If we have a 16 cell battery, each cell of which is an 80 ampere hour cell, the ampere hour capacity of the entire battery will be 80, the same as that of one of its cells, since the cells are all in series and the same current passes through all cells. The watt hour capacity of the battery will be 32 times 80, or 2560. The ampere hour capacity is computed for the 8 hour rate, that is, the current is drawn from the battery continuously for 8 hours, and at the end of that time the battery is discharged. If the current is not drawn from the battery continuously for 8 hours, but is used for shorter intervals intermittently, the ampere hour capacity of the battery will be somewhat greater. It seldom occurs that in any installation the battery is used continuously for eight hours at a rate which will discharge it in that time, and hence a greater capacity is obtained from the battery. Some manufacturers do not rate their batteries at the 8 hour continuous discharge rate but use the intermittent rate, thus rating a battery 30 to 40 percent higher. Rated in this way, a battery of 16 cells rated at 80 ampere hours at the 8 hour rate would be rated at 112 ampere hours, or 3584 watt hours.
In determining the size of the battery required, estimate as nearly as possible how .many lamps, motors, and heaters, etc., will be used. Compute the watts (volts X amperes), required by each, Estimate how long each appliance will be used each day, and thus obtain the total watt hours used per day. Multiply this by 7 to get the watt hours per week. The total watt hours required in one week should not be equal to more than twice the watt hour capacity of the battery (ampere hours multiplied by the total battery voltage) at the eight hour rate. This means that the battery should not require a charge oftener than two times a week.
The capacity of a battery is often measured in the number of lamps it will burn brightly for eight hours. The watts consumed by motors, heaters, etc., may be expressed in a certain number of lamps. The following table will be of assistance in determining the size of the battery required:
| No. | Type of Appliance | Watts Consumed | Equivalent Number of 20 Watt Lamps |
|---|---|---|---|
| 1 | 16 candle power, Mazda lamp | 20 | 1 |
| 2 | 12 candle power, Mazda lamp | 115 | 3/4 |
| 3 | Electric Fan, small size | 75 | 4 |
| 4 | Small Sewing machine motor | 100 | 5 |
| 5 | Vacuum cleaner | 160 | 8 |
| 6 | Washing machine | 200 | 10 |
| 7 | Churn, 1/6 h.p. | 200 | 10 |
| 8 | Cream Separator, 1/6 h.p. | 200 | 10 |
| 9 | Water pump 1/6 h.p. | 200 | 10 |
| 10 | Electric water heater, small | 350 | 18 |
| 11 | Electric toaster | 525 | 26 |
| 12 | Electric stove, small | 600 | 30 |
| 13 | Electric iron | 600 | 30 |
| 14 | Pump, 1/2 h.p. | 600 | 30 |
From the foregoing table we can determine the current consumption of the various appliances:
| No. | Watts | Amps at 32 Volts | Amps at 110 Volts |
|---|---|---|---|
| 1 | 20 | 0.625 | 0.18 |
| 2 | 15 | 0.47 | 0.14 |
| 3 | 75 | 2.34 | 6.80 |
| 4 | 100 | 3.125 | 0.90 |
| 5 | 160 | 5.00 | 1.44 |
| 6 | 200 | 6.25 | 1.80 |
| 7 | 200 | 6.25 | 1.80 |
| 8 | 200 | 6.25 | 1.80 |
| 9 | 200 | 6.25 | 1.80 |
| 10 | 350 | 11.00 | 3.20 |
| 11 | 525 | 16.4 | 4.77 |
| 12 | 600 | 18.75 | 5.40 |
| 13 | 600 | 18.75 | 5.40 |
| 14 | 600 | 18.75 | 5.40 |
The following tables show how long the battery will carry various currents continuously:
