Multiple battery questions (starter and coach house)

[Jennifer Rose Towing]

One of the places I have less knowledge on is multiple batteries in a setup so I am looking for help. I do understand the systems of a single battery, testing and charging. I do not feel an isolator would be a good fit for what I am looking to do.

The short description is that I want to add a 35 amp hour battery to the vehicle (like a house battery in an RV) and looking for options to charge it.

This is the long version and setup. All of the aftermarket/upfit items I have added to the vehicle have a wiring system that can be isolated from the vehicles battery via a large solenoid. Currently I can power all of the aftermarket stuff via the vehicle charging system or a 110v supply via generator while isolated without issue. What I want to do is add a 35 amp hour battery to power the isolated side when needed without the generator (or vehicle running).

2 main issues are charging the battery and wanting to limit that charge to around 5 amps. I can easily remove the battery to charge it separately but would prefer to find a fairly inexpensive way to charge it from the vehicles charging system while driving (solar is not an option).
The thoughts I have but have not been able to find much on are... Finding some type of charger that runs off 12 volts. I figure if a trailer brake away battery can be charged without a large battery isolation system then I should be able to do exactly the same. I do not believe the smaller batteries are designed to take the higher voltage of a vehicle charging system once topped off. I would think a 2-5 amp battery charger that runs on 12 volts vs 110 would be great. I have been looking a little but have a lot to learn on them.

Seeking input and advice so I can get the system going in the next couple of weeks.

 

[Sparky_911]

If it's available, tap the trailer wiring to charge the house battery while the vehicle is running. The only concern is you have to tamper with stock wiring and risk shorting something out if it cross shorts (you will need to hit the 12v+ and 12 ground from the trailer plug). Be mindful that the amp specs from the trailer wiring vary from manufacturer (Ford, GM, Dodge, etc). Might check your specific vehicle to see what power it sends out.

Your next best option would be to find a 12v to 12v charger. NOCO and Renogy make them and range from 40-200 dollars and deliver 2A-200A at 12v.

The third option would be to pick up a small 12vdc-110vac inverter (say 400w?) and then pick up a separate 110vac-12vdc charger ala NOCO or battery tender brand. Run it off the vehicle battery to keep the house battery topped off (plus 110vac if you need it elsewhere). It's a bit more complicated and not as efficient as a 12v-12v setup, but would solve the isolation issue if you're concerned about any cross over or short outs. Cost would be slightly higher than the above 2nd option. But it does open up your options for charging amps (ranging from 2A-10A).

 

[NoLimitSquads]

Dont know if this helps, but.. when we build EV squads, or vehicles with a secondary battery, we use a DC-DC charger. The ones we use are bluetooth capable and you can set the charging parameters. I set them up so they only charge when they see voltage higher than a 12v battery’s resting voltage. So like 12.9v. On the gas vehicles, they also have an onboard AC-DC charger for charging the vehicles main battery. So when that charger is plugged into the wall, the voltage is also higher than 12.9 and it will start the DC-DC charger.

The DC-DC chargers that I use are very high amp though. So if you could apply this same thing, but find a much smaller DC-DC charger, it would probably work exactly how you want it to.