I use a UPS. Disconnect the battery and extend the cables. Most UPS's charge at 12 volts and have all the circuitry to charge and don't overcharge the battery. I disconnected the buzzer and replaced it with a light so the sound doesn't drive me nuts when charging a battery that's completely dead. I have a UPS on my workbench that also doubles as a 12 volt power supply and an AC power strip.
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