Saturday, June 10, 2023


Ask Dave: New batteries: Which should I get and how do I connect the cables?

Dear Dave,
I just bought a used Thor with two house batteries and they are not holding a charge. I need to install new batteries but the compartment seems like a rat’s nest! What batteries are the best and how do I keep all the cables straight? —Terry

Dear Big T (he’s a friend of mine!),
Wow, you do have a rat’s nest and some really bad-looking batteries! First, these are two 6-volt deep cycle batteries connected in series, which means they are positive to negative creating a 12-volt bank. Other issues here are how swollen the cases are. That means they have gotten hot and/or overcharged, judging by the enormous amount of corrosion at the terminals. It’s ironic these batteries have a “quick disconnect” fill hole system. That is the two bar system that can be squeezed together to remove the caps rather than take each one out individually to check the fluid level. I doubt these were ever checked as they have not been maintained properly.

Choosing the right battery

To start with, you need to decide what battery or batteries are right for the way you use your RV. You can simply swap these out for two new deep-cycle 6-volt lead-acid batteries. But you need to make sure you maintain them properly. That means not only checking fluid levels and keeping the terminals clean, but also properly charging them.

I would imagine your Thor has a simple distribution center that has a built-in charger/converter. This will only charge the batteries at 13.6 volts until they reach 12.6 volts and shut down to a maintenance charge of 13.2 volts. This will not break up the sulfation that coats the plates. Eventually the batteries will become sulfated to the point they are not holding a charge again. I would recommend looking at a multistage charger at this point. You could go with Absorbed Glass Mat (AGM) batteries, which are sealed and are less affected by sulfation.

6-volt versus 12-volt batteries

Then there is the discussion between 6-volt and 12-volt batteries. 6-volt batteries typically are available in higher amp-hour ratings, have more cells/plates/acid, and last longer as they provide more “cycles”. However, you do need two batteries vs. 12-volt batteries.

The big question is how often do you boondock or dry camp? This will help determine the best battery for you and how long you need the batteries to provide power before needing to be recharged. This is amp-hours.

Lead-acid batteries can only be drained down to approximately 50 percent. So a battery rated at 105 amp-hours can only provide about 50 amp-hours. It is hard to determine the exact amp-hours you will require as it depends on several factors. Those include how many 12-volt components you will be running and how long. Items such as interior lights, LP appliances (especially the furnace), and how long you will be boondocking should be taken into consideration.

How many amp-hours do you need per day?

According to Go Power!, a solar company we have worked with for several years, the average Class A unit will require approximately 100 amp-hours per day. This, of course, is just an estimate as every RVer is different in what they use per day, the ambient temperature, and how many people are RVing. But let’s just use 100 amp-hours as a number, so a lead-acid battery rated at 105 amp-hours will not last a full day without needing to be charged. If you plan to boondock, visit this page and use their calculator to factor in the appliances and everything that will get you started on the process to determine the right battery size. If you are not planning to boondock much at all, I would suggest just putting in one 12-volt deep cycle battery and saving some money.

Another option is Lithium, which can be drained to almost 100 percent. So a 100 amp-hour battery will last twice as long, but will cost 5-6 times as much money. Keep in mind if you do go Lithium, you can’t just swap out your two 6-volt batteries with Lithium without checking the charger/converter or solar panels.

Keeping the cables in the correct location

Let’s assume you are going to simply change the batteries with two new 6-volt deep cycle batteries. You have two batteries connected in series, which means a cable connects a positive terminal on one battery with a negative terminal on the other. Check out the photo below, the arrow shows the “jumper” cable that connects these in series.

From the photo, I cannot tell for sure which terminals are positive or negative. However, you should be able to wipe off the top case of the two front terminals shown and see the + or – or possibly a “NEG” or “POS”. From what it looks like, the left side battery has the positive post in the front as it seems to have red markings on the cables. This is just a guess—make sure you verify the post. The post on the right battery on the front then should be negative—but, again, verify that.

How to make the switch

Remove the two cables connected to the negative cable first and zip tie them together or tape them and mark with a – or NEG.

Disconnect the two cables on the left side positive terminal and zip tie or tape and mark with + or POS. These are the only cables you need to worry about as all the other rat’s nest cables are connected to the BIM solenoid and other components and not connected to the batteries. Just deal with the two positives on the post, two negatives on the post, and the jumper cable from POS to NEG on the back.

If you use just one 12-volt battery, connect the positive, then negative, and you will not need the jumper.

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.

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Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


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Thomas D
1 month ago

Id go to hardware store and buy some colored cable ties( plastic handcuffs) put a black one on everything that shows — ( minus) and red ones that are on the + terminals. They become permanent unlike tape and then you wont get anything mixed up

1 year ago

Snapping a picture with you phone before you disconnect anything is a great help.

1 year ago

I seem to always question myself about the wiring. I guess I need to label things instead of guessing all the time. I’m going to need new batteries this year and like your questioner best batteries for toy hauler who seldom ever stays in a park usually a week at a time boondocking. Have solar, lights, furnace, tv, radio, about it. Would stay 12v only. Just what type of battery? Not lithium. I understand many variables just best guess. Thanks

Roger S
1 year ago

I am surprised that Dave did not suggest cleaning the cable terminals and the rest of the battery compartment. The amount of corrosion and other debris just shouts “no maintenance ever!” Good housekeeping in this compartment can keep the 12v system in good shape. Corroded terminals are a good place to start.

Bob p
1 year ago

A simple wrap of tape you can write on wrapped around each cable with the + or – should suffice.

1 year ago
Reply to  Bob p

Red and black electrical tape work wonders to identify the wiring. From the picture, it appears that the positive is on the left side (red mark on the wires) and negative is on the left side. The jumper also appears to be marked. Just verify that the red goes to positive and the black negative. Remark the wires with the tape.

1 year ago

buy a label maker and label everything. you will be glad you did.

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