Schumacher battery maintainer SEM-1562A analyzed

Thread Starter


Joined May 11, 2018
Some time ago I bought this battery maintainer for use on a small 12V lead-acid battery. I found the battery had little charge after some winter months on the maintainer, but seemed fine on a cheap transformer-rectifier charger, and on the machine's own charging circuit. I measured the maintainer's open circuit voltage in the mid-12V range, which I consider too low for proper "maintenance." Accordingly, I decided to extract the circuit and see if I could change it appropriately. I have attached a schematic, assembly diagram, and functional description. This battery maintainer is well made and is a more sophisticated device than many I found online, and so worth understanding. I hope some find this interesting or useful.



Joined Nov 6, 2012
Replace R11P and R3P with a 50K, 20-turn Trim-Pot.
Connect Charger to already charged Battery and measure the Battery-Voltage.
Adjust the new Trim-Pot to achieve exactly 13.8-Volts.

If You live where the weather gets really cold, You may want to bump the Voltage up a little.
Attached is a document outlining Maintenance-Voltage vs Temperature.
Temp Compensation .PNG