LiFePO4 Charging: AC to DC Charger: Multiple batteries?

[Dave in AZ]

Doesn't the DC DC charger prevent the batteries from accepting too much charge and overloading the alternator and also keeps the charge voltage in the acceptable range?

What Peter said, but also YES. You can overstress your alternator with a battery bank and wires that will pull a lot of amps, pushing alternator to 75-100% output for long periods of time. For some folks with motor homes with 250A or bigger alternators, designed to output a lot, no problem. But for my Tacoma, for example, it has 130A alternator that I could easily overstress. AZ 130f in sun under hood..I have a dc dc to ensure I never pull more than 30A extra from it.

 

[DaveInDenver]

@DaveInDenver Hugely helpful and confirms my very uneducated suspicion. I have an uneven draw issue with my two battery setup, although I suspect it may be due to a bad BMS.

The BMS is unlikely to be related. You don't strictly need a BMS if you control the charging and discharging. BMS provides safety so that you don't go over- or undervoltage on a cell and can watch their temperature.

If you stay within voltage and temperature boundaries a BMS is mostly idle and the cells are at the mercy of topology and workmanship (e.g. cable length, termination quality, etc.) putting them into a battery and the battery into the bank. That's not to suggest you should run without one but the BMS doesn't excuse poor practices when you're worried about performance and lifespan. It just protects on the worst abuse of the cells.

Some BMS have balancing circuits but they are often passive so only help during charging and only marginally at that. An active balancer may be useful if your cells are imbalancing badly since they work much harder at it and during charge and discharge (they can not just burn off a strong cell but can add energy to a lagging cell), but that's usually a separate device and not part of most of BMS.