Wiring Batteries in Parallel

Wonder if I could get an opinion from one of the electrical gurus out there.

I'm preparing to wire a second 12-volt battery in parallel with the original. (I'm aware of the advantages of two 6-volts in series and will probably do that later, but for now I have two almost new 12-volts and will use them, at least for the moment.)

Our 07 3124 KB has 10 gauge wire from the battery's positive terminal to the loads and 8 gauge ground from the negative terminal to the frame. (The battery on this model is mounted on the TM's tongue frame.)

Here's the plan: After disconnecting the ground first and positive afterward, I would use 10 gauge from the positive of the second battery to the positive of the original. Then use 8 gauge from the negative of the second battery to the negative of the original. After that, re-connect the positive of the original to the load, then re-connect the original ground.

My questions: 1) Is 10 gauge sufficient from positive post to positive post? 2) Can I safely use only the original ground for both batteries, since they have become one battery in effect? Or does the second battery require a separate ground?

I haven't been able to find any specifics on doing this, so would appreciate any expert help that's available.

Thanks,
Jon

1) I believe the thickest 12 volt positive wire in the TM is 10 gauge. Thus 10 gauge between the batteries is fine. You can also use 10 gauge for the negative wire between the batteries with no problems. I'd use whatever you have that is cheap and handy and at least 10 gauge.

2) No need for another ground. In fact, you probably *shouldn't* do an extra ground for a lot of reasons I won't get into here. Use the original one.

Keep the wires short, prevent them from rubbing against any wires of opposite polarity or the frame. It doesn't hurt to fuse the batteries (if you get a sudden short inside one battery it can prevent a fire), but it probably isn't necessary.

Also, use two identical batteries bought at the same time. You don't want to add a new 12 volt battery and and old 12 volt battery, as it will reduce the life on both batteries. It will also drain the new battery, and never allow the new battery to obtain a full charge. I know that probably isn't what you want to hear.

I added a second battery to my TM when I bought it new. I used short wires of the same gauge as the wire tht was already on the TM to connect the two batteries in parallel (positive to positive and negative to negative). I used marine grade wire because the wires are exposed to the weather. I connected the hot lead from the TM to the positive post of one battery and the negative and ground wires to the negative post of the opposite battery. I had been told that this is the way solar systems are connected when there are two batteries. It spreads the load better between the two batteries. Makes sense to me, based on what little I remember from studying to be an electrical engineer oh so many years ago.

Today I have a switch. It allows me to have either one of the two batteries, both batteries or no battery at all. I like being able to disconnect the batteries with a switch instead of pulling the fuse. I also like the ability to disconnect one battery if I were to suspect that it had a problem.

Thanks, guys. Appreciate the info.

Wayne, where did you mount your battery selector switch? Is it a standard marine type switch or is there something better for RV applications?

For my 2720 the batteries are on the tongue. I mounted a weather proof electrical box under the front edge of the TM on the street side. I've been meaning to take pictures and post them thinking that someone else might be interested in my solution to mounting a battery disconnect switch.

I mainly use it to kill the batteries when in storage in my garage.

PopBeaver, I would really like to see some pictures of your battery configuration and switch.

It has been a long time, but I finally took a couple of pictures of my battery switch.

It is mounted in some sort of weather proof (resistant) electrical box that I bought at the hardware store. I made the two L brackets by bending some aluminum straps 90 degrees. The blue thing at the top is some masking tape that is holding the cover open for the picture.

You can see two sets of wires that are wrapped. One set is from the charger/controller. I just wrapped them up to make it look neater. The other bundle is from the pair of batteries to the switch. There are three wires, one for each battery and the other to bring the power back.

The switch has four positions, battery 1, battery 2, both batteries in parallel and disconnect.

It is not convenient to access, but then I do not want strangers to be tempted to mess with it. I wanted it to be out of the weather, so it is not only in a box, it is tucked up under the front of the TM.

If you look closely you will notice that one of the screws is missing where it was bolted to the battery tray. I installed the bolts pointed down, so the excess was on the lower side. Well, going out of a driveway on the first trip after I built it, the screws dragged on the ground. That was a surprise. The battery tray never touched the ground, but the excess half inch of screw (machine number 8) certainly did. Apparently TM engineered the battery tray to be as close to the ground as they could get without hitting the driveway. They did an excellent job on that design.

I disconnect the batteries when stored in the garage and for any extended stop on the road when the fridge is running on 12 volts. No one has any reason to use the switch except me.

The box is not enclosed on the back side, which is facing forward as I have it mounted. I have a piece of sheet metal inside with caulk to close that off. For fear of the wires touching the sheet metal there is a piece of plastic between the sheet metal and the switch.

Great pictures, thank you Wayne!

I just posted this on another thread, but it seems to be appropriate here as well, so I will replicate it here for those that find this thread in the future, but not the other one:

The reason I got the switch that allows me to use one battery at a time is because if I think one battery has a problem, or I want to measure the voltage across a single battery, all I have to do is rotate the switch.

If all you want is a way to disconnect a single battery, or all batteries, then a simple on off switch will suffice.

I got my switch at the local West Marine store. Looking at their web site, it looks like the one that I bought is BLUE SEA SYSTEMS # 350A Compact Battery Switches which has a price of $39.99.

I also bought some marine wire. I used that instead of auto wire because it is tinned to reduce corrosion. The price is not very different. Trailers are not exposed to moisture as much as a boat is, but my wire is more exposed than wiring in a car.

If I was paying someone to install it, I would specify that all new wire be marine wire. For existing wire just go ahead and use it as is.

I used crimp on terminal lugs at the end of each wire. Then I soldered the wire to the lug. I don't fully trust crimp on connections in an environment with a lot of vibration.