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Calculating charge times
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), Cold-Cranking Amps
(CCA), or both.
BATTERY SIZE/RATING
Motorcycle, garden
SMALL
tractor, etc.
BATTERIES
200 - 315 CCA
CAR/TRUCKS
315 - 550 CCA
550 - 1000 CCA
MARINE/DEEP CYCLE
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 Percent-of-Charge Tester. The following
table will help you convert hydrometer readings to
percent of charge values.
SPECIFIC
PERCENT
GRAVITY
OF CHARGE
1.265
100%
1.225
75%
1.155
25%
1.120
0%
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
NOTE: The Reserve Capacity can be obtained from
the battery specification sheet or the owner's manual.
6 - 12 AH
12 - 32 AH
40 - 60 RC
60 - 85 RC
85 -190 RC
80 RC
140 RC
160 RC
180 RC
PERCENT OF
CHARGE NEEDED
0%
25%
75%
100%
2
• Marine, or deep-cycle batteries are usually rated in
Reserve Capacity (RC).
• NR means that the charger setting is NOT RECOM-
MENDED.
Find your battery's rating on the chart below and note
the charge time given for each charger setting. The
times given are for batteries with a 50-percent charge
prior to recharging. Add more time for severly dis-
charged batteries.
CHARGE RATE/CHARGING TIME - HOURS
2 AMP
2 - 4
4 - 10
11 - 14
14 - 18
18 - 35
18
27
30
33
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
(2/15/50 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 deep-cycle battery.
Example:
Amp Hour Rating x % of charge needed x 1.25 = hours
Charger Setting
100 (AH Rating) x .50 (charge needed) x 1.25 = 4.16666
15 (Charger Setting)
100 x .50 = 3.3333 x 1.25 = 4.16666
15
You would need to charge your 100-Ampere Hour
Battery for a little more than 4 hours at the 15-Amp
charge rate using the above example.
12
15 AMP
50 AMP
NR
NR
NR
NR
90 - 120 min.
30 - 40 min.
120 - 130 min.
40 - 45 min.
2.5 - 4.5
45 - 90 min.
130 min.
NR
3.5
NR
4.0
NR
4.5
NR
of
charge
hours
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