Battery Capacity

The capacity of a battery is measured in ampere- hours. The ampere-hour capacity is equal to the product of the current in amperes and the time in hours during which the battery is supplying current. The ampere-hour capacity varies inversely with the discharge current. The size of a cell is determined generally by its ampere-hour capacity. The capacity of a cell depends upon many factors, the most important of which are as follows:

1. The area of the plates in contact with the electrolyte

2. The quantity and specific gravity of the electrolyte

3. The type of separators

4. The general condition of the battery (degree of sulfating, plates buckled, separators warped, sediment in bottom of cells, etc.)

5. The final limiting voltage

Battery Ratings

Battery ratings were developed by the Society of Automotive Engineers (SAE) and the Battery Council International (BCI). They are set according to national test standards for battery performance. They let the mechanic compare the cranking power of one battery to another. The two methods of rating lead-acid storage batteries are the cold-cranking rating and the reserve capacity rating.

COLD-CRANKING RATING. - The cold- cranking rating determines how much current in amperes the battery can deliver for thirty seconds at 0F while maintaining terminal voltage of 7.2 volts or 1.2 volts per cell. This rating indicates the ability of the battery to crank a specific engine (based on starter current draw) at a specified temperature.

For example, one manufacturer recommends a battery with 305 cold-cranking amps for a small four- cylinder engine but a 450 cold-cranking amp battery for a larger V-8 engine. A more powerful battery is needed to handle the heavier starter current draw of the larger engine.

RESERVE CAPACITY RATING. - The reserve capacity rating is the time needed to lower battery terminal voltage below 10.2 V (1.7 V per cell) at a discharge rate of 25 amps. This is with the battery fully charged and at 80F. Reserve capacity will appear on the battery as a time interval in minutes.

For example, if a battery is rated at 90 minutes and the charging system fails, the operator has approximately 90 minutes (1 1/2 hours) ofdriving time under minimum electrical load before the battery goes completely dead.

Battery Charging

Under normal conditions, a hydrometer reading below 1.240 specific gravity at 80F is a warning signal that the battery should be removed and charged. Except in extremely warm climates, never allow the specific gravity to drop below 1.225 in tropical climates. This reading indicates a fully charged battery.

When a rundown battery is brought into the shop, you should recharge it immediately. There are several methods for charging batteries; only direct current is used with each method. If only alternating current is available, a rectifier or motor generator must be used to convert to direct current. The two principal methods of charging are (1) constant current and (2) constant voltage (constant potential). Constant current charging is be used on a single battery or a number of batteries in series. Constant voltage charging is used with batteries connected in parallel. (A parallel circuit has more than one path between the two source terminals; a series circuit is a one-path circuit). You should know both methods, although the latter is most often used.

CONSTANT CURRENT CHARGING. - With the constant current method, the battery is connected to a charging device that supplies a steady flow of current. The charging device has a rectifier (a gas- filled bulb or a series of chemical disks); thus, the alternating current is changed into direct current. A rheostat (resistor for regulating current) of some kind is usually built into the charger so that you can adjust the amount of current flow to the battery. Once the rheostat is set, the amount of current remains constant. The usual charging rate is 1 amp per positive cell. Thus a 21-plate battery (which has 10 positive plates per cell) should have a charging rate no greater than 10 amps. When using this method of charging a battery, you should check the battery frequently, particularly near the end of the charging period. When the battery is gassing freely and the specific gravity remains constant for 2 hours, you can assume that the battery will take no more charge. 2-5