Quote:
Originally Posted by Bill
Rick -
If the batteries are connected in parallel, is there any guarantee that a given charge current will divide equally between the two parallel batteries? I can imagine (but I have no experience) that when you first turn on the charger, small differences in the batteries could result in a large current imbalance, meaning one battery will charge harder than the other. This will even out, of course, as the initially-lower battery comes up to match the other. But in an extreme example, an initial 100-amp charge might divide unequally - say 75 amps/25 amps. Could this be dangerous to the batteries? Will the BMS in each battery take care of it?
I think you are telling me above that the BMS will take care of it, but you seem to attach some caveats to that statement. Will the BMS forcibly reduce the current in the 75-amp battery, down to something acceptable? Or will it simply disconnect the battery that is being charged too fast - in which case, that battery doesn't get charged at all.
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Answer #1: LFP internal battery resistance is very low, but possibly unequal (between parallel batteries) by small amounts. The BMS itself is typically contributing more resistance than the cells themselves. The State-of-Charge in battery battery packs will eventually equalize, when one approaches 100% charge before the other (because the Charge State versus Voltage curve becomes non-linear as SOC rises, and the less-charged battery will be favored).
There is definitely a risk of unequal charging among parallel batteries, and it is pretty much guaranteed to happen if the resistance on the charging path through one battery (the resistance of battery-specific "+" and "-" leads from the "bus" connections + and -) ever becomes significantly different from the resistance on the path through the other battery. Even if those leads are the same length and same size, that could maybe happen from a loosening terminal. And any internal failure of ONE battery (though very unlikely with BattleBorn batteries, or my "home-built battery packs) will definitely cause the other to be charged with excessive current, if excessive charging current is ever applied.
It is also possible, for a short period, when one battery has reached "balancing" voltage start and the other (although configured to trip over to "balancing" at the same voltage), has not yet reached that internal Voltage on all its internal cells or strings. This would also cause nearly all available charging current to be directed into only one battery.
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Answer #2: Nearly all BMS units do not provide any kind of active "current limiter" for bulk charging. When charging occurs, it only has a choice between 3 connection states (state-1, fully connected for charging at up to maximum allowed current; state-2, connected to only drive an internal 'balancing circuit', which sends very small amounts of current down the balancing wires; or state 3, charger "input" disconnected, due to reaching Voltage or Current limits for state-1 or State-2).
In the previous post I was explaining that some BMS units do not auto-recover gracefully from temporary over-current situations - requiring intervention on a command interface (a cellphone 'controller', computer interface controller program, a physical 'reset' button, or a disconnect-reconnect procedure (in my brand of BMS, 'Daly' (also called Dahlgreen on some labels), 30/30/30 disconnecting and then reconnecting the balancing wires header performs a physical reset.
The biggest worry in all of this would be: In cases of extreme over-current, the actual circuitry might "fry" before detecting that excess current is present.