OEM Accessory Battery to Renogy AGM hookup

Hey there! I’ve finally narrowed it down to 2 Renogy AGM 100Ah batteries. My vehicle (2011 Ford Starcraft Shuttle bus) already has an accessory battery that I want to tie into to charge my AGM’s. Can I just hard wire them together or do I need a controller in the middle to charge the AGM’s properly?
Also, as far as inverter size goes, I have a 1000W already from previous happenings, but would that waste too much and I should get a more appropriately sized inverter? I’m thinking 300W since we are only using Whynter refrigerator, charging laptops, cell phones, charging lights, etc.


They say it’s not good to mix different types, ages, or capacities of batteries… Maybe remove the original house battery and replace it with one of the AGMs, then link the other to it for a 2 battery system.

The smallest inverter that will handle the job is best too.


"Always avoid expensive solutions to cheap problems." ~ OffGrid

Ok, I kinda thought that after I posted. I will need a charger to go between alternator and AGM’s (correct?), which ones are people using? I’d like alternator to be primary and generator/shore power as secondary charging source.
Since it’s crunch time for me to make our departure date (Sept. 14th) I’m trying to get all my equipment ordered ASAP. Tell me if I’m missing something:

  • AGM Battery Charger
  • AGM Battery balancer (I already purchased 2 - 100Ah batts.)
    I already have a charger that will control generator/shore power charging for AGM.


I’m not an expert on AGM batteries, even though I have had them… I just treated mine like any other battery, just maintenance free, and I never had a problem. No special charger or anything else, they were used when I got them, and still lasted me 5+ years for under $20.

I strongly suspect that all this special charging & special maintenance stuff is just designed to get people to spend unneeded extra money. When I had solar, the flooded/AGM/Lithium switch on the controller had no wires attached to it. It was window dressing to make it look fancier than it actually was, and of course cost substantially more.

I learned my lesson on expensive batteries, chargers, and controllers. Now I buy cheap used junkyard batteries, and the cheapest automatic battery charger off the shelf, with no extra AGM or lithium controls. Average cost of about $30 for a battery charger, and I can run it off my generator or shore power if it happens to be available.

I currently have 110ah AGM batteries for both my starter battery, and my house battery connected to my starter battery via a plain old solenoid. Nothing special, and no known problems. Bought them used, 5+ years ago, and they’re still going strong. Occasionally the external low battery cut-offs will shut them down, but other than that, I give them zero thought. Still plenty of power to start my rig, and be on my way while everything charges back up to full.

I also totally disagree with the people who say you can’t get a full battery charge while driving. 99% of the time, that is my only charging source, and it works perfectly, even just doing short trips around town most of the time. AND… I can park in the shade!


"It is always cheaper and easier to conserve power than it is to make it." ~ Road Warrior

So just wire up the AGM’s directly to the alternator for my power-in, connect an inverter for my power-out, and have a battery charger that could connect to shore or generator…! Ok, I’m just talking myself through it at this point. Let me know if I’m wrong.

You should use an isolator between the starting battery and the house batteries to allow charging with the alternator, but not allow the house batteries/loads to discharge the starting battery. Also, check the charging voltage. If the voltage is above about 14.8 volts you’ll ruin the AGM batteries.

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Thanks, the isolator is definitely on my list and I forgot to include! I checked the voltage today on my “wheel chair lift” battery leads (that I’ll be connecting my batteries to) and it’s giving out 13.99 volts. This seems good to me… Ya?

Yes, I think that voltage will work. Just be sure it doesn’t go too high when engine RPM is increased. The regulator should control it. When you get an isolator, get a good quality one that doesn’t drop the voltage. Some use diodes that will drop the voltage too much.

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Last question I haven’t gotten a strait answer on: I already have a 1000W inverter; would that waste too much energy compared to a smaller sized inverter (circa 300W) or is it negligible and I shouldn’t worry about it at this time? (T-13 days till departure EEK!)


My apologies, I thought I answered the inverter question, perhaps it was in a different thread…

I suggest the smallest inverter possible, with a little bit of leeway. ie: If your maximum loads will be 250 watts, go with a 300 watt inverter.


"Beat Murphy's Law with a KISS! (Keep It Stupidly Simple)" ~ Van_Dweller