BATTERIES: 

Preventive Maintenance, Charging and Equalization

WARNING: 

* THIS ADVICE DOES NOT APPLY TO SEALED BATTERIES OR ANY BATTERIES OTHER THAN FLOODED (WET) CELL LEAD-ACID BATTERIES. EQUALIZATION INVOLVES OVERCHARGING WHICH CAN PERMANENTLY DAMAGE SEALED BATTERIES.

* EQUALIZATION CHARGING WILL PRODUCE AN ABNORMAL AMOUNT OF GASSING. BATTERY GASSES (HYDROGEN AND OXYGEN) ARE POTENTIALLY EXPLOSIVE. TAKE EXTRA PRECAUTIONS AND KEEP UNAUTHORIZED PERSONNEL AWAY FROM BATTERIES.

* Turn off sensitive loads during equalization. They will be exposed to higher than normal voltage. Most inverters have a high voltage shutoff (see specifications for your inverter.)

This bulletin describes preventive maintenance and recommended charging procedures to maximize battery life. The leading cause of premature battery failure is improper charging and poor battery maintenance. Equalization is very important in alternative energy applications and must be preformed correctly, but only as required.

Preventive Maintenance

When a battery is first received the cell acid levels should be checked and the battery should be put on charge. After removing from charge the specific gravity readings of each cell should be recorded and kept for the life of the battery.

All metal components attached to battery terminals, plus all nuts and bolts, should be coated with a thin layer of petroleum jelly (Vaseline) or other non-hardening sealant BEFORE assembly. This will stop the corrosive effects of acid exposure.

Preventive maintenance involves, at a minimum, checking the cell electrolyte level for correct volume once a month, and equalizing once every six months. The cells should be watered back to the original electrolyte level which is 1/4 to 1/2" below the bottom of the vent well (tube inside the battery cell with slots on each side).

Distilled or de-ionized water is preferred, but local water (not chlorinated) maybe acceptable if it is not "hard" or does not contain high iron levels. Very clean rain water or melted snow can be used, if it is not collected or stored in metal containers.

A recommended preventive maintenance program can be summarized as follows:

1. Water each cell to original level as required
2. Equalize as required, or once every six months
3. Record the specific gravity readings of each cell every three months.

Occasionally cleaning the battery terminals and case / cover is recommended. A weak solution of household baking soda (sodium bicarbonate) and water can be used to neutralize any spilt or spattered acid (100 g per liter or 4 Oz per pint). Make sure the vent caps are securely tightened and NO soda solution gets into the battery cells.

Good record keeping is important. Review of these records can help to determine the "health" of the battery and can prove invaluable if system problems develop.

State of Charge and Charging

The truest measure of a battery's state of charge is the SPECIFIC GRAVITY of the battery acid. The following shows the approximate state of charge at various specific gravities at 77ºF (25ºC). Temperature correction factor is -0.010 V per 10º F (-0.010 V per 5º C).

% Charged Specific Gravity

100%       1.255 - 1.275 

75%        1.215 - 1.235 

50%        1.200 - 1.180 

25%        1.165 - 1.155 

0%         1.130 - 1.110

Hydrometers can be difficult to use and at best accurate to +/-0.005 points. 

We recommend a three step charging procedure. Recommended voltage settings are as follows:

             (Volts per cell)     12V            24V           48V

Equalization    2.58-2.67      15.5-16.0     31.0-32.0     61.9-64.1 

Absorption      2.37-2.45      14.2-14.7     28.4-29.4     56.9-58.8 

Float           2.20-2.23      13.2-13.4     26.4-26.8     52.8-53.5

Equalization

Equalization is required to mix the battery acid and bring every battery plate to an equal charge. Equalization should only be performed when required or once every six months in a renewable energy system. Equalization is required when the cell specific gravities vary from highest to lowest by 0.015 (1.245 - 1.260 at full charge).

After a season (several months) of generally low state of charge (SOC) or failure to reach full SOC frequently, equalization will probably be required. After a season of abundant energy (long durations of full SOC) equalization will generally not be necessary.

The exact particulars (such as charging time and currents) are dependent on the charging system. However, the point is to bring the batteries up to the equalization voltage and continue charging for 1-2 hours at a low current, without excessive heat. The final or finishing charging current should be 3-7% (we recommend 5%) of the 20 hr capacity in amps. If battery temperature exceeds 125°F the battery should be taken off of charge and allowed to cool before equalization is continued. When two consistent specific gravity readings are taken a half hour apart, the battery is equalized.

It is recommended to water the battery cells before or half way through the equalization. This is to assure the water is completely mixed into the electrolyte. Do not fill higher than the recommended level, or overflow may result during equalization because of excessive gassing.

For more information, contact:   

SADHANA ENERGY DEVICES