Simple dual battery setup ?

[allyncooper]

I have a small enclosed trailer I'm slowly outfitting into an offroad camper. I plan on putting a small 35 AH AGM battery in it that will power some small loads ( satellite radio and small powered speakers pulling only a few amps, a 24" LED monitor for movies, and a few LED's for lighting. No fridge or other big loads. Plan on using a 120v smart charger when grid power is available, charge with a small generator or perhaps solar when not.

What would I need for a simple and economical (OK, cheap) dual setup to charge the camper battery from the truck 45amp generator? Obviously not looking to use the camper battery for truck starting (have a jumper battery pack that works fine for a backup). Just thinking that if I'm on the road for a while then make camp I can always have a topped off battery in the camper when I arrive. TIA.

 

[4x4junkie]

My idea would be to set up a simple charge relay that energizes when the ignition is on/engine running (take a look at the $50 battery isolator thread for some ideas), and run a cable from that out to your trailer battery (using a dedicated connection such as an Anderson PowerPole connector or similar to connect between the vehicle & trailer).
35Ah is not a very large battery, so something like #4 AWG cables should be sufficient to charge it (for a larger 100Ah or bigger batt you'd be looking at 1/0 AWG cabling). These wire sizes might sound like overkill, however the smallest voltage drop (0.5V even) will cause a significant increase in your recharge times. With such a long run of wire, you definitely want to minimize that as much as possible.

 

[anickode]

For a small AGM, a generic 30 amp relay triggered by the ignition or retained accessory circuit would do the job.

10 gauge wire from the main battery to the AGM. 30 amp Fuse at BOTH ends, and the relay to kill the connection when the vehicle is off.

The trailer aux 12v is a good way to go too if you have one.

 

[Stitebunny]

The simple solenoid plan works great for me. I use a switch in the cab to turn it on or off. That's as fancy as I need. I charge the leisure batteries running down the road and isolate them in camp. When I return home I plug it into a real charger to get a full charge for the next trip.
I think 9 days has been the longest trip using it. Running a fridge 24/7, a propex at night and a few lights now and again.

 

[4x4junkie]

The conductor between your truck & trailer battery should be 10 AWG at minimum.
.

10 gauge wire from the main battery to the AGM. 30 amp Fuse at BOTH ends, and the relay to kill the connection when the vehicle is off.

I foresee this trailer battery being at least 15, maybe 25 feet away from the truck's battery...
At (lets say) 25 amps of charge current (about what I'd expect a 35Ah battery to need), a 25 foot run of #10 wire will incur a 10% voltage drop at 14.4V. That means at 25 amps, less than 13 volts will be making it back there to charge the battery (less if the OP's alternator is an older unit that operates at a lower voltage).

I strongly suggest going bigger than that on your wire if you want your battery to recharge within a reasonable amount of time, especially if it's had a deep discharge (#6 awg minimum, #4 ideally).

 

[allyncooper]

Thanks to all for your replies. 4 x 4 Junkie - thxs for the $50 battery isolator thread, good information there. As some have described their setups, they use a manual switch ( turn it on when engine running, off at camp) and that's what I would like. Distance between vehicle battery and trailer battery would be about 18 ft. (prolly at least 20' of wire per install run). 4 x 4 Junkie - I would tend to agree I need more than #10 wire, more like #6 or even #4 as you recommend. So if I would use #4, what amperage fuses should I use? And can you recommend a solenoid ?

One more question. Do I need some type of charge controller at the trailer battery? What keeps the trailer battery from overcharging if it's topped off? I will be installing a solar charge controller for solar charging at some point, can this be utilized in the dual battery setup or not? TIA.

 

[4x4junkie]

I would fuse each end of your cable run at least 80% of what your alternator's rating is (you said 45A, so somewhere between 40-50 amps, which is fine with #4 or #6 wire).

A charge controller isn't required to charge from your alternator (the alternator itself is regulated).

A solar charge controller also can be left hooked up to the system while the alternator charges. It will simply shut the current off from the panel when it detects the alternator's voltage coming in (but will resume charging after you shut the engine off and/or shut the switch off, isolating the batteries).

As for a solenoid (relay), I would say anything having a 50-amp or better contact rating should be fine for your setup. Only thing is to make sure it's rated for continuous duty (so starter relays are likely out).

Hope that helps

 

[mep1811]

Check this system . I like it very much. Why use such a small battery?

http://www.lslproducts.net/ToadChargePage.html

 

[allyncooper]

Why use such a small battery?

No fridge or other big loads. Just 2-3 LED lights, sat radio, LED monitor (occasional use).