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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.

enter image description here

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  • \$\begingroup\$ 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. \$\endgroup\$
    – Kartman
    Commented May 10, 2022 at 23:56
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    \$\begingroup\$ 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. \$\endgroup\$
    – DrMoishe Pippik
    Commented May 11, 2022 at 0:24
  • \$\begingroup\$ Could possibly use three 12V chargers if their secondaries are floated from the line. \$\endgroup\$
    – hacktastical
    Commented May 11, 2022 at 1:45
  • \$\begingroup\$ "Per say"? Did you mean "per se" ? \$\endgroup\$
    – Solar Mike
    Commented May 11, 2022 at 4:25
  • \$\begingroup\$ 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. \$\endgroup\$
    – EJE
    Commented May 16, 2022 at 16:22

2 Answers 2

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I tried this, but finally I decided it's not worth the effort. This schematic was my draft for the test:

Prototype battery matrix switch

There may be errors in the schematic, don't trust it! Danger to come!

To your question: I used it in a large electric scooter and wanted to charge it with an all available 12V car battery charger. It has a balancing effect. Different aging of the batteries was a problem in the scooter but since I charged them in parallel they could no longer drift apart. The decision against it was the cost of this hardware, the extra fuses, save mounting place for the switches, the danger to blow up the charger if the switches are set to 36V.. The microcontroller was an atMega88, just to control proper sequencing of the takeover. It never left the prototype phase.

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The problem is, if you mis-sequence your switch/relay connections, you will dead-short the battery and fuse the relay contacts so you'll never get them disconnected. Make/break sequence will be everything.

Generally, this is very difficult and bizarre to do. The normal way to do this thing is spend a few bucks on a 36V battery charger. If your source is 12V, then use a 36V charger that accepts 12V. (or a COTS AC inverter and AC-36V battery charger, all in stock and sold 24x7 at your nearest truck stop). So I guess the motivation is to avoid that modest expense??

The right way will have a big advantage: you will be able to operate the 36V load at any time, since use of the load can operate concurrently with battery charging. Any reasonable quality battery charger can support this.

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