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Are my boat batteries dead?

2014 December 1

Dear Boat Doctor, 

I am having a problem with the batteries in my boat. I can charge the house bank up to 12.75 volts or so, but once I am away from the dock, the batteries quickly discharge. 

Jim Vandenburg 

Old Saybrook, Connecticut Dear 


Battery problems can be frustrating, and if the quantity of questions I’ve been getting about them is any measure, you should feel secure in the knowledge that you are not alone. 

Batteries just get old, and unfavorable treatment can make their short life even shorter. It is important to keep a battery charged because batteries left in a partially charged state for a long time can be damaged. 

As batteries age they go through a process called sulfation, which is a build-up of material on the negative plates of the battery. If this material builds up thick enough the performance of the battery can be impacted. When this material is sufficiently present it can present a false voltage, making the battery appear more charged than it really is. This, in turn, causes the battery charger to lower its output current which prevents the battery from being charged, and the cycle continues. 

Sulfated batteries can be recovered if the buildup is not too bad. It takes an equalizing charge with a higher voltage and lower current. Most good-quality smart chargers will have an equalization setting. It may be worth giving that a shot, and if it doesn’t work, it may be time to look for new batteries.  

Bob Pingel, a Coast Guard-licensed captain, runs Custom Line Splicing LLC. Send your questions to boatdoctor@sailingmagazine.net.