Auxiliary Battery For Refrigerator

First, a little background to this project. We have been camping for a few years in our Camp Inn, using our Dometic refrigerator in our Nissan Xterra, plugged into the factory outlet in the back and it has served us well. However on a recent outing in Sequoia National Park, we went on a hike one day directly from the campground without starting the car for a full day. In the morning, the battery was low enough it would not start the car. We had done this before, but this time the battery couldn't take it. Luckily we were camping with my son and his family and a quick jump and we were going again. However, if we had been out in a remote place (think Death Valley) by ourselves, it would have been a big problem. The battery in the car was only 3-1/2 years old, but although it was a high quality AGM battery, it was an ordinary starting battery. Automotive starting batteries have evolved for a hundred years and the current batteries are excellent for their intended purpose: Start the engine and then immediately recharge, but also built to be cost competitive. They are not happy being discharged even half way, while this is apparently working, it shortens the life of the battery and reduces its capacity. I suspect our usage with the fridge had affected our battery leading to this situation.

This prompted me to work on a better solution. I considered replacing the battery with a "deep cycle" one which is happier about being discharged. Instead, to be better prepared for the isolated boondocking we like to do I decided to add an auxiliary battery specifically to run the refrigerator, leaving the primary battery to its intended purpose, starting and running the car. I also replaced the car main battery to start with a fresh reliable power source.

The auxiliary battery is contained in a box and will only be put in the car when the fridge is there. At other times the box will be stored in my shed with the fridge, connected to a battery maintainer to keep it at optimum charge. I considered trying to mount the auxiliary battery in the car, but there is no good place to put it, and it really is not needed unless the fridge is there.

I started by purchasing an Optima Deep Cycle marine battery, model D34M. I built up a box system using a box I had left over from a previous project, two power point sockets, a circuit breaker, switch, and a volt meter. It also has a 50 amp connector with #6 wire to connect it to the car.


Here is the wiring inside the box. I added a block of wood to keep the battery in place, plus it provides a place to keep the documentation for all the parts.

Under the hood I added a battery isolator and a 50 Amp circuit breaker. The isolator is a Battery Doctor unit that keeps the two batteries disconnected when the car is not running, charges the auxiliary battery when it is running, plus includes a function that allows a "jump start" feature allowing the auxiliary battery to be connected to start the car if the main battery is too weak for the job.


I then ran a #6 cable from the circuit breaker under the hood to the rear cargo area of the car, with a 50 Amp connector matching the one on the battery box. When we are ready to go, the auxiliary battery box goes in the back with the fridge, plugs into the car 50Amp connector to keep it charged, and the fridge plugs into one of the sockets on the box. There is a second power point for other stuff, such as charging a phone or whatever is needed. Also, a solar panel setup could be plugged in here to keep it charged. The voltmeter keeps track of the state of charge.


For about $350 I now have peace of mind that we can keep our food supply fresh and be confident we can start the car when we are ready to move on.

 

Cool setup!

Cary

 

An additional use for this setup is when camping in bear country, the fridge and the battery box can be put in a bear box over night. No danger of the big furries trying to break into your car or trailer.

 

As requested, here is the schematic for my setup:
AuxBattery.pdf

The specific parts I used are:
Aus Battery: Optima Batteries 8016-103 D34M BlueTop Starting and Deep Cycle Marine Battery
Battery Isolator: WirthCo 20092 Battery Doctor 125 Amp/150 Amp Battery Isolator
50 Amp Breaker: Bussmann CB185-50 CB185 Series Automotive Circuit Breaker
50 Amp Connector: Battery Quick Connect/Disconnect Wire Harness Plug Connector Recovery Winch Trailer 6-10 GAUGE
Switch: 15 Amp Toggle Switch (from my junk box)
15 Amp Breaker: Potter Brumfield Circuit Breaker 15 Amp 120v Push Thru W28XQ1A-15
Power Point Sockets: Blue Sea Systems 12 Volt Dash Socket
Meter: Blue Sea Systems Mini OLED DC Voltmeter with 12V/24V DC
Battery Box: Attwood Standard Battery Box, Vented, 24 series
Battery Maintainer: Schumacher SEM-1562A-CA 1.5 Amp Speed Charge Battery Maintainer
Charger Adaptor: Battery Tender Cigarette Lighter Adaptor
Large Wire: 6 Gauge Premium Extra Flexible Welding Cable 600 VOLT - BLACK - 25 FEET - EWCS
Small Wire: #14 miscellaneous wire from my shop
Miscellaneous terminals from my shop

Everything here (execpt where noted) was purchased from Amazon. If you really want to build something like this, I am happy to help with even more details.

Most important is to be safe. Automotive batteries have a lot of energy and while the voltage is not high enough to do you any harm, if you accidentally short out the battery, things will get very hot in a big hurry. Circuit breakers are important to protect everything. All terminals should be covered.

 

My big concern with this is the amperage capacity of a "DC plug mounted in the truck bed". If the auxiliary battery is low on charge and the TV is started, there can be a lot of current applied to charge the battery. Definitely add the battery isolator and some circuit protection appropriate for the wire size you use - either a circuit breaker or fuse.

 

Thanks so much for the links and additional detail. I think I am going to have a go at this. I purchased this battery box from Amazon. I like it because it has two female cigarette lighter ports, external terminals, battery power indicator, nice carrying handle and circuit breakers. The ports and external terminals will make it easy to attach a solar panel.
Amazon.com : MinnKota Trolling Motor Power Center : Electric Trolling Motors : Sports & Outdoors

Also got a battery tender to keep it charged at the house.
Amazon.com: NOCO Genius G1100 6V/12V 1.1A UltraSafe Smart Battery Charger: Automotive

Thank again Rotus, I think this is a good use for my used battery that still has some life left and as you mentioned, will allow me to put the fridge and battery in the bear boxes that are so common in areas that we camp in the So Cal area and Eastern Sierras.

Gratefully

Randy

 

Randy - that battery box looks just the part, essentially everything I purchased and assembled in pieces, but already put together. I would be a bit nervous about the exposed wing-nut terminals, a stray camp chair with a metal frame could make things interesting; some kind of cover would be prudent.

Ken - Yes, the solar panel will charge the aux battery. It will not charge the car battery as the battery isolator prevents charging (as well as discharging). Actually, the isolator I used has a manual override button that can be used to "jump start" the TV battery from the aux, I suppose this function could allow a solar panel connection to charge both; interesting thought. When the trailer is connected, both aux and camper batteries (as well as the TV battery) will be charged.

One other thing to mention - a battery even in a box, is a heavy thing that can become a projectile if you have an accident. It can also move around in unexpected ways during normal driving which can be troublesome. Some way of tying it down while driving would be a good idea. Also, anything except a sealed battery should not be in the same space as living things. There is a lot of energy inside a battery, as well as some nasty chemicals - be safe!

 

I have been wondering if it would be possible to "split the line" coming off our Zamp solar panel to charge both the Camper and Auxillary batteries. Did some google searches but didn't come up with much. Thoughts anyone? We don't drive our tow vehicle much if at all once we are setup. Thanks

Randy

 

I don't think this is possible, at least not without adding massive inefficiency. To elaborate on the last statement, powering an inverter and two battery chargers would work, but I don't think that is practical. I can imagine some fancy electronics, but nothing comes to mind that would be off the shelf.

 

Indeed, that is exactly why I put that connector there. Several times when fiddling with it, I wanted to disconnect the cover, so I added the connector. (Confession - I have almost as much automotive electrical and mechanical bits in my shop as my wife has thread and fabric in hers. )

 

Excellent! Happy Camping.