FIGURE. Maintenance-free batteries.
In a maintenance-free battery there is no provision for the addition of water to the cells. The battery is sealed. It contains cell plates made of a slightly different compound than what is in a conventional battery. The plate grids contain calcium, cadmium, or strontium to reduce gassing and self-discharge. Gassing is the conversion of the battery water into hydrogen and oxygen gas. This process is also called electrolysis. The antimony used in conventional batteries is not used in maintenance-free batteries because it increases the breakdown of water into hydrogen and oxygen and because of its low resistance to overcharging. The use of calcium, cadmium, or strontium reduces the amount of vaporization that takes place during normal operation. The grid may be constructed with additional supports to increase its strength and to provide a shorter path, with less resistance, for the current to flow to the top tab.
FIGURE. Maintenance-free battery grids with support bars give increased strength and faster electrical delivery.
Each plate is wrapped and sealed on three sides by an envelope design separator. The envelope is made from microporous plastic. By enclosing the plate in an envelope, the plate is insulated and reduces the shedding of the active material from the plate.
FIGURE. Construction of a maintenance-free battery.
The battery is sealed except for a small vent so the electrolyte and vapors cannot escape. An expansion or condensation chamber allows the water to condense and drain back into the cells. Because the water cannot escape from the battery, it is not necessary to add water to the battery on a periodic basis. Containing the vapors also reduces the possibility of corrosion and discharge through the surface because of electrolyte on the surface of the battery. Vapors leave the case only when the pressure inside the battery is greater than atmospheric pressure.
Note: If electrolyte and dirt are allowed to accumulate on the top of the battery case, it may create a conductive connection between the positive and negative terminals, resulting in a constant discharge on the battery.
FIGURE. One cell of a maintenance-free battery has a built-in hydrometer, which gives indication of overall battery condition.
Some maintenance-free batteries have a built-in hydrometer to indicate the state of charge. A hydrometer is a test instrument that is used to check the specific gravity of the electrolyte to determine the battery’s state of charge. If the dot that is at the bottom of the hydrometer is green, then the battery is fully charged (more than 65% charged). If the dot is black, the battery state of charge is low. If the battery does not have a built-in hydrometer, it cannot be tested with a hydrometer because the battery is sealed.
Note: It is important to remember that the built-in hydrometer is only an indication of the state of charge for one of the six cells of the battery and should not be used for testing purposes.
Many manufacturers have revised the maintenance-free battery to a “low maintenance-battery,” in that the caps are removable for testing and electrolyte level checks. Also, the grid construction contains about 3.4% antimony. To decrease the distance and resistance of the path, that current flows in the grid, and to increase its strength, the horizontal and vertical grid bars do not intersect at right angles.
FIGURE. Low-maintenance battery grid with vertical grid bars intersecting at an angle.
Hie advantages of maintenance-free batteries over conventional batteries include:
- A larger reserve of electrolyte above the plates.
- Increased resistance to overcharging.
- Longer shelf life (approximately 18 months).
- Ability to be shipped with electrolyte installed, reducing the possibility of accidents and injury to the technician.
- Higher cold cranking amps rating.
Hie major disadvantages of the maintenance-free battery include:
- Grid growth when the battery is exposed to high temperatures.
- Inability to withstand deep cycling.
- Low reserve capacity.
- Faster discharge by parasitic loads.
- Shorter life expectancy.