Photos by Capt. Vincent Daniello

Battery Blues

Quick troubleshooting when batteries go afoul.

My father called. “The boat smells like battery acid and the batteries are hot,” he said. Neither are good things. My father has reasonable mechanical skills, but basic electrical understanding evades him and I’m 1,500 miles away. He shut off the batteries and charger, and I called in an electrician. With just a multimeter, this one was easy to diagnose.

Dad’s boat has seven automobile-size batteries in three banks. “The first step for any battery troubleshooting is to isolate the batteries,” says William Hart, owner of Marine Electronics Services of Jupiter, Florida ([email protected]). “In this case, I removed one end of each of the positive cables that interconnect the batteries.” (Tip: Wrap the ends with electrical tape to prevent shorts.)

Once isolated, a standard test of battery health measures voltage while applying a load to the battery, but a load test is valid only with fully charged batteries, which these weren’t. “A quick test is just to measure the voltage of each battery with a multimeter,” Hart says. “Let them sit for a while. An hour is plenty of time. After that, a discharged battery should read about 10.7 volts. Anything under 10 volts is a red flag.” 

Of the seven batteries on my father’s boat, both batteries in one bank measured about 12 volts, and both batteries in another bank were more than 11 volts. The house bank, with three batteries, proved to be the problem with two batteries at 7 volts and one under 3 volts. “Insulators within the battery can break down,” Hart says. “A short in a single cell takes the other cells down, and when batteries are connected in parallel that bad battery will pull others down with it.” The same holds for batteries wired in series for 24-volt systems. 

All seven batteries were three years old—about the typical life of “wet” lead-acid batteries. It is a little unusual, although not unheard of, to see an old battery short like this. More often, this type of failure happens within the first weeks of a new battery’s installation.

“In a pinch, just eliminate the bad battery,” Hart says. In this case it was the middle battery of three, so removing its interconnecting positive cable and spanning across that battery with the other cable would have the boat up and running. But since we planned on replacing all seven batteries, Hart simply connected one known-to-be-good battery to each of the three battery-charger outputs to ensure the charger was functioning properly. 

“Twelve-volt systems should charge at 13.8 volts,” he says. “Some chargers have a boost mode that goes higher. That’s okay for a maximum of about 8 hours. By then the charger should be back down in float mode.” Similarly, 24-volt chargers should remain under 28 volts except when in boost mode.

It’s also a good idea to test each engine’s alternator output. With the battery charger turned off, measure voltage at each battery bank and then start one engine. If voltage doesn’t jump from under 12.7 volts to around 13.8 volts, that alternator isn’t working properly.

Rather than replacing batteries at his hourly rate, Hart refers customers to one of several local companies that specialize in replacing batteries. Treasure Coast Battery and Alternator (www.treasurecoastbatteryfl.com) replaced all seven batteries the day I called for just a bit more than what the batteries would cost at the marine hardware store. We upgraded from maintenance-free wet batteries to longer-lived Absorbed Glass Mat batteries.

This article originally appeared in the August 2012 issue of Power & Motoryacht magazine.