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In my vehicle, I'm wishing to install a separate 12V/20Ah battery to power electronics (network gear, cb radio, etc).

I was told to not run two different sized batteries as they would overcharge the smaller.

My goal is to have the smaller / aux battery charged while the engine/alternator are running. When the engine is off, the equipment of course would still be usable without killing my truck's main battery.

I have found a device (Automatic Charging Relay) that may help me. Am I wrong? Is there a safe way to accomplish my goals?

Blue Sea Systems SI-ACR Automatic Charging Relay
https://amzn.com/B000OTIPDQ

ExpertPower EXP12200 12/20Ah Battery
https://amzn.com/B00KC39BE6

Thanks!

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The relay you selected will do the job. This is a very common configuration for dual battery emergency vehicles with large auxiliary loads that must be kept active when the ignition is off. It is also common with boats, RV's, etc.

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"Overcharging" per se is not the same issue.

When the small battery is used with vehicle power off, on restart it's voltage may be lower or much lower than the vehicle "12V" supply. Charging currents via the relay may be "extreme". Worst case you might expect damage to the relay and/or the auxilliary battery.

One partial solution is to place a small resistance between vehicle system and auxilliary battery. It would need to be adequately power rated and arranged so as not to be an ignition hazard.

An eg 1 Ohm series resistor will limit maximum current to maybe 4A and dissipate 16W. The charge time for a very discharged 20 Ah battery would be "many hours".
A say 0.2 Ohm resistor (or long wires) will provide some protection and much shorter charging times.

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