Monday, September 25, 2023


Dueling dual batteries? Here’s a fix for your tow vehicle

Does one of your batteries feel like this? Photo: Sarah_Ackerman on

Many diesel tow vehicles have dual starting batteries—it takes a lot of muscle to turn over that starting motor and fire up the glow plugs. But how do you take care of dual batteries?

The routine as far as physical care is much the same as in taking care of a single battery. Keep an eye on the electrolyte levels, keep the terminals clean and tight, and don’t leave the headlights on or you’ll run down your batteries.

Dual batteries are connected in parallel

Nevertheless, when it comes to doing a voltage check to see if your batteries might be getting a bit weary, babysitting the twins can be a bit different. Remember, your dual batteries are connected in parallel, meaning positive post to positive post, and negative post to negative post. In this type of connection, your batteries “talk” to each other—they share. If one of your batteries is “getting ready to go,” then it will take its twin right along with it.

A fully charged 12-volt starting battery should read 12.63 volts at rest. Connecting up your digital volt-meter to one of them will show the shared voltage—or typically should. To really check on them, disconnect one of the negative battery posts, then check the voltage on each individual battery. The read voltage on each battery should be very close. If not, then one of your batteries is beginning to “give up the ghost.” The weaker of the two will start to drain the stronger one.

Check for dirty terminals

Before condemning the “weak sister,” check for dirty terminals or a loose connection. If that’s the case, you may be able to revive the situation by just fixing the problem, and letting the alternator bring the voltage up. After cleaning the battery posts, give them a good coating of grease, or spray on battery terminal corrosion protector.

But if you need to replace one of your twins, don’t try and “cheap it out.” Dual batteries should be replaced in pairs, with the same size, capacity, and preferably maker. It’s not an inexpensive proposition and, frankly, it hurts. But better to spend your money wisely than get stranded in the middle of nowhere.


Russ and Tiña De Maris
Russ and Tiña De Maris
Russ and Tiña went from childhood tent camping to RVing in the 1980s when the ground got too hard. They've been tutored in the ways of RVing (and RV repair) by a series of rigs, from truck campers, to a fifth-wheel, and several travel trailers. In addition to writing scores of articles on RVing topics, they've also taught college classes for folks new to RVing. They authored the book, RV Boondocking Basics.


  1. This is the issue with charging batteries that are connected in parallel, the strong one will always take the charge. If given enough of space to install 1 large battery instead of 2 smaller ones I would take that route.

  2. Nanci, you did a good job but, I would like to add, 12.63 volts at rest is a fully charged battery. The at rest is 30 minutes or more after shutting the engine off. With the engine running, the voltage should be 13.8 volts or more up to 15.2 volts which tests your charging system to see if your charging system is properly functioning. Without getting too technical, these voltages are the most important. 12.1 volts the battery is discharged and anything less than 12.1 volts means you have a dead cell. There are charts available on-line or any service manual which give you the state of charge of your battery by reading your voltages with a good digital volt meter. Dual batteries need to be isolated when trying to diagnose a problem.

    I used to spend about a week teaching battery/charging system diagnosis and maintenance.

  3. Also, attempt to get batteries from the same production run, and, ideally, same day. a stronger lead acid in a pair situation will destroy the weaker one.

  4. Luckily I found out about replacing both batteries at the same time while I was in an auto parts store parking lot having a bad battery replaced. A diesel, specifically Cummins, mechanic stopped in to pick up parts and saw what I was doing. He told me that both should be replaced at the same time. I’m glad I listened and did but the guy at the parts store had never heard that.

    • I had to replace both at the same time as well. My Chevy 3500 batteries died shortly after I got to my sister’s cottage in Northern Ontario from my home base in Toronto. Sucked to spend the money for two batteries, but the timing was perfect! Have to look at the glass as half full in life!


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Sign up for the

RVtravel Newsletter

Sign up and receive 3 FREE RV Checklists: Set-Up, Take-Down and Packing List.