(All articles reproduced in this collection originally appeared in the Techtalk series in the club newsletter Telltale. Titles, authorship, and publication dates are reproduced as originally published.)
by Tom Winlow and Marcel Laroche, Techtalk Series, NSC Telltale, April 2001
Last month, we reviewed basic electricity theory. Now, let’s look at some of the components of a boat’s electrical system.
Batteries
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 sulphuric acid in very specific proportions. The sulphuric acid reacts with the lead plates in the battery to produce electricity. As the chemical reaction goes on, lead sulphate forms on the lead plates, and the amount of sulphuric acid gradually decreases as the battery discharges.
Charging the batteries transforms the lead sulphate back into sulphuric acid, and the mixture (electrolyte) is restored to near its original level.
As the process is repeated many times, lead sulphate gradually builds up on the plates, and the amount of sulphuric acid in the electrolyte decreases. This decreases the ability of the battery to take a full charge and, therefore, to produce electricity.
When a battery is recharged, some of the water (H2O) in the electrolyte is transformed 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 hydrogen 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 battery 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. The device measures the specific gravity of the electrolyte in each of the 6 cells. A low specific gravity indicates that the proportion of water to acid is too high. This means that the battery needs to be charged some more, in order to turn the lead sulphate into sulphuric 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 SULPHATED).
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 (SULPHATED/ DAMAGED PLATES), and battery replacement is probably required. Take it to a battery shop and have them test it.
Batteries are rated in Amp Hours (AH), Cold Cranking Amps (CCA), Marine Cranking Amps (MCA), and Reserve/Minutes (Res./Min). Amp Hours (AH): 100AH means that a battery can deliver a steady 5 amps 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. Cold Cranking Amps (CCA): 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. Marine Cranking Amps (MCA): Is the same as CCA, but is measured at 32F. It is always higher because a warm battery always delivers more power than a cold one. Reserve Minutes (Res./Min): A rating of 180 means that the battery can supply a 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) for 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 sulphuric acid, it is able to supply more amperage than a deep cycle battery. However, since the plates are 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 sulphate 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 mixture is mostly water, while a fully charged battery electrolyte mixture contains more sulphuric 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.
Tom Winlow and Marcel Laroche