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Use a cord that plugs into your 12v power port or cigarette lighter socket. Same thing as running wires from your battery.
Without somehow using the transformer/converter, that is the charger, don't you run the risk of overcharging? There will be nothing to indicate that the charge is complete. Or am I overthinking this?
The charging current should be no more than 25% of the battery capacity. A 12v 10aH battery should be charged at no more than 2.5 amps. A 7aH battery no more than 1 3/4 amps. I don't know how you will control this amperage when hooked directly to your battery while the engine is running. It is fairly easy to ruin one of these small batteries by charging too fast.
X2. It would be nice to attach the charger as well. Thought maybe a female plug (plugged into the male on the charger), then wired to the connector desired (cig. lighter or USB). But the charger is designed to convert from 120 to 12. Not sure if it would work without converting (12 to 12). Another option? Buy a lithium. Lasts for days.