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I want to charge a small 22AH AGM auxiliary battery just to turn on some light signals. I would like this battery to be charged by the alternator while the car is running. The problem is: the battery manufacturer suggests not to exceed 6A during charging (it damages the battery and could cause it to explode) and the alternator generates quite a bit more of those 6 amps.

Any solution?

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Instead of using the alternator, use a charger appropriately sized to charge the AGM battery. The charger would run off of the car battery and the alternator would charge the car battery. 6A works out to 72 watts, which in the grand scheme of things is not a lot of power draw on the system. Done this way, you'd have no fear of overcharging your AGM battery, but you would need to ensure you turn off the charger when you shut the car down, or it would drain the car battery (easily done by powering it using a power source which shuts off with the ignition switch).

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