Tip & Trick: Dual Battery Setup with Automatic Charge Relays

I know, I know…we have gone totally geeky on the blog lately with all of these electronic upgrade posts. Most of which I don’t totally understand because this is not in my skill set, but we wanted to share how we are making our trailer life a little bit better for us and if it can help someone else, that makes it worth sharing. This is going to be the last post about batteries and relays and crazy cords and sensors and bus bars and all of that stuff for a while. Promise. Unless my husband comes up with something else that is completely fantastic. We are doing a few other updates that we will post on, but it is more lifestyle and what works best for us. More on that later.

We do not like camping with a generator and nobody likes camping next to a generator, so this is how we solved the problem of dry camping and still powering our travel espresso maker. I am serious. I understand, we are only camping by name and not by practice at this point. My husband is not the man we love without his coffee though and this was really important for him (and it seems my large Bialetti doesn’t do it for him…sigh).

Maybe this application will be important for something I need in the future too. We’ll see! Here is his long write up.


While our Airstream is new to us we are experienced “campers with a trailer”. As you may be aware we previously owned a CampInn teardrop. Several of the places we visit have full hookups but based on our camping style and some locations not having shore power I knew we would need to upgrade the electrical system to fit our needs. This is a long thread but I hope it gives others some ideas for their setup.

First upgrade was to upgrade of the converter, then installing 235 AH batteries in the stock battery box location, followed by the 200 Watt of Solar.

My final project (for now) was to install a second battery bank. When we first got the trailer I noticed a substantial amount of room under the bed. I’m not talking about taking up the space of the valuable storage unit under the bed, but behind that next to the fender wells, water heater and stock inverter. My idea – install a second set of batteries here.IMG_0905-001

In the diagram you can see the house and the secondary “AUX” battery bank, it’s the AUX bank which will install under the bed. One key point in my desired setup is that the two battery banks are not directly joined together. The house batteries power the trailer as normal and this second set will power the inverters.

In this setup if we choose to use the inverters, which are a luxury, we do not drain our important house battery. The switch seen has two inputs and two separate outputs: Off, On and emergency.

  • Off = both inputs are disconnected from their outputs.
  • On = input 1 is connected to output 1 and input 2 is connected to output 2.
  • Emergency = connects #1 and #2 together.

While this setup ensures that the batteries are kept separate it also poses a challenge in respect to charging. If #1 and #2 are not connected then each battery would need its own charge connection. This problem is solved with an automatic charge relay – ACR for short (BlueSea product). The ACR is a little device that senses charge voltage on either battery and when sensed creates a parallel connection sharing the charge between batteries. No charge sensed, no connection and the batteries are completely isolated.

In my case I disconnected the old stock inverter positive and fed that into the switch and then to the ACR. As I have solar charging (or converter when plugged in), when the ACR senses a charge voltage on the house battery cable it triggers and shares that charge to the second battery bank. However, this in itself poses a possible problem. There could be a scenario where I have solar charge current the ACR has triggered and connected the house to aux batteries in parallel and you use the inverter. In this scenario the inverter will pull current from both the aux battery and house battery through the ACR connection. The ACR has a capability which I used to solve this problem too. It has an inhibit or isolate function which via a switch enables manual override – disconnecting the ACR. Of course you would have to remember to press the button but even in the worst case scenario it just means you create a drain on both batteries.

Finally, I wanted a setup that if we accidentally drained the house batteries (light left on, furnace, etc.) that we would have a way to use the aux batteries in their place. In this scenario you would disconnect the house batteries (re-settable fuse in my case) and turn the switch to emergency which would connect the aux battery to the house circuit via the cables which used to run to the inverter.


I’ll cover the 12 volt system and later discuss why I have two inverters and how they are both connected, but first, lets look under the bed – look at all that space! Removing the wooden bed base confirmed the amount of space I had to work with after first looking through the access panels to check things out. Silver box at bottom of the picture on the right is the stock installed 1000 watt inverter. At the top is the water heater.



First I remove the stock inverter wiring and fuse, cut a small chunk out of the panel support as this is where the second battery bank would sit as seen in the photo on the left.



This cardboard box in the picture on the right was around the same size as one of the 6 Volt batteries I planned to install which meant everyone was looking good so far, and everything would easily fit.



Next a test fit of moving the stock inverter and a place for a second larger 2000 Watt inverter. You can see the new location in the picture on the left here.

At this point you are probably scratching your head as to why I plan to keep the stock 1000 Watt inverter. My reasoning was quite simple – it was there, I didn’t want to waste it, I didn’t want a button on the wall of the Airstream interior that didn’t do anything. But hindsight being 20/20 and all that, I should have just scraped it.

IMG_0649-001I wanted a larger 2000 Watt inverter as I have a guilty pleasure that I wanted to enable… my travel espresso machine. That and this inverter is better than the stock one. In my setup you get to keep both. More on that later.

The switch was mounted at the end of the bed – it is sort of hidden low down and in a space between the end of the bed and the fridge in our Airstream. While it is out of sight it is still easily accessible when we need to get to it.



In the photo on the left you can see the back of the switch inside the storage area with only the positive cable from the front batteries connected.



For the new cabling I purchased a mixture of 1/0 flexible welding cable for the 1000 Watt inverter and ACR runs. For the bigger 2000 Watt inverter I purchased 2/0 cable. I’ve done a couple of other projects with this cable and love working with it.



This cable lug installer is cheap and easy to use – just hit it with a hammer. It is one of the best tools I have found and I’ve installed over 100 lugs with this one.


IMG_0661-001On the right you can see the start of the 12v wiring. #1 input is from the house battery which in turn is connected to the solar charger and converter charger. The #1 output goes only to the ACR. The #2 input will, in the end, be connected to the second battery bank with 2/0 cable. You can see a couple of connections on output #2 – one goes to the ACR, one goes to the 1000 Watt inverter via a 100amp fuse and one to the 2000 Watt inverter via a 200amp fuse.

IMG_0654-001As for the inverters, I really don’t need two inverters but as I mentioned before I have two now (the stock 1000 watt inverter and the the 2000 watt inverter I added). The output of each inverter is connected to the inputs of an automatic transfer switch. This switch seamlessly switches between 110v inputs.

2017-01-27_16-38-07_877The automatic transfer switch’s output is connected to the trailer’s inverter sockets. The 2000 watt inverter has priority. Regardless of the state of the 1000 watt inverter, if you turn on the 2000 Watt inverter the automatic transfer switch will connect it to the trailer inverter sockets. This allows us to continue to use the stock inverter switch, which is more conveniently placed.

Perhaps in the future I’ll disconnect the stock 1000 Watt inverter and route in a shore power line to the automatic transfer switch instead. I would switch the input so shore power would have the priority as well. If you look closely in the picture you can see I used plug and sockets to make the connections so it’s easy to disconnect and reconnect.

2017-02-02_15-35-42_459.jpgI installed the inverter remote power switch in the storage cupboard under the bed which you can see on the left.

At the top of the image you can also see the small battery voltage meter. While I installed an AH meter on the house battery I didn’t bother for the AUX set.

IMG_0726-001Moving on to the second battery bank. There is a lot to see in these next pictures (note the negative connection post, this joins the negative from the house battery to the aux battery and inverters). You can see the ACR unit partly installed, it has 50 amp fuses on it’s connections and a negative fused connection which is required for it to function. The isolator connection is not installed here but it connects to the middle spade on the ACR. You run 12v to this middle connection to disable the ACR relay.



The batteries are securely attached to the trailer floor via a strap and you can see the fused copper bus bar I built connecting the two 6v batteries in series above and to the left. This fuse is a fail safe and acts as a battery disconnect if needed.

This completed the secondary battery install.




On the left you can see a photo of all the components in place. On the right from a different angle.




The disappointing thing as an electrical engineer is that all you see once the bed frame is back on is view of the battery (show in the picture on the right) and disconnect fuse from the access panel. This however is something my wife really likes and assures me everyone else will appreciate it also!

The only visible part of the install is the switch and inverter panel, and both of those are out of sight, hidden by the bed frame and the storage compartment door.

I did replicate the 12 volt connections in the rear storage area as well.

IMG_0756-001This has two uses, first is the black battery box is a portable backup battery (22 AH). If we screw up and kill both house and aux banks, this can also power the trailer in an emergency. The aux battery switch would be turned to off and this switch to emergency with the house battery fuse in disconnect position.

The second use is that the cables unplug from the battery backup box and can be plugged into our portable solar panel with integrated charger. The idea behind this is that we can use the portable solar as either a solar charge boost or if the trailer is undercover. The cables for the portable solar are long enough to reach about 20 feet from the trailer. Of course the back storage door has to be open when you are connected here. You don’t want to leave your portable solar while you are away anyway so we don’t feel this is a concern.

Something you don’t see it in this picture, but I did was connect a homemade extension cord to the second output of the 2000 Watt inverter and the other end of that is also in the storage area. You can unroll it and have a 110v connection out the back of the storage unit. No idea what I would use this for but that didn’t stop me installing it just in case.

This is the conclusion of our Dual Battery set up instructions. Feel free to comment with questions!



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