Add Batteries to Camper

Let's make a live link to your blog to help others: Blog linky to circuit diagram Better effort on the diagram. Like said, only you have to understand it.

One further caveat: A 300 W inverter is not going to power a compressor fridge. I tried a 600W and it didn't work. Currently, I have a Xantrex 1000 W Pro Watt and it powers a small dorm fridge well.

Thanks for the info. I updated the diagram with voltages and the fuses you mentioned. I don’t see where I have high and low voltages crossing.

To westend’s questions:
My list is on the cheap end. I’m not above spending the extra money where needed. Are their particular items not to go cheap? I’m thinking battery and charger. Is there not much benefit in other areas, like that isolator or monitor?

The battery would be called on power a small fridge and a few LED lights. There is a microwave and air conditioning unit on the A/C side but those won’t be run off the battery. The transfer switch would run everything off shore power when available instead of routing through the charger and inverter.

There is a dedicated battery for the emergency brake but it is not nearly big enough to power anything else for long.

There are 12 volt lights powered off the truck battery. I intend to keep them separate.

There trailer is 2005 Cherokee White Cloud. Came stock so all 120V stuff was added aftermarket. The trailer is used for weekend camping so two days off power at the most. The truck would always be available to charge the batteries.


To DrewE’s questions:
I substituted hot for plus and neutral for minus on the 120V side of the diagram

There are two A/C circuits in the trailer; one for the air conditioner and one for everything else. The everything else is a fridge, a microwave, and a few lights. I would need a bigger inverter to put that circuit continually through the charger but seems like the best way.

My truck is a 2013 Silverado with a 7 pin trailer connector. I’ll have to check to see if it is already switched

What surge protector would you recommend? The A/C panel is 30 amp.

DC wire runs will be short. The whole system will be in a single cabinet. I made a new diagram with the changes noted above and more fuses. It’s on the blog.


Thanks for the help. I’m feeling better already.

Your diagram is a little confusing to me because it mixes 120V and 12V circuits without a lot of indication of which is which. Also, 120V AC circuits don't have a positive and a negative leg; there's a hot and a neutral (and a protective ground), but the polarity of them with respect to each other alternates. However, I think I'm able to suss out what you're trying to do.

You have one rather important problem on the 120V side: the charger/converter circuit is not connected to a branch circuit from your electrical panel. That's almost certainly not acceptable per the NEC. However, connecting it to the panel after the transfer switch does lead to some potential difficulties to solve: you don't want the charger to be on when running off of inverter power, as you'll just waste battery power trying to recharge the battery from itself (with the power waste coming from the inherent inefficiencies in the inverter and the charger).

The ideal solution there is to have only selected circuits switched between the inverter and shore power with the transfer switch. There may also be other things you don't want off the inverter power--commonly in a camper these might include an absorption fridge, an electric water heater element, an air conditioner, or (with a 300W inverter, at least) a microwave. If you partition your electrical circuits appropriately you can make it inherently impossible to run them from inverter power.

A simpler and cheaper solution for small setups like this is to just have some outlets dedicated to inverter usage. This eliminates the need for a transfer switch entirely. If you're plugged into shore power, you can continue to use the inverter outlets off the inverter indefinitely since the charger will carry the load. (A larger inverter might require a bit more care and consideration in that regard...with a 300W inverter and a 45A charger, you'll still be charging the batteries a little even with the inverter running full bore.)

The battery isolator is quite possibly unnecessary in your situation, assuming you're using a 7 pin trailer connector to get the charge current. If the trailer connector power is switched on the truck (some are and some aren't), you won't be able to deplete the truck battery by using power in the trailer. Most trailers do not have an isolator setup.

The surge protector you show is of rather limited value. It's basically the same as a surge protector power strip with a GFCI added on. It does not provide any protection against sagging or moderately high voltages like an EMS unit does, nor against wiring errors in the outlet you plug into.

In terms of wire sizes, etc: for the AC side, this is pretty simple as it's defined by the NEC. For 30A circuits, you use 10 gauge wire (or heavier); for 20A, 12 gauge wire (or heavier); and for 15A or less, 14 gauge wire (or heavier). Given the length of the wire runs you have in a trailer, heavier wires are a waste of money. The current rating of the circuit is basically determined by whatever circuit breaker is upstream protecting it.

For DC circuits, there are two considerations. The first is fire safety and heating, which basically means not exceeding the safe carrying capacity of the wire size. The second is voltage drop, which one wants to minimize as much as practical to avoid wasting power. The current carrying capacity of the wire does not vary with wire length, but the voltage drop does, so for longer wire runs you often want or need to use heavier wire than would be necessary for short runs.

The wiring should be protected (on the positive side, assuming a standard negative chassis ground) as close to the power source as practical with a fuse or circuit breaker of some sort that is not larger than the safe current carrying capacity of any wire it protects. In situations where there are more than one possible power sources, it's necessary to analyze the circuit somewhat to figure what the total current at any point could be and figure protection appropriately. What you want to ensure is that a short to ground at any point would not cause any wire to carry more current than it safely can.

The charger has output fuses built in. One common approach at the battery bank is to have a large fuse at or near the battery positive terminal for everything in the RV(in my motorhome, this is a 175A fuse). From there you have wires going to whatever large or special things need connection, with fuses or circuit breakers appropriate to the situation. For the inverter, the documentation probably gives a suggested size, maybe around 50A. For the connection to the charger, 50A would also be appropriate. For connection to other 12V circuits (which you haven't shown, but I would urge you to think about having...things like interior lights, 12V appliances, exhaust fans, convenience 12V receptacles...), usually there's a separate fuse/distribution panel that has maybe a 50A line feeding it, often with a master disconnect switch. Sometimes the charger/converter connects to this panel rather than more directly to the battery.

On a trailer with electric brakes, you should have a breakaway switch that turns the brakes on from the trailer battery should the trailer become disconnected from the tow vehicle. This would be powered pretty much directly from the battery. If you already have a setup like this, it presumably has its own battery somewhere, and it would be simplest in my opinion to get rid of this separate battery and use the main trailer battery. (Among other things, doing so would simplify the charge line wiring from the truck.)

Not a very good circuit diagram of typical RV electrical. You have the 120 V side mixed in with the 12 V circuits. But, if you understand it, that's all that matters.

First, just about everything on that list except the converter I would not buy. It appears that you are buying on low price and that's not always the best choice.

Second, what are you powering and is it essential to have a transfer switch? IMO, transfer switches are problematic and if only powering a few devices, aren't essential.

Third, why do you not already have a battery? In just about all states, any trailer over a certain weight needs a battery to power a break-away emergency braking switch.

Fuses and circuit protection: You will need one circuit breaker or fuse typically mounted within 18" of the battery. Most travel trailers use a self-resetting CB of about 40 amps. You will need a fuse between the inverter and the battery. Mfg of your inverter will have a size recommended. You will need a circuit breaker or fuse between your truck battery and the isolator. This should be in the order of 50 amps or smaller, depending on load and wire size.

You don't mention any 12 V distribution. All travel trailers have a 12 V distribution panel. Each circuit on there has it's own fuse.

If you post the name, year, and model of your trailer, advice can be more specific. If you wish to know why some of the items listed are deemed less than good, ask about them particularly. Initially, that battery is not Absorbed Glass Mat construction. It is Sealed Lead Acid. There are better solutions available.

Also, any advice on some electrical installation bears on how you use your trailer. Share that with us, please.

A couple of links for education: 12 volt side of life

Golden rules (has pertinent wiring info)