Calculating charge time
The Chart Method
Use the following table to more accurately determine the time it will take to bring a battery to full charge. First, identify where your battery fits into the chart.
•Small batteries — motorcycles, garden tractors, etc.
— are usually rated in Ampere Hours (AH). For example; 6 to 12 AH, or 12 to 32 AH.
•Batteries in cars and smaller trucks are usually rated in Reserve Capacity (RC),
•Marine, or
•NR means that the charger setting is NOT RECOMMENDED.
Find your battery’s rating on the following chart and note the charge time given for each charger setting. The times given are for batteries with a
| BATTERY SIZE/RATING |
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| CHARGE RATE/CHARGING TIME - HOURS | |||||
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| 2 AMP | 15 AMP | 50 AMP | ||||
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SMALL |
| Motorcycle, garden | 6 | - 12 AH | 2 | - 4 | NR | NR | |
BATTERIES |
| tractor, etc. | 12 | - 32 AH | 4 - 10 | NR | NR | ||
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| 200 | - 315 CCA | 40 | - 60 RC | 11 | - 14 | 1½ - 2 | 1/2 - 5/8 |
CAR/TRUCKS |
| 315 | - 550 CCA | 60 | - 85 RC | 14 | - 18 | 2 - 2½ | 5/8 - ¾ |
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| 550 | - 1000 CCA | 85 | 18 | - 35 | 2½ - 4½ | ¾ - 1½ | |
MARINE/DEEP CYCLE |
| 80 RC | 18 | 2½ | NR | ||||
| 140 RC | 27 | 3½ | NR | |||||
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| 160 RC | 30 | 4 | NR | ||
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| 180 RC | 33 | 4½ | NR | ||
The Hydrometer or Electronic Method
To find the time needed to fully charge your battery, determine the battery’s charge level with a hydrometer or electronic
SPECIFIC | PERCENT | PERCENT OF |
GRAVITY | OF CHARGE | CHARGE NEEDED |
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1.265 | 100% | 0% |
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1.225 | 75% | 25% |
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1.155 | 25% | 75% |
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1.120 | 0% | 100% |
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When you know the percent of charge and the Amp Hour (AH) rating of your battery, you can calculate the approximate time needed to bring your battery to a full charge.
To convert Reserve Capacity to Amp Hours, divide Reserve Capacity by 2, and add 16:
Amp Hours = Reserve Capacity + 16
2
To calculate time needed for a charge:
•Find the percent of charge needed. (A battery at 50 percent charge that will be charged to 100 percent needs another 50 percent (.50).
•Multiply the Amp Hour rating by the charge needed (.50) and divide by the charger setting (2, 20 or 60 amps).
•Multiply the result by 1.25 and you’ll have the time needed, in hours, to bring the battery to full charge.
•Add one additional hour for a
Example:
Amp Hour Rating x % of charge needed x 1.25 = hours
Charger Settingof charge needed
100 (AH Rating) x .50 (charge needed) x 1.25 = 4.1625
15 (Charger Setting) | hours |
100 x .50 = 2.5 x 1.25 = 3.125
15
You would need to charge your
12
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