Charging accessory batteries

I've converted my 5th gen 4runner into a camper. I've added solar power with 2 batteries stored under the sleeping platform in the back. I'd like to charge the batteries with the vehicle alternator while driving. I've found plenty of how to videos describing installing a dual battery charging relay that involves running cables up to the front battery and installing a charger/isolator/etc. It seems like a simpler solution would be to just use an AC to DC charger plugged into the 110v AC outlet in the back of the 4runner. Why do I not see others using this option to charge accessary batteries? Is there some problem with this method?
thanks.

 

To charge battery, you will need more than 13V... Alternator will provide sufficient voltage. just need cut off circuit. $69.99 for this kit. very simple installation.

 

Picture previously posted by @PCRIDE4RUNNER
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3.3 amps available when parked.
1.2 amps when the vehicle is in motion. (Thats a slow charge rate but if it suits your needs.) Go for it.

 

The alternator is designed to recharge the starter battery. A secondary battery used to power items should be an AGM type battery, as opposed to a wet cell starter battery. The reason is an AGM is designed to be drained and recharged many times. The alternator on a car is designed to charge a starter battery and not an AGM type battery. That is why you need a Redarc type battery manager between the starter battery and the secondary AGM battery. The battery manager handles the proper recharging duty of the AGM battery to ensure it is charged to it's maximum. That is why 4Runner170V correctly stated that a battery isolater is needed. As it ensures the secondary battery is properly charged.

Also, using the 12v to 120 AC plug in the back of the 4R is ok for short periods of time, but if not careful you will kill your battery, assuming the car is not running. Using the electricity while the car is running will work the alternator hard, for which it is not really designed. It is designed to keep the starter battery charged. The modern alternator will go into rest mode once the starter battery is refreshed to an appropriate level. The alternator has circuitry to stop charging when a certain voltage is achieved. That circuitry will keep a secondary battery from being fully charged. Again, this is why you need a battery manager/isolater to handle the charging of the secondary battery.

Running a secondary battery is a costly solution, but the AGM battery is designed to work as a continuous power source and be recharged fully, and it serves the purpose of running accessories.

I hope this helps. You can also check the Toyota owner's manual regarding use of the 120 plug.