Buy Boats, Sell Boats, Review Boats

Posted by on Dec 23, 2012 in Boat Equipment, Boat Maintenance, Powerboating, Sailing, US |

Electrical Panel Voltmeters Can Sometimes Mislead

Your meter can give you a surface charge reading showing decent voltage even when the battery plates are not deeply charged.

Question: Recently I ran into a problem with my boat that seems kind of strange to me. As I usually do when I first get on board, I checked the electrical panel board-mounted voltmeter for my boat’s battery. The DC voltage reading was over 12 volts, so I assumed that my battery was fully charged and ready to go. Next I tried to start the boat and heard the engine starter kick in and crank the engine. But, after a few cranks all I could hear was the classic clicking noise an engine starter makes when a battery is dead. How can I get a voltage reading of over 12 volts and end up with a dead battery? Is my panel board meter lying to me?

Open Circuit Voltage Testing

Here we are checking the “open circuit voltage” across the battery’s positive and negative posts. The voltage reading may be inaccurate until a load, such as your boat’s engine starter motor, is activated and the voltage reading maintains itself above about 10.5 volts minimum.

Answer: Your meter may not be lying at all; it may be reading what we refer to as a “surface charge” in your battery. The term is one that confuses most people because they really don’t have a very good understanding of what goes on inside a battery as it charges and discharges.

The process of bringing a battery up to full charge is an electro-chemical reaction. The idea is to turn as much as possible of the lead sulfate, which form the plates of a discharged battery, into lead and lead dioxide, which form the plates of a fully charged battery. This chemical reaction takes a bit of time to occur.

Initially the chemical reaction only takes place on the surface of the battery plates, where they are in direct contact with the electrolyte. It takes some time for this chemical reaction to start to penetrate deep into the plates. This is where the term “surface charge” comes from. Essentially the process of recharging is incomplete, and there’s just enough of a surface charge present to trick your voltmeter. As you put a real load on your battery, like engaging your engine’s starter motor, you break down that surface charge quite rapidly, as you’re asking your battery to deliver a considerable amount of electrical current (amps) quite quickly. It’s just not going to happen; the battery does not have enough of a charge to maintain the electrical load long enough to get your engine started.

So, does this mean your battery is ruined? Not necessarily. It may simply not have been recharged in your absence. Does your boat have a shore power-supplied battery charger? If so, was it turned on when you last left the boat? Or, if it doesn’t have a plug-in charger, maybe the alternator charging system on your boat has failed, leaving the battery flat.

Either way, the first step is to get the battery connected to a good charger ASAP and attempt to get it up to a truly full charge. Then re-test the battery by putting a load on it. A good quick check is to measure the voltage across the battery posts as shown in the photo above, while you are cranking the engine. If the minimum voltage you read is greater than about 10.5 volts, then you are good to go with your battery.

- Ed Sherman