Battery Charge

This article provides information on your battery, how to care for it, and how to measure the state of its charge

Boat Batteries

Most boats are powered by some form of storage battery. The lead-acid battery is most common, although there are many more exotic and more expensive options available. The principal load on the battery is operating the electric motor which will start the engine. Once started, the engine generally supplies sufficient current to handle the load demand, as well as maintain and restore the battery charge. On most trailerable boats the electrical demand from instruments, radio, depth sounders, fish finders, etc., is much lower than the starting load. Thus, the typical installation will find a lead-acid "starting" battery, designed to provide very high current for short periods, as opposed to a battery designed to have the capacity for long, deep discharge cycles. Batteries designated for marine applications are often ruggedized somewhat because of the harsher ride they experience. They generally have different style connections than automotive batteries, and often have handles attached to permit their easy removal.

Battery Chemistry 101

The chemical reaction in a lead-acid battery which produces the voltage difference between the two terminals in each cell has the wonderful property of being able to be driven in the opposite direction by the application of a voltage greater than the cell's potential. This causes current to flow into the battery, resulting in an increase in the acidity of the electrolyte and thus the storing of electrical energy. The greater the charge in a lead-acid battery, the greater the concentration of the acid in the electrolyte. As the battery becomes more discharged, the electrolyte loses its acidity and approaches a neutral pH. The specific gravity of the electrolyte in a lead-acid battery when fully charged will be 1.265.

The typical marine lead-acid battery has a terminal voltage of "12" volts. The battery actually consists of six cells wired in series, each cell capable of producing 2.15 volts, creating an aggregate voltge of 12.90 volts. When a cell is approximately half discharged its voltage will be 2.03 volts, and the battery terminal voltage will be down to 12.36 volts. When the cells are almost completely discharged their voltage will be 1.75 volts or less, and the battery terminals will be 10.5 volts or less.

Measuring Charge Condition

There are two indices for measuring the condition of the charge on a lead-acid battery: the terminal voltage or the specific gravity of the electrolyte. Some batteries are completely sealed and thus it is impossible to use the specific gravity method. With the advent of low-cost and accurate digital voltmeters, measuring the terminal voltage may be the easiest and best indicator of a battery's charge.

The table below shows the percentage of discharge (from full charge) as a function of the battery terminal voltage. The exact voltage varies as a function of the temperature of the electrolyte. These values are for a nominal room temperature and a typical lead-acid six-cell battery.

Percent Discharge Terminal Voltage
1 % 12.9
10 % 12.8
20 % 12.65
30% 12.5
40% 12.35
50% 12.2
60% 12.0
70% 11.9
80% 11.7
90% 11.4
100% 10.5


Winter Storage

In northern climates many boats are stored over the winter and not used. During this down time the condition of the batteries on board should be checked and maintained.

During the winter storage period my boats are located off-premises in unheated storage sheds, and thus my personal practice is to remove the batteries from the boat and bring them home and indoors. There are several reasons for this:

I store them at room temperature on the floor of the "boat" closet. There is an old superstition that storing batteries on a concrete floor causes them to self discharge. I keep a layer of carpet between the batteries and the concrete. It keeps the batteries cleaner, and who knows about that self-discharging myth.

When a battery is to be stored and not used for a long period, it is wise to first charge it to near 100% capacity. All batteries will self-discharge at some rate. A battery stored in a highly discharged state may self-discharge to a near zero-charge state over the course of the winter. It is better to begin with a well-charged battery in the fall and end up with a usable battery in the spring.

Before charging the battery, I check the level of the electrolyte, adding distilled water as needed to bring each cell to full. When water is added the state of charge is always reduced, so add the water first, before charging begins.

Before putting them away for the long winter, each battery was briefly charged using a standard automotive style charger. These work well with lead-acid batteries. If you are using a more exotic battery, you may need to use a more sophisticated charger.

For charging I prefer to use the lowest setting possible. On my charger this produces about 2 amperes of charging current or less. I let the battery slowly build a charge with this small current. This slows the chemical reaction rate, reducing the amount of heat built up and the rate of gas released. There may also be additional chemical and mechanical reasons why a slow charge cycle is preferred.

Anecdotal Data

This fall I removed the three batteries from my boats, briefly charged them, noted their voltage, and put them in the closet. Four and a half months later, I checked their voltages. Here are the results:

Voltage Measurements
Date Volts Discharge Volts Discharge Volts Discharge
10/21/00 12.48 Vdc 35 % 12.42 Vdc 38 % 12.65 Vdc 20 %
3/4/01 12.33 Vdc 45 % 12.32 Vdc 45 % 12.50 Vdc 30 %

The measurements of voltage were made with a Fluke Digital Multimeter having an accuracy of about 3%. All measurements were made with an open circuit on the battery, that is, no load other than the tiny drain of the meter itself. The batteries appear to have self-discharged only approximately ten percent over the winter.

Spring Preparation

After enduring the long winter, I dragged the batteries out and took the measurements shown above. I also checked the fluid levels, and all three showed very little sign of evaportion or other fluid loss. Then I charged each battery individually at the 2-Amp rate for as long as 24 hours. With my simple charger the current tapers as the voltage rises, so most of the charge was at a lower than 2-Amp rate. After a few days moving the charger among the three batteries, I had their terminal voltages over 12.8 volts, indicating only a slight discharge from theoretical maximum charge. When measuring the battery terminal voltage, there must be a brief rest period between the removal of the charging voltage and the measurement of the battery voltage. Immediately after charging the terminal voltage will be as high as 13.2 volts (or more), but it will quickly reach an equilibrium at the new charge state. Allow about 20 minutes for this (lower) voltage to appear; it is the true indicator of the charge.

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Copyright © 1999, 2000 by James W. Hebert. Unauthorized reproduction prohibited!

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Author: James W. Hebert
This article first appeared March 11, 2001.