Custom RV 12 Volt Battery Box

by | Sep 29, 2015

Any fan of Star Trek remembers the commands by Captain Kirk demanding “More Power!” and the eventual reply from Chief Engineer Scott “I’m givin’ her all she’s got Captain”. Well that was the state of our two 12 volt golf batteries we use for inverter power. We knew quickly we did not have enough RV batteries and determined we need to expand our power needs with a custom RV 12 volt battery box.

Our 2014 Heritage Gateway fifth-wheel is equipped with a residential refrigerator. I’m not here to debate the pros and cons of a 120 volt frig versus the traditional LP gas model, needless to say, we preferred the latter due to less heat generated and larger capacity. It does come with a downside – it needs power and that power is 120 volts AC, better know as house current. Some state you can close up a residential frig for several hours with no power and all will be safe until you reach your destination. That may be true for a short haul, but this is very impractical when traveling cross county where overnight stays may not support an electrical hook-up. Plus, I don’t want my ice cream to melt!

Solution, a couple deep-cycle batteries and an inverter that can handle the current surge generated by the start of the generator’s compressor. Testing our set up, I found the frig operates at .7 amps or 85 watts, but the compressor has a “kick” lasting about a second in time, requiring 800 to 900 watts. This kick can drive some inverters crazy, looking like a fault in the AC circuit causing the inverter to trip or shut off.

Our set up from the factory worked well for a full days pull. Initially we had a problem with our brand of inverter not supporting the initial compressor demands, but Heartland offered good support and sent a replacement at no charge. Since I was going to be in the front bay changing out the inverter and also concerned with some of the marginal factory wiring, I decided on the custom RV 12 volt battery box project. Joan and I are starting our travels and plan to be on the road 9 months of the year. As the old adage goes: “Sometimes you can’t get there from here.” holds true and a stop along the way may not lead itself to a shore power hook-up (insert Walmart or truck stop here). We wanted the ability to have inverter power for at least 2 days without a generator and still have the frig, lights, water pump, 12 volt fan, etc. available for overnight boondocks. We looked at our current battery capacity and determined we needed a total of 4 batteries.

Custom RV Battery BoxNext challenge. Fitting four batteries in the front 5th wheel bay. I searched throughout Googleland and wasn’t able to find a solution at a reasonable cost that would house 4 batteries length ways across the front of the bay. I felt this was very important to keep the weight evenly distributed. Solution, build a custom RV 12 volt battery box. With $22 of 3/4 inch plywood sheathing, we ended up with the set up to the right. We protected the wood from rot using a plastic spray-on paint. Basic latches on the front and side to keep the lid air tight with simple 1 inch, plastic conduit fittings at the top and bottom to allow air to flow through the box and vent hydrogen gas.

Custom RV Battery BoxLet me stop here and briefly discuss battery venting. When a battery charges, each cell will produce hydrogen gas. This gas is extremely combustible and you don’t want it building up in the bay of your RV much less leaking into your living quarters. If you want to see the affect of hydrogen gas, watch this old film clip of the Hindenburg Disaster where this massive air ship was destroyed in less than a minute. Batteries and hydrogen gas is not something we need to fear, rather understood and properly managed. For our installation, we ensured the lid on our battery box was tightly secured and good air flow was supported with a vent to the outside of the 5th wheel.

We mounted the box with a series of carriage bolts and added the batteries. Those with bad backs beware! These things are heavy! We’re using 12 volt golf cart batteries, also known as deep cycle flood which can be found from discount stores to specialty distributors in a wide range of amp capacities. Next is the connection. Wiring is critical – use too small of a wire and you generate heat. Heat requires amps to create and this wastes battery capacity. Connectors are equally important – if they wiggle, then replace. I found a great source for 2/0 welding cable along with quality connectors and a hydraulic cable crimper. When crimping cable, please don’t use a hammer and chisel or a set of vice grips. A good crimper can be purchased for less than $20. When you have hundreds of dollars in batteries, make sure you’re getting all the amperage available with quality cables, connectors and the installation.

Custom RV Battery BoxLast item was the connections between the box and the outside cables. In this pictures, you see a thin sheet of plexiglass with 2 Moroso Thru Panel Connectors. A little silicone around the edge of the plexiglass provides an airtight connection and the plexiglass provides electrical isolation for the box.

The results have been great. We recently ran the frig for 36 hours on the new house batteries. Along with the time on the road, we stopped for the night with the frig running, also using the house lights (all LED), the water pump and a Fan-Tastic Endless Breeze standalone fan. At the end of 36 hours, the batteries were reading 12.35 volts, roughly 70% capacity remaining.

Cost of the box was $79 and adding the cables and connectors the total project was $158. If you’re interested in the parts list or need additional information, feel free to send us a message using our Contact Us page. We’d love to hear from you.

I love RV life. Jerry

Useful links for your project.

Wire Resistance and Voltage Drop Calculator. Remember, as voltage decreases, amperage demand increases. Use the lowest estimate for your battery string at discharge.

TMS Cablle Lug Crimper TMS Wire Battery Cable Lug Terminal Crimper 16 Ton Hydraulic Cable Crimper With 11 Dies and Carrying Case. Provide a factory style crimp to attached lugs to battery cables.
Moroso Red Thru Cable Connector Moroso Red Thru Cable Connector 3/8 inch insulated power stud.
Moroso Black Thru Cable Connector Moroso Black Thru Cable Connector 3/8 inch insulated power stud.

Pico 4539C 2/0 AWG Battery Cable 3/8" Tubular Lug Ring Pico 4539C 2/0 AWG Battery Cable 3/8″ Tubular Lug Ring Solid Copper Closed End Lug Rings, 3/8″ Stud Size, Solder or Crimp, 2/0 AWG Cable.
TEMCo WC0046-20' (10' Blk, 10' Red) 2/0 Gauge AWG Welding Lead & Car Battery Cable TEMCo WC0046-20′ (10′ Blk, 10′ Red) 2/0 Gauge AWG Welding Lead & Car Battery Cable 325 Max Amps (up to 50 feet), Rated up to 600 volts, -50°C to +105°C (-58°F to +221°F)
Battery Vent Kit and Hose Battery Vent Kit and Hose <strong> Vent for outside of RV with hose to connect to battery box.

Battery Vent Flange for Box Battery Vent Flange for Box

latch hardware Latch Hardware Wood Box Toolbox Cabinet Metal Spring Loaded Latches Catch Toggle Hasp 

Flex Seal Spray Rubber Flex Seal Spray Rubber Seeps into cracks and holes. Dries to a watertight, flexible, rubberized coating.

RV AccessoriesNot sure about what RV accessories you need as you start RV travel? Check out our RV Accessories page where we have listed a wide range of products from essential items to those “nice to have” RV products to make RV life more enjoyable. Every item listed is being used or has been used by Joan and I as we’ve traveled across the country in our RV.

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