Battery Capacity and Rating

Resistance

The resistance corresponds to the size of the hole controlling the rate of flow of the water coming out the bottom of the jug. A battery’s voltage is directly related to current flow by resistance via the Ohm’s Law equation you met in Chapter 6: V 5 IR

where V is voltage in volts, I is current in amps, and R is resistance in ohms. Actually, there are two resistances: the external resistance of the load (the light bulb in this case) and the internal resistance of the battery. The battery’s internal resistance is important in battery efficiency (heating losses), power transfer, and state-of-charge determinations.

Power

Electrical power is defined as the product of voltage and current:

P 5 VI

where V is voltage in volts, I is current in amps, and P is the power in watts. To use a 100-watt light bulb instead of a 50-watt light bulb requires twice the amount of power from the battery—twice the current at the same battery voltage. If the Ohm’s Law equation is substituted into the previous equation, P 5 I2R

this equation defines the power losses in the resistances in the circuit—either external load or internal battery.

Efficiency

Battery efficiency is

Efficiency 5 Power Out/Power In

The principal battery losses are due to heat. These come from resistance and chemical sources: internal resistance of the battery determines its heating or PR losses when charging and discharging; chemical reaction between the lead and the sulfuric acid produces heat (called an exothermic reaction) during charging; and chemical reaction absorbs heat (called an endothermic reaction) during recharging.

While PR losses are present whether charging or discharging—because they are proportional to the square of current flow—battery heat rise is higher during charging (because PR heating losses add to the internal heat-generating chemical reaction) and lower during discharging (because IR heating losses are balanced by the internal heatabsorbing chemical reaction). Given the PR relationship, charging or discharging at a lower current rate obviously contributes to keeping battery losses lower.

Battery Capacity and Rating

Capacity and rating are the two principal battery-specifying factors. Capacity is the measurement of how much energy the battery can contain, analogous to the amount of water in the jug. Capacity depends on many factors, the most important of which are

• Area or physical size of plates in contact with the electrolyte • Weight and amount of material in plates

• Number of plates, and type of separators between plates • Quantity and specific gravity of electrolyte • Age of battery • Cell condition—sulfation, sediment in bottom, etc. • Temperature • Low voltage limit • Discharge rate

Notice the first four items have to do with the battery’s plates and electrolyte—its construction; the next two items concern its history; and the last three depend on how you are using it at the moment. We’ll get into all the details, but keep in mind that the most truthful thing you can say about battery capacity is: it depends.

Battery capacity is specified in ampere-hours. A battery with a capacity of 100 ampere-hours could in theory deliver either 1 amp for 100 hours or 100 amps for 1 hour. This doesn’t help you any more than would drawing a straight line on a map if someone asked you for a destination. You need the second coordinate, the second factor-rating.

A battery’s rating is the second specifying factor. It refers to the rate at which it can be charged or discharged. It is analogous to how fast the sink will fill up with the water from the jug. In equation form: Battery Rating 5 Capacity/Cycle Time

In this equation, the rating is given in amperes for a capacity in ampere-hours and a cycle time in hours. In practical terms, a battery with a capacity of 100 ampere-hours that can deliver 1 amp for 100 hours (known as a C/100 rate) would not necessarily be able to deliver the much higher 100 amps for 1 hour (known as a C/1 rate). You can only get the water out of the jug so fast.

Requesting 10 amps from a fully charged 100 AH capacity battery reflects a C/10 rate; this same request reflects a much lower C/40 rate from a 400 AH battery. In other words, smaller batteries have to deliver energy faster in relation to their size, or larger batteries have lower discharge rates in relation to their capacity.

Capacity of commercial batteries is standardized by the Battery Council International (BCI) into several usable figures. Two figures, a 20-hour capacity and a reserve capacity, are usually given for every battery depending on its application.

• 20-Hour Capacity—This is a battery’s rated 20-hour discharge rate—its C/20 rate. Every battery is rated to deliver 100 percent of its rated capacity at the

C/20 rate, if discharged in 20 hours or more. If a battery is discharged at a faster rate, it will have a lower ampere-hour capacity. • Minutes at 25 amps Reserve Capacity—This is the number of minutes a fully charged battery can produce a 25-amp current. This is the automotive starting battery rating that tells you how long your starter battery will power your automotive accessories if your fan belt breaks and disconnects the alternator; in other words, how many minutes you have to get to the nearest gas station. • Minutes at 75 amps Reserve Capacity—This is the number of minutes a fully charged battery can produce a 75-amp current. This is the golf cart battery rating, because 72 minutes translates to about the amount of time it takes to