Important Information On Marine Batteries

It seems that everything electrical on our boats has improved dramatically over recent years. We now have electronic computer controlled engine management systems, advanced electronic navigation and fish finders. We even have LED lights and GPS speedometers! The heart of our boats electrical system, the humble battery has also been changing lately. It used to be all marine batteries were of the simple open vented lead acid type that had been around since the early 1900s. This type of battery requires a regular check of the fluid (known as electrolyte) level in the cells and add distilled water as necessary. You do that don't you?

But times are changing and now we have Sealed Lead Acid, Gel and Absorbent Glass Mat (AGM) batteries. So what does this mean for boat owners? The most common type of advanced marine battery is the Sealed Lead Acid type. These are often called "maintenance free" because their sealed construction means that there is no longer a necessity to check the cells and add distilled water. This is a great feature especially when the battery is difficult to get access to. Because of this these "Maintenance Free" batteries, for example, are excellent for use as a trolling motor battery on a bass boat.

But there is something very important to know about "Maintenance Free" sealed lead acid batteries and that is how you go about testing one. The usual way to test a lead acid battery was to carry out a "high-rate resistance discharge test" using a resistance type battery tester. These simple testers apply a massive resistance of three times the battery cold cranking amps (CCA) rating for 20 seconds and in the case of a 12-volt battery a reading of greater that 9 volts indicated a good battery.


A "resistance type" of battery tester in use. Note the resistance coils glowing red inside the tester!

This type of test can be extremely dangerous on a sealed lead acid battery. If one of the cells is low on electrolyte, below the cell plates, a load that great could cause arcing inside the battery and a possible explosion! This can cause serious injury and sulfuric acid burns to the person carrying out the test! Because of this sealed "maintenance free" batteries must be tested with an electronic conductance tester. This type of tester does not put a load through the battery as a resistance type battery tester does. Instead they send a low level electrical signal through the battery and measures the batteries internal resistance. From this the electronic tester is able to calculate the health of the battery and whether any of the cells has failed. This is a very accurate way of testing any lead acid battery open or sealed and much safer! Because of this Sterling Aero Marine Services always uses an electronic conductance tester to test boat batteries.

One more important thing to know about batteries installed in boats with electronic computer controlled engines, (which is almost every marine engine nowadays! Cranking the engine puts a tremendous amperage load on the battery (this is why the battery cables are so large). There is always a voltage drop when cranking the engine. On older non-computer controlled engines (the type that still used contact breaker points) they would still start with a cranking voltage as low as 9.6volts. Most modern computer controlled engines require a minimum cranking voltage of 10.5 volts. Anything less than this could cause the engine's electronic control unit to work erratically causing a variety of strange problems! The moral here is just because your tired and discharged battery can just about turn over your fuel injected motor it probably can't start it!

Marine Battery

A "electronic conductance" type of battery tester in use. A much safer way of testing batteries!

Marine battery tester

The display of a "electronic conductance" type of battery tester showing a good battery.

Just like in modern computer controlled automobiles the humble battery has become an incredibly important piece of equipment. Make sure yours is in good condition! By today's standards even the excellent "Maintenance Free" batteries are inexpensive enough to be an excellent retrofit in our boats. Sterling Aero Marine Services can carry out all your battery testing, servicing and replacement requirements.

Happy Boating!
Larry Simpkins
Sterling Aero Marine Services