Circuit to charge three lead acid batteries in parallel and discharge them in series

I have three 12V lead acid batteries. I want to charge them in parallel and discharge them in series. I designed a circuit using switches and SPDT switches. The simulation works, but I was hoping a more experienced EE could shed some light on possible real world circuit issues as I'm not really a power systems guy.

The circuit is extremely simple but I won't take any chances with power systems since the 36V system will be powering BLDC motors and circuits I saw on the web for charging in parallel and discharging in series are scarce. I'm a bit worried about parallel battery charge balancing though battery 2 will be the point where the charger will be connected to aid with that problem, so the approximate amperage per battery should be okay.

I plan to build the circuit using relays to replace all the switches and an MCU will be handling switching order.

The circuit works when SW4 and SW5 are in the closed position and SW1 and SW2 are in position B with a charger replacing R1; the batteries charge in parallel @12V.

When SW4 and SW5 are in the open position and SW1 and SW2 are in position A with a load replacing R1; the batteries discharge in series @36V.

• Do not rely on a MCU to enforce relay operation! Use interlocked contactors to ensure the batteries cannot be short circuited. As well, each battery should be fused. May 10, 2022 at 23:56
• Be careful that all batteries have equal capacity are equally charged, or as they're drawn down, the weakest one could be reverse-charged, destroying the battery and possibly releasing flammable hydrogen. BTW, this circuit is approximately equivalent to a Cockroft-Walton voltage multiplier. May 11, 2022 at 0:24
• Could possibly use three 12V chargers if their secondaries are floated from the line. May 11, 2022 at 1:45
• "Per say"? Did you mean "per se" ? May 11, 2022 at 4:25
• 1) What is the Capacity Ah of each battery? 2) What is the maximum running current when operating at 36Vdc? Each battery should use a Fuse. An UnderVoltage protection , interrupting the current drain is practically a Must a have to avoid damaging the weakest battery.
– EJE
May 16, 2022 at 16:22