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What is the most efficient way of charging an AGM battery using a car alternator? I have a 12V plug connected to alligator clips which can be used in the 12V sockets inside the vehicle. Is this the best option or is it best to use jump start cables to connect directly from car battery to AGM terminals (positive to positive and negative to negative)? How many amps of current would be achieved with each of these methods?

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I would connect the battery to the vehicle battery with the jumper cables, assuming that the battery is a similar capacity, then start and run the engine at about 2000rpm.

If you use the 12v sockets you risk blowing a fuse or damaging something.

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AGM cells are more fussy about reliable charging vs temperature and have a reduced float voltage to prevent grid corrosion, gassing and water loss.

Therefore at room temp do not exceed 13.6V and reduce to 13.4 on float.

Jumper cables to battery on low idle should achieve this. (e.g. 1000 RPM depending on vehicle) Higher RPM may rise to 14.2 and hopefully, not much more, yet this should be avoided for AGM batteries.

If you plan on doing this often, I would use a current shunt of the jumper cable and measure voltage drop on the wire, but not at any connection points (Kelvin method) with a mV DMM calibrated from an ammeter.

https://batteryuniversity.com/index.php/learn/article/charging_the_lead_acid_battery

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