More and more wind and solar installations are incorporating battery banks as a means of letting the wind or solar generators maintain a level output to the grid in the event of temporary interruptions such as passing clouds or a sudden lull in the wind. Unfortunately, today’s best batteries still have a finite life expectancy. You can anticipate about five years of life, after which performance drops because of internal electrical leakage and/or series impedance. But with good maintenance procedures, battery life can be extended significantly, reducing the cost of ownership.
All batteries lose charge over a period of time because of an inevitable internal parallel conductance. And batteries are inherently inefficient. The larger the battery bank, the greater the loss for a given time period. For this reason, the battery bank should be carefully sized to the end use.
Most wind and solar electrical generation systems use lead-acid batteries, other types being more costly. In these, the electrolyte is a mixture of sulfuric acid and water. As the charge increases, the electrolyte becomes more acidic. As the battery discharges, the electrolyte becomes less acidic, as if it had been diluted with water.
The ideal float voltage is the lowest voltage setting that will maintain the battery at full charge. The higher the voltage, the more water the cell will consume. Electrolyte levels should be carefully monitored. If the electrolyte level goes down, there is less electrical storage capacity. More important, the top area of the plates is exposed to a mixture of moist air and acid fumes, causing them to rapidly oxidize. They swell, reducing the space between them and decreasing internal parallel impedance, which equates to less ability to hold a charge.
When a lead acid battery is left in a discharged state for an extended period of time, the plates become coated with a sulfur compound. This process is known as sulfation and as a result the battery will not take a charge. A slow dc charge over an extended period will sometimes remove the sulfur coating, giving the battery new life. Also, lightly tapping on the outside of the case will sometimes cause the coating to fall off the plates, accumulating harmlessly at the bottom of the battery.
The amount of charge and condition of the battery cannot be directly measured by means of a multimeter. To check the condition of a battery, first make sure it is fully charged. Then briefly connect a heavy load and see if the voltage drops too much.
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.
Many battery makers recommend reventive maintenance that includes checking the cell electrolyte level for correct acid volume monthly and equalizing once every six months. Cells should be watered back to the original acid level, usually 0.25 to 0.5-in. from the bottom of the vent well (tube inside the battery cell with slots on each side). Distilled water is preferred but local water (not chlorinated) maybe OK if it is not “hard” or does not contain a lot of iron. Use of non-distilled water can cause mineral build-up in the battery cell.
A hydrometer is used to measure the amount of charge in a battery. A rubber squeeze bulb draws out a sample of electrolyte so that its specific gravity can be measured. A gauge is calibrated to indicate percentage of charge. The truest measure of a battery’s state of charge is the specific gravity of the battery acid. That said, hydrometers can be difficult to use and are at best accurate to ±0.005 points.
Periodically, batteries should be given what is known as an equalization charge. This is generally 10% higher than the charging voltage and it is applied for eight hours. The minimum equalization voltage is highly recommended unless a sulfation problem is suspected and a corrective equalization is necessary. Equalization results in a longer battery life by intentionally creating bubbles at the plates, which reduces sulfation and mixes the electrolyte, preventing layering, a condition where there is a more acidic mix near the top, making for shortened battery life.
Equalization charging is not needed for moving vehicles, where bumps in the road prevent layering and control sulfation.
Finally, it is good practice to cccasionally cleane the battery terminals and case. A weak solution of baking soda and water can neutralize any spilled acid (100 g per liter or 4 oz per pint). But vent caps should be securely tight: No soda solution should get into the battery cells.