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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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13.8V could be an absolute voltage limit.

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

Since AGM cells reduce in ESR with rising temperature and also reduce in cell voltage, thus the max current rises from a fixed voltage regulator. This is why temp-compensation is necessary over a wide ambient temp. If over-voltage occurs with high current, the temperature rise can exceed unity gain for heat rise vs heat reduction from reaching full charge and result in thermal runaway and catastrophic failure. (e.g. excess H2 in a sealed box is explosive with O2 and spark or the battery acid over-heats)

DIY

You could put a long wire or resistance heater in series to limit current and monitor voltage manually or use a proper charger.

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  • \$\begingroup\$ The alternator on my car (a 2016 Honda Civic LX) puts out 14.5 V regardless of the RPM, according to the voltmeter I installed. Is my car weird? \$\endgroup\$ Apr 10, 2022 at 13:08
  • \$\begingroup\$ If the voltage is between 13 and 14.5 volts, then your alternator is probably fine. Higher than 14.2 reduces sulfation to increase lifespan from high ESR due to elevated ambient temp. but also increases acid evaporation which shortens lifespan, so it depends on ambient. (-2mV/'C) It also depends on AGM, SLA vs. flooded acid, flooded acid tolerates a higher voltage of 14.5 while AGM does not! @TannerSwett \$\endgroup\$ Apr 10, 2022 at 15:12

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