DIY Electric Car Forums banner
1 - 1 of 1 Posts

4,095 Posts
I like your explanations, and differing internal resistance may well explain the behavior I am seeing while I am attempting a manual top-balance before reverting to normal use and pack charging. I have brought all cells up to a 'final voltage' cell by cell, but they just don't seem to stay balanced the way they did 18,000 miles ago.

What I am looking for now is a way to identify each cell's IR, and some guideline on how much the IR can vary cell to cell to 'stay in balance' with repeat charge cycles in series. I'd like to limp along for a while if there are just a few cells out of spec, or consider replacing just a few cells rather than the entire pack.

what equipment would I need to establish a cell IR?
method for measuring?
acceptable range of difference to minimize 'drift' with repeat charge cycles?
I would do a full discharge to 2.3 volts each on every cell in the pack resting voltage. Then do a charge and watch your pack as it charges and record your voltages and when the first one comes to your max something like 3.8 volts then that is the one with the least capacity. Check the others and see where those voltages are sitting. This will allow you to pull out the low capacity cells and replace them. This is why I always bottom balance. That way if you end up with an extreme discharge the cell are all at the same voltage. Bottom balancing will allow you to use your true capacity from your lowest capacity cell. The rest of the cells will just have to go along for the ride. If a cell is really low capacity you should replace it with ones that are more like the rest in capacity. You can only use the lowest capacity anyway. So if you top balance and drive your vehicle extreme discharge you risk that low imbalance and causing a cell to go empty before the others and kill a cell or two and even damage surrounding cells. Ask me how I know.

My John Deere Mower is now using Leaf Modules. I bottom balanced them and they are very nicely balanced cells. I charge to a lower than max top voltage and stop before I go to low. That will keep my pack equal. If I need to pull off 12 volts I will do so from a DC DC converter that utilizes the full pack voltage and will not imbalance a cell or group of cells. So when my pack goes low they all go low and will never have a reversal event. Monitor the cells. Yes. Top balance. Nope. Bottom only.


Use the revoltx to fully discharge to 2.3 volts constant current constant voltage just like you do when you charge them. That way you know they are all starting out the same state of charge. Low capacity cells will fill first. So if your low cell goes to to 3.8 real early while all the others are still like 3.2 volts then that cell should be removed.
1 - 1 of 1 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.