Battery Basics

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Do you use an electric starter to get your boat motor running? Well so do I, and trust me, that means your boat battery is your BEST friend when you’re out on the water. Without it, you won’t leave the dock side or launch ramp and perhaps more importantly, if you shut the motor down, without it, you won’t be coming back anytime soon.


So that means it’s probably worth your time and effort to keep your marine battery (or batteries) in tip top shape. Batteries require periodic servicing, so a definite maintenance program will help ensure extended life. A failure to maintain the battery in good order can prevent it from properly charging or properly performing its job even when fully charged. Low levels of electrolyte in the cells, loose or dirty cable connections at the battery terminals or possibly an excessively dirty battery top can all contribute to an improperly functioning battery. So battery maintenance, first and foremost, involves keeping the battery full of electrolyte, properly charged and keeping the casing/connections clean of corrosion or debris.

If a battery charges and tests satisfactorily but still fails to perform properly in service, one of three problems could be the cause.

  • Leaving an accessory on overnight or for a long period of time can discharge a battery.

  • Using more electrical power than the alternator/regulator, stator assembly and/or lighting coil can replace will slowly drain the battery during motor operation, resulting in an undercharged condition.

  • A defect in the charging system will allow the battery to discharge without replacing the lost voltage. A faulty stator assembly or lighting coil, defective regulator or rectifier or high resistance somewhere in the system could cause the battery to become undercharged.


There are a couple of things you need to know about batteries. First, they store electricity and pack quite a punch. Never allow objects to short between the 2 battery terminals as the sudden discharge of voltage can be enough to weld tools together. Second, they are filled with acid and non-sealed batteries frequently bubble, froth, leak or otherwise sweat a small amount of that acid, so watch you hands, face and clothing around them. Lastly, when discharging the electrolyte gives off HIGHLY explosive hydrogen gas. So ALWAYS avoid sources of ignition such as flame, sparks or cigarettes around a battery, especially one that is being charged.


The most common and important procedure in battery maintenance is checking the electrolyte level. On most batteries, this is accomplished by removing the cell caps and visually observing the level in the cells. The bottom of each cell normally is equipped with a split vent which will cause the surface of the electrolyte to appear distorted when it makes contact. When the distortion first appears at the bottom of the split vent, the electrolyte level is correct. Smaller marine batteries are sometimes equipped with translucent cases that are printed or embossed with high and low level markings on the side. On some of these, shining a flashlight through the battery case will help make it easier to determine the electrolyte level.

During hot weather and periods of heavy use, the electrolyte level should be checked more often than during normal operation. Add distilled water to bring the level of electrolyte in each cell to the proper level. Take care not to overfill, because adding an excessive amount of water will cause loss of electrolyte and any loss will result in poor performance, shortened battery life and will contribute quickly to corrosion.

HERE’S A TIP, never add electrolyte from another battery. Use only distilled water. Even tap water may contain minerals or additives that will promote corrosion on the battery plates, so distilled water is always the best solution.

Although less common in marine applications than some other uses today, many sealed maintenance-free batteries also require electrolyte level checks, through the window built into the tops of the cases. The problem for marine applications is the tendency for deep cycle use to cause electrolyte evaporation and electrolyte cannot be replenished in a sealed battery. It should be noted though, that more and more companies are producing maintenance-free batteries for marine applications and they are having some success.

The second most important procedure in battery maintenance is periodically cleaning the battery terminals and case. Dirt and corrosion should be cleaned from the battery as soon as it is discovered. Any accumulation of acid film or dirt will permit a small amount of current to flow between the terminals. Such a current flow will drain the battery over a period of time.

Clean the exterior of the battery with a solution of diluted ammonia or a paste made from baking soda and water. This is a base solution that will neutralize any acid that may be present. Flush the cleaning solution off with plenty of clean water.

ANOTHER TIP: Take care to prevent any of the neutralizing solution from entering the cells as it will quickly neutralize the electrolyte (ruining the battery).

Poor contact at the terminals will add resistance to the charging circuit. This resistance may cause the voltage regulator to register a fully charged battery and thus cut down on the stator assembly or lighting coil output adding to the low battery charge problem.

At least once a season, the battery terminals and cable clamps should be cleaned. Loosen the clamps and remove the cables, negative cable first. On batteries with top mounted posts, if the terminals appear stuck, use a puller specially made for this purpose to ensure the battery casing is not damaged. NEVER pry a terminal off a battery post. Battery terminal pullers are inexpensive and available in most parts stores.

Clean the cable clamps and the battery terminal with a wire brush until all corrosion, grease, etc., is removed and the metal is shiny. It is especially important to clean the inside of the clamp thoroughly (a wire brush or the brush part of a battery post cleaning tool is useful here), since a small deposit of foreign material or oxidation there will prevent a sound electrical connection and inhibit either starting or charging. It is also a good idea to apply some dielectric grease to the terminal, as this will aid in the prevention of corrosion.

After the clamps and terminals are clean, reinstall the cables (negative cable last); do not hammer the clamps onto battery posts. Tighten the clamps securely but do not distort them. To help slow or prevent corrosion, give the clamps and terminals a thin external coating of grease after installation.

Check the cables at the same time that the terminals are cleaned. If the insulation is cracked or broken or if its end is frayed, that cable should be replaced with a new one of the same length and gauge.


A quick check of the battery is to place a voltmeter across the terminals. Although this is by no means a clear indication, it gives you a starting point when trying to troubleshoot an electrical problem that could be battery related. Most marine batteries will be of the 12 volt DC variety. They are constructed of 6 cells, each of which is capable of producing slightly more than two volts, wired in series so that total voltage is 12 and a fraction. A fully charged battery will normally show more than 12 and slightly less than 13 volts across its terminals. But keep in mind that just because a battery reads 12.6 or 12.7 volts does NOT mean it is fully charged. It is possible for it to have only a surface charge with very little amperage behind it to maintain that voltage rating for long under load. A discharged battery will read some value less than 12 volts, but can normally be brought back to 12 volts through recharging. Of course a battery with one or more shorted or un-chargeable cells will also read less than 12, but it cannot be brought back to 12+ volts after charging. For this reason, the best method to check battery condition on most marine batteries is through a specific gravity check or a load test.

A hydrometer is a device that measures the density of a liquid when compared to water (specific gravity). Hydrometers are used to test batteries by measuring the percentage of sulfuric acid in the battery electrolyte in terms of specific gravity. When the condition of the battery drops from fully charged to discharged, the acid is converted to water as electrons leave the solution and enter the plates, causing the specific gravity of the electrolyte to drop.

It may not be common knowledge, but hydrometer floats are normally calibrated for use at 80°F (27°C). If the hydrometer is used at any other temperature, hotter or colder, a correction factor must be applied. Also, remember, a liquid will expand if it is heated and will contract if cooled. Such expansion and contraction will cause a definite change in the specific gravity of the liquid; in this case the electrolyte.

A quality hydrometer will have a thermometer/temperature correction table. By measuring the air temperature around the battery and from the table, a correction factor may be applied to the specific gravity reading of the hydrometer float. In this manner, an accurate determination may be made as to the condition of the battery.

When using a hydrometer, pay careful attention to the following points:

  • Never attempt to take a reading immediately after adding water to the battery. Allow at least 1/4 hour of charging at a high rate to thoroughly mix the electrolyte with the new water. This time will also allow for the necessary gases to be created.

  • Always be sure the hydrometer is clean inside and out as a precaution against contaminating the electrolyte.

  • If a thermometer is an integral part of the hydrometer, draw liquid into it several times to ensure the correct temperature before taking a reading.

  • Be sure to hold the hydrometer vertically and suck up liquid only until the float is free and floating.

  • Always hold the hydrometer at eye level and take the reading at the surface of the liquid with the float free and floating.

  • Disregard the slight curvature appearing where the liquid rises against the float stem. This phenomenon is due to surface tension.

  • Do not drop any of the battery fluid on the boat or on your clothing, because it is extremely caustic. Use water and baking soda to neutralize any battery liquid that does accidentally drop.

After drawing electrolyte from the battery cell until the float is barely free, note the level of the liquid inside the hydrometer. If the level is within the charged (usually green) band range for all cells, the condition of the battery is satisfactory. If the level is within the discharged (usually white) band for all cells, the battery is in fair condition.

If the level is within the green or white band for all cells except one, which registers in the red, the cell is shorted internally. No amount of charging will bring the battery back to satisfactory condition.

If the level in all cells is about the same, even if it falls in the red band, the battery may be recharged and returned to service. If the level fails to rise above the red band after charging, the only solution is to replace the battery.

An alternate and even BETTER way of testing a battery is to perform a load test using a special Carbon-Pile Load Tester. These days most automotive and many marine parts stores contain a tester and will perform the check for free. Essentially, a load test involves placing a specified load (current drain/draw) on a fully-charged battery and checking to see how it performs/recovers. This is the only way to test the condition of a sealed maintenance-free battery.


If the boat is to be laid up (placed into storage) for the winter or anytime it is not going to be used for more than a few weeks, special attention must be given to the battery. This is necessary to prevent complete discharge and/or possible damage to the terminals and wiring. Before putting the boat in storage, disconnect and remove the batteries. Clean them thoroughly of any dirt or corrosion and then charge them to full specific gravity readings. After they are fully charged, store them in a clean cool dry place where they will not be damaged or knocked over, preferably on a couple blocks of wood. Storing the battery up off the deck, will permit air to circulate freely around and under the battery and will help to prevent condensation.

Never store the battery with anything on top of it or cover the battery in such a manner as to prevent air from circulating around the filler caps. All batteries, both new and old, will discharge during periods of storage, more so if they are hot than if they remain cool. Therefore, the electrolyte level and the specific gravity should be checked at regular intervals. A drop in the specific gravity reading is cause to charge them back to a full reading.

In cold climates, care should be exercised in selecting the battery storage area. A fully-charged battery will freeze at about 60°F below zero. The electrolyte of a discharged battery, almost dead, will begin forming ice at about 19°F above zero.

Fig 1. A large part of battery care is making sure the connections remain clean and tight. Also, be sure the battery is properly secured when the boat is underway

Fig 2. Battery terminal cleaning tools are available to clean battery posts

Fig 3. Battery tools also contain a brush for cleaning the large battery cable ends

Fig 4. A handy way of testing the battery is using a hydrometer to read electrolyte specific gravity

Fig 5. Small marine batteries will require a hydrometer which uses a smaller electrolyte sample

Fig 6. ONLY add distilled water to a battery once it is in service

Fig 7. Be sure to properly charge and maintain your battery anytime the boat will not be used for more than a few weeks

This product was added to our catalog on Monday 20 November, 2006.


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