you pack that last 5% in, and that is when it really starts warming up.

My solution is to completely avoid the two ends of the spectrum. To do this you really want some sort of BMS. You can do it manually, but it is a pain. My version of a BMS is not really a Battery Management System as much as a Battery Monitoring System though.

Let me start by explaining the charging side. Normal lithium cells go through the same 2 stage charging as a lead acid battery goes through. The first stage is constant current. You charge your cells at say 10A until they hit say 3.6V. After that, it switches to constant voltage, and the current drops to maintain that 3.6V. This second stage is where you get that last 5% and where your heat comes from. I would suggest giving up that 5% and just eliminating the constant voltage stage. You loose 5% battery capacity, but you gain cell life. So, now you're using this on a full pack of batteries, this means you need to monitor every cell's voltage. With a bottom balanced pack, your weakest cell is going to hit 3.6V first. Once that cell hits 3.6V you are done. You are always limited by your weakest cell. This is how it should be so you don't beat that cell to death. That is pretty much it for charging.

Some guys do this all manually. They identify their weakest cell, and they watch the pack voltage at that point where that lowest cell hits 3.6V. They program their charger to shut off at whatever pack voltage it is at when that weak cell hits 3.6V. This works, but doesn't account for any cell drift or degradation. Thus I like monitoring each cell a bit more. However, it does work.

The discharge side IMO has to be handled with a amp hour meter / counter. Its best to capacity test all of your cells in your pack so you know what is the weakest, and you know that you have 100Ah or whatever to use, so you can say once you hit 70Ah used, you're done. This also gives you a great 'fuel gauge' that is dead on. There is no guessing like with a voltmeter.

If you check out my bms thread, I have a system pretty much designed. I never really got my Ah counter working with enough accuracy where I was comfortable using it. However, the charging side of it worked great and it was simple and cheap to build.

http://ecomodder.com/forum/showthrea...tem-20445.html

I agree with you in every respect:
1) As soon as the weakest cell hits 3.6V, turn everything off.
2) Rely on an Ah counter as your fuel gauge. Even better if your Ah counter has the ability to derate your controller when things are almost spent.
This seems to be a good one for only $75: Instrumentation :: JLD404 Programmable Battery Meter - Thunderstruck Motors - Electric Vehicles, Electric Vehicle Accessories and Electric Vehicle Components. We have electric motorcycles, electric bicycle batteries and controllers.

I have very limited electronics knowledge, so coming up with a device for each cell that trips the charger when the first one hits 3.6V will be a challenge, but this is what I will do.

Now another charging question. Let's use the Kelly 72V/10A charger that I used to own as an example. It had a constant max voltage of 88.2V, and on 25 cells that is an average of slightly more than 3.5VPC. I imagine that once 88.2V is reached, most cells will be right around 3.5VPC but a few will be lower (lets say 3.4V) and few will be higher (lets say 3.7V).

Once the charger holds 88.2V for, say, 10 minutes and is pulling back the current more and more until it decides to switch to float (82.8V), what will the voltage do on the higher voltage, weaker cells? Will they stay at 3.7V or will they continue to increase past 4.0-4.5V (because they are full) while the stronger cells catch up, eventually causing a disaster?

My only experience so far is with lead acid batteries, which simply start to bubble and gas when the voltage goes high. No big deal.

Maybe this is a stupid question or a moot point, but I cant help but ask.

What about those with an ev who use lead acid and 6 or 12 volt batteries. Are or do they do any routine checks to see if they had a low battery or a battery with a bad cell?

I believe with lead acid you can trickle the whole pack making all the cells equalize, but still one bad cell ruins the over all output.

I have instrumentation to monitor each of my six 12V banks while I am driving. And since I charge with six 12V chargers, I guess you can say I am "top balancing". Since they are AGM batteries you cannot perform a high voltage equalization charge anyway...at least not with my particular batteries. I think you can with an Optima battery, but it shortens the batteries lifespan each time you do.

It is impossible to bottom balance lead acid because:
1) voltage sag is substantial even as low as 30% DOD, so it would not be accurate
2) you never want to go greater than 50% DOD or cycle life goes through the floor
3) bottom balancing would prevent stronger cells from ever being fully charged; lead acid batteries do not remain happy for long if they are never fully charged

I used celllog8m devices. They're made for R/C stuff, but they'll monitor the voltage of 8 cells at once. They can be setup to send an alarm signal for high voltage. With my BMS, this signal was sent to an arduino (brain) and the arduino then cut power to the charger.




EDITED:

The answer is no. Once your charger hits the constant voltage phase, the high/weak cells voltage WILL continue to increase very quickly. But, since the charger is monitoring voltage, it should also decrease amperage very quickly to compensate. This is why the manual way I talked about above works out.

The real disaster happens when you hit 3.6V and you're still in constant current mode. That voltage will just start shooting up and that cell will start heating up until something bad happens.

I can verify that. I was precharging the 24 volt packs that make up the 48 volts for the enginer phev kit I have. I was using a cell log and a regular 12/24 volt battery charger. I saw one cell near 3.6, but rest were lower. I walked away to go do something and came back. I heard a beeping noise and saw the cell that was 3.6 volts was 3.9. I stopped it immediately and the high cell went right down.

Balancing
 

I top balance with all my Lithium systems because

1) It's a bit easier technically and I understand it.
2) It ensures all the cells reach 100% soc at least when first fitted.

I monitor the voltage of every cell and keep them in a safe operating voltage window when driving/charging.

As soon as the weakest cell starts falling to my cutoff voltage threshold then I declare the pack exhausted. And vice versa when charging.

I don't top balance every cell every day to 100%.
As soon as the first cell reaches full when grid charging I declare the pack full.
I actively balance once a week or so and the cells tend to keep well in sync.

Been doing it since 2003 with my own BMS, several lithium packs and conversions. :thumbup:

To make a fuel gauge I fully charged my pack, set an Ah counter going and then drove it normally until it was exhausted. I then used that Ah figure -5% and calibrated it to a 0-100% range display. Works for me.

No problems since 2003, you must be doing something right. :thumbup: