Using 7v and 2 cells are only an example to simplify things. I will be series charging 20x200ah cells at a time. My charger is capable of charging 12v-80v at 1/10th volt increments and amperage limiting to 5-60amps and the varying adjustments above can be set for 1 min to several hours and also can be set on a percentage basis and also can be varied according to temp variations. The above can also be adjusted in both Absorb and Float settings. The charger will also throttle down to 1/10th amp when preset voltage is reached no matter what voltage the batteries start at. Anyway it is a solar MPPT charger/controller running off my solar panels and digital/computerized.
But my question is if I am charging x number of cells at the proper voltage (3.5-3.7v), can a bad cell "run-away"? In Lead, the bad cell/battery will boil or get hot ------- so what will a Lithium do? thanks
francis
This would be the route of the whole BMS debate. The ideal thing to do is make sure the cells are at the same SOC, now you can choose to bottom balance, top balance, or if the cells haven't been cycled yet trust good old Jack R. and just wire them in series and charge them (I don't quite trust that one) So I would top/bottom balance them depending on your goals and use of the pack, if you will have a BMS, will it shunt, monitor etc?
With lithium there is potential to do exactly as you mentioned, a cell that hits the top first could "run-away". I used a few small packs of 100AH TS at work, (we replaced LA with Lithium in a battery backup setup charged from an alternator) We top balanced the cells because they would commonly be in a float charge off the alternator, and the system it was powering could shut down at any low voltage level we programed. I was impressed though that after top balancing there was virtually no drift after many test cycles. At the bottom, a couple cells would drift as they hit the knee, but we tweaked the software to shut down as the first cell hit the knee. But every single charge cycle the low cell would come up and end at the same voltage as the other cells every single time.
So there are benefits to top and bottom balancing but it depends on your application.