Tecktalk – Basic Electricity – Batteries

Apr 2001 Telltale Page 18 - by Tom Winlow and Marcel Laroche.

Last month we reviewed basic electricity theory.

Now let's look at some of the components of a boat electrical system.


A battery is an electrical reservoir, which you draw from when at anchor and that you refill with a charging device.

The wet cell battery (the same as in your car,) contains a mixture of water and sulfuric acid in very specific proportions. The sulfu-ric acid reacts with the lead plates in the battery to produce elec-tricity. As the chemical reaction goes on, lead sulfate forms on the lead plates, and the amount of sulfuric acid gradually de-creases as the battery discharges.

Charging the batteries trans-forms the lead sulfate back into sulfuric acid and the mixture (electrolyte) is restored to near its original level.

As the process is repeated many times, lead sulfate gradually builds up on the plates, and the amount of sulfuric acid in the electrolyte de-creases. This decreases the ability of the battery to take a full charge and therefore to produce electricity.

When a battery is re-charged, some of the water (H2O) in the electrolyte is trans-formed into hydrogen (H2) and oxygen (O). These gases escape through the breather holes. This is why you must add water. There is no such thing as a maintenance free battery! It must also breathe to prevent pressure buildup.

Ideally, a battery should be recharged at 13.75 to 14.25 volts, in order to achieve its fully charged voltage of 12.5 to 12.6 volts. If the charging voltage is higher than 14.25 volts, more water will be transformed into hy-drogen and oxygen, and more frequent topping-up with water will be required. In this case, check the output voltage of your alternator while it is charging the battery. When topping up, add distilled water until the plates are covered up plus about ¼ inch. If the charging voltage is much lower than 13.75 volts, the bat-tery will not reach its' full charge capacity.

If the alternator output is normal, but you find that the battery is always weak, check each cell with a hydrometer ($5.00 item). The device measures the specific gravity of the electrolyte in each of the 6 cells. A low specific gravity indicates that the pro-portion of water to acid is too high. This means that the battery needs to be charged some more, in order to turn the lead sulfate into sulfuric acid, thereby restoring the proper water + acid ratio in the electrolyte. If after a 24-hour charge from a battery charger, the specific gravity is still low, the battery is probably worn out (PLATES ARE SULFATED).

A worn out battery can still indicate 12.5 volts, with no electrical load on it. Turn on 3 or 4 cabin lights for about 15 minutes, then turn them off and re-check the battery voltage. If the battery voltage has gone down to below 12 volts, the battery is probably worn out. If the specific gravity in a cell is low while the others are normal, it indicates a problem with that cell (SULFATED/DAMAGED PLATES), and battery replacement is probably required. Take it to a battery shop and have them test it.

Batteries are rated in AH, CCA, MCA and Res./Min. AH - amp-hours. 100AH means, that it can deliver a steady 5amps for a period of 20 hours (5X20=100AH), or 10 amps for 10 hours before the battery voltage drops to 10.5 Volts. This is a testing method used to compare batteries. CCA - cold cranking amps. The number of amps a battery can deliver for 30 seconds at 0F before the battery voltage drops to 7.2 Volts. This is an automotive rating. MCA - marine cranking amps. Is the same as CCA, but is meas-ured at 32F. It is always higher because a warm battery always delivers more power than a cold one. Res./Min - reserve minutes. A rating of 180 means that the bat-tery can supply steady 25 amps for 180 minutes (3 hours), before the battery voltage drops to 10.5 Volts. This also means that it can supply a load (2 cabin lights + stereo) of 5 amps (1/5 of 25 amps) five times longer, or 15 hours.

The voltage rating is the same for all batteries, whether it is a deep cycle, starting or a car battery. The only thing that varies is the amperage ratings.

A deep cycle battery, generally has a lower AH rating than a starting battery of the same size, but is able to withstand many more discharges than a starting battery. A deep cycle battery has thicker and consequently fewer plates than a cranking battery of the same dimensions. This more robust plate construction allows it to be discharged deeper and more often than a starting battery before the lead in the plates disintegrates.

A starting battery has more but thinner plates. Having more plates, therefore more surface area exposed to the sulfuric acid, it is able to supply more amperage than a deep cycle battery. However, the plates being thinner, they will disintegrate after fewer discharge cycles.

Once a battery is discharged, recharge it as soon as possible. Leaving a battery discharged is very harmful, as the sulfate builds up over time and is harder to remove in the charging cycle. If left discharged over the winter, it may not take a charge in the spring. A discharged battery is more likely to freeze than a fully charged battery, because the electrolyte mix-ture is mostly water, while a fully charged battery electrolyte mixture, contains more sulfuric acid and is less likely to freeze.

The battery is the heart and soul of the electrical system; take good care of it. We will look at generators and alternators next month.