On our solar-powered off grid system we have 4 massive 60lb 6v 232 Ah Lead Acid golf cart batteries hooked up in series to make a 24v system. Every couple of weeks on a sunny day we have to equalize the pack. It’s a simple enough process, we check the water levels in the batteries and then set the charge controller to a 2 or 4 hours equalize and it pumps 31v DC into a 24VDC setup. The batteries boil and overcharge and are generally stressed out but in the end they all end up at about the same charge level.
With Lithium batteries it’s not that easy. You can’t just overcharge the batteries to get them to balance out. Lithium chemistry will often self destruct in a catastrophic way when you overcharge it. So how does the pack balance itself?
There are 2 kinds of BMS, ‘balancing’ (smart) BMS and ‘monitoring’ (stupid) BMS. The stupid BMS just monitors the cells and shuts off power if any of your cells reads voltages that are too high or too low or if you short out the battery terminals. The smart BMS pays attention to which cells have how much power and it will charge the cells that need it. When you discharge all the cells drain out at pretty much the same rate, but when it is charging the cells can charge at different rates.
How do I know what kind of BMS I have? If you bought a 20Ah battery and after a few months you’re getting only 11Ah of power before it shuts down then in all likelihood you bought a stupid BMS. Many of the cheaper battery packs available on Alibaba and fleabay are lowend monitoring BMS units, but don’t worry. You can likely fix the problem yourself.
When working on a large battery pack be aware there is no ‘off switch’ everything is live all the time. If you’re accident prone then I would advise you take your battery to an ebike shop like Boxy Bikes in Ithaca NY to have them repair your batteries for you. If you try to cut through wires that hold both the power and the ground wires at the same time you’re going to make a lot of sparks and take a pretty big chip out of your wire cutters. I recommend using Nitrile gloves when working with batteries ‘just in case’.
The first thing to do is to run the battery down till it shuts off then check the voltages on all the cells. Carefully remove the heat shrink-wrap from your pack and uncover the BMS (it should be on the top). There is a pin block with the same number of wires on it as the number of cells in your pack (If you have a 16S pack there will be 16 wires). Carefully test the power at each of the wires and write them down. Under no circumstances should you have the red probe touch two wires at one, you’ll be shorting a cell and it will be very, very bad. Pretend you are playing that game ‘operation’ where one wrong move and it will blow up in your face.
More than likely one cell will have a much lower voltage than all the others. For ease of measuring I usually leave the black probe on the large ground solder joint and just carefully move the red probe between jumpers. Because the cells are hooked up in series you’ll get a bunch of numbers that increase sequentially (ie 3.3,6.6, 9.9 etc.). To get the power level of an individual cell just subtract that number from the number to it’s left.
It’s much safer to find the low voltage cell and charge it up to the same level as the other cells when the battery is not fully charged. Once the cell is at about the same voltage as it’s neighbors you can then just hook it up to the bulk charger and it should be good to go.
There are two ways to charge the low cell, one is with variable voltage power supply set to very low power (3.3v) and low amps (100-250Ma), the other is with an old cellphone charger. I recommend pulling out the large pin block connector and charging the individual cell through the pin block. Older cellphone chargers were all pretty much set to charge a single Lithium cell and the voltage/amp range should not fry your cell.
Once the pack is fully bulk charged check the voltages on all the cells again, it should be close to 3.65 and all the cells should be about the same voltage. It is hard to get the proper heat-shrink tubing for large packs like this so your best bet is to harass whoever you bought the pack from and get some replacement heat shrink tubing from them.
When re-shrinking your battery be careful not to apply to much heat. Heat is what kills Lithium batteries and you want to make sure you battery gets exposed to as little heat as possible.
With a little luck your pack will stay in sync for another couple of months. If you find your pack is frequently falling out of sync try a longer charging cycle on the bulk charger. Sometimes if you leave a stupid BMS on a bulk charger for several days in a row the low voltage cells will even out themselves. Then again, sometimes not.