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.