Southeast Marine Services :: Technical Information Document Library

Now that you are the proud owner of a new bank of batteries, you need to pay attention to their care. For most people, battery maintenance is an unpleasant job so it is usually ignored, a situation which can have consequences arranging from unpleasant to catastrophic. Your batteries are the heart of your electrical system. Failure to maintain them will assuredly result in disappointing performance. Batteries seldom die of old age, most are murdered.

What do I need? Some basic tools are necessary to keep your batteries performing throughout their expected life.

Specific gravity should be checked once a month for the first three months and then once every three months thereafter. Specific gravity should be recorded; a form can be downloaded at; http://www.semarine.com/store/home.php?cat=230. Forget about hydrometers with floating balls or a swinging needle. Get a certified hydrometer which is long enough to read easily, or better yet an optical refractometer which requires only one drop of electrolyte from each cell, will last a life time or two and will survive dropping. Besides which you don’t have to buy a new pair of pants every time you check your batteries.

The following table provides battery state of charge at various specific gravities.

For every 10 degrees above or below 78 degrees F. add or subtract 0.004 from the specific gravity reading. For example if the hydrometer/refractometer reading is 1270 and the temperature is 68F. 1.270 .004 = 1.266 SG

Fig.2

Since checking specific gravity can be an unpleasant job, there is another way to determine your battery’s condition on a day to day basis. Although voltage is not as accurate a measurement as specific gravity which should be checked quarterly, it is helpful. Note that the values are at open circuit, that is, no load condition which means the battery has been allowed to sit with no load for at least 6 hours.

Since in the real world, open circuit voltage is not a likely condition, how can we determine the condition of the battery when it is being used? Before answering that question a little battery theory is helpful. Batteries are rated according to the service for which they are intended, but all ratings are a formula of discharge in amps over time. For example, starting or cranking batteries are rated in cold crank amps, CCA. House batteries are usually rated according to a 20 hour discharge rate. For example a 100 amp hour battery at the 20 hour rate can be discharged at 5 amps for 20 hours that does not however mean that it can be discharged at 10 amps for 10 hours. At the latter rate the battery will have a capacity of 83 amp hours. In other words, the capacity of a battery is determined by its rate of discharge. Temperature also plays an important role. The 20 hour discharge rate for larger batteries or battery banks can be easily determined.

Capacity of Battery or battery bank in amp hours/100 x 5 = Discharge rate

(600 AH/100)=6 (6x5) =30 amp discharge rate.

The importance of the above is that to use voltage as an indicator of battery condition the 20 hour discharge rate should not be exceeded. Referring back to Fig.2 note that 50% discharge (the desired maximum for longest battery life) at no load is 12.2 volts. As a practical matter if the discharge rate is below the 20 hour maximum (30 amps in our hypothetical 600 amp bank), and the battery reaches 12 volts it is time to recharge. You have not actually reached 50% discharge but you are close.

The accuracy of the meter is important. Since we only have a difference of one volt in a 12 volt battery, 12.7-11.6 volts it is important that the meter read to.01 volts (1/100th of a volt) to provide accuracy in the final digit.

The Shoreline 12-EXB is well suited to this purpose. Analog meters other than expensive expanded scale models are not accurate enough. It is also handy at this stage to install an ammeter to determine discharge rates as discussed above as well as providing monitoring of charge rates. Again the Shoreline 200 AB ammeter is well suited.

Some commonly asked questions:

How high should the electrolyte level be? If your battery has a vertical tube extending down from the top about ¼” below the bottom of the tube. Other wise, depending upon the space available, ½ to ¾ inch above the top of the plates. Never allow the plates to become exposed to air.
Do I have to use distilled water? Only, if you do not want to buy another set of batteries soon. Tap water usually has dissolved minerals that are injurious. However any water is better than no water. Clean rain water is a reasonable substitute. Bottled drinking water is usually not distilled and in fact may be from a source rich in the minerals we don’t want in the water.
How often should I charge the batteries? Batteries left unattended and with out a float charge should be completely charged every 30- 45 days. Batteries left on a float charge should be completely charged every 90 days.
How often should I add water? A battery that uses more than an ounce or two of water per cell every 30-45 days is being over charged, float voltage should be reduced. Hydrocaps, a catalytic recombining cap will reduce watering to once or twice a year assuming charger settings are correct.
Should I equalize my batteries and how often? As a general rule batteries should be equalized when specific gravities vary between cells by 0.015, if they seem sluggish or lacking capacity(this may be also be a sign that the batteries are on their last legs) or if they have been sitting at float voltage for more than 90 days. Equalization s a broadly used term and for a better understanding and more specific information see the Author’s paper at (link)

Keep the batteries clean, batteries self discharge at varying rates depending upon the alloys used in the plate construction. Dirty batteries self discharge more rapidly. Tighten the vent caps and wipe down the tops and terminals of the batteries with the soda solution, a tooth brush will get under the terminals and other hard to reach spots. You will probably observe some gas bubbles, this is carbon dioxide released from the soda. Rinse the soda solution off with clean water and absorb with paper towels or rags. Do not allow the soda solution or rinse water to enter the cells. Clean the batteries at least once a year. Coat the terminals with an anti corrosion compound, you can either buy it or make a life time supply with one pint of linseed oil, one pint of 30 weight motor oil and 6 table spoons of soda. Stir well and apply to the terminals it will be gooey at first but the linseed oil will dry it in a few days to a varnish like substance.

A few safety notes, always wear rubber gloves, long sleeves and eye protection when working with batteries. Keep the area well ventilated and don’t smoke, no open flames and do not turn switches on or off, start pumps or other motors. Keep your battery terminals covered with an insulating material. Battery terminal boots are available at most marine outlets. An inexpensive but effective alternative is a piece of rubber or vinyl hose slit lengthwise and placed over the terminal and attached cable. Another cheap way is a square of old inner tube rubber plastic wire tied over the terminal. Enforcement officials have been known to issue citations for uncovered battery terminals. Be especially careful using wrenches around battery terminals, the Author dips his wrenches in vinyl except for the end actually used on the bolts. You can also insulate your wrenches with rubber electrical or silicon tape. Do not attach a multitude of cables to your battery terminals, one cable with a class T fuse of appropriate value installed within 7” of the battery terminal, should be connected to the positive battery terminal and thence to a busbar of appropriate amperage. All other load, charge and sensing connections should be at the busbar. The negative cable should be the same but no fuse is necessary.

The above procedures will go along way towards preventing battery problems and early failure.

The Author is an internationally recognized authority on battery chemistry and physics.

Battery Maintenance Guide.