@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.
