You won’t know if you are charging to 80% of your battery capacity
Mine counts Amps over time. It knows the SoC.
The cell voltages often won’t differ until the weak cell is in its knee, this is where active balancers and most passive balancers miss the point. They try to balance at this point and overheat bypass resistors, or fail to control voltage rise in the weak cell.
The Batrium for example will bypass current until the bypass gets hot - then will turn off the charger. In practice it works a lot better to turn the current to a level that won’t overheat the bypass, and keep charging the rest of the cells.
Correct, and the whole point of this thread is to show and tell about a BMS that does not do that nonsense. I suggest you read at least the initial posts from me at the start of this thread so you know what I am doing. Once you get caught up, I can point you to other clarifying comments if this is not making sense.
My resistor is 50Ohms and will dissipate the heat just fine.
It’s not ideal, the lower charge rate you can average- the better. It’s better to charge at 1amp for an hour than it is to charge at 60amps for a minute.
I suspect you are discussing the diminishing current at the top of the charge. My dumb solar controller will be shut off by the BMS when it is capable of delivering more Amps than the batteries will accept without going above 3.5V. This means I cannot trickle in the Amps necessary to bring the pack to the rated 100%. But, again, I have no intention of running my cells up there regularly. I intend to stop at N% SoC.
I am totally interested to hear a use case where it is necessary to eek out the max rated capacity of the pack every day. I suspect most people that are concerned about this, have lost themselves in a bout of anality that is pointless for their use case.
However, you might be pointing out a use case where the charger is capable of delivering more than say .4C. For example, a large PV array that will grab enough sun during the winter and clouds to charge at say .3C and keep your off grid setup running. In full sun, that array will deliver a huge, and pointless, 1C. You want to avoid stressing out the batteries with that higher charge rate. In other words, you'd rather charge over 5 hours, than 1. I can accomplish the goal of turning off portions of the array with a cool upgrade to my software.
I will spend nothing on my setup to do this. I have 4 DSSR20's. Each is just a diode and a relay that turns on/off the PV array. I have no MPPT. Each can handle 30A, but the PVs attached to each won't deliver more than say 22A. Right now all 4 are controlled via 1 relay on my BMS. It is trivial to separate 2, or all 4, so that the BMS can control the 2 halves of the array separately.
After I update my software to handle this, the user will specify the maximum Amps allowed for the total. The BMS will turn off charge sources in your ranked ordering until Amps going into the battery is below that maximum.
In summary, your concern is really about eeking out the full rated capacity of a battery pack, which is rather pointless and stresses Lithium batteries. With lead-acid, you do want your battery regularly at full charge, but with lithium you really don't want to do that. Or your concern is about balancing where again, the benefit is to eek out the max rated capacity, or the BMS cannot stop at a %SoC.
And if your concern was the interesting situation where you have more charge capacity for rainy days and want to not use them on sunny days, then my BMS provides an excellent solution (when I update the software, after someone states they want this feature).
