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12
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EEBC500B-INT-Z
13. 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 – motorcycle, garden tractor, etc. –
are usually rated in Ampere hours (Ah).
For example: 6, 12, 32 Ah, etc.
• Batteries in cars and smaller trucks are usually rated
in Reserve Capacity (RC), Cold-Cranking Amps
(CCA), or both.
• Marine or deep-cycle batteries are usually rated in
Reserve Capacity (RC).
• NR means that the charger setting is NOT
RECOMMENDED.
Find your battery 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 severely discharged
batteries.
BATTERY SIZE/RATING
CHARGE RATE/CHARGING TIME
(in hours, unless otherwise stated)
MAINTAIN
(4 AMP)
FAST
(15 AMP)
SMALL BATTERIES Motorcycle, garden tractor, etc.
6-12 Ah
1-2
NR
12-32 Ah
2-5
NR
CAR/TRUCKS
200 - 315 CCA
40-60 RC
5½-7¼
1-1½
315 - 550 CCA
60-85 RC
7¼-9¼
1½-2
550 - 1000 CCA
85-190 RC
9¼-17½
2-3½
MARINE/DEEP-CYCLE
80 RC
8¾
1¾
140 RC
NR
2¾
160 RC
NR
3
180 RC
NR
3¼
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
GRAVITY
PERCENT OF
CHARGE
PERCENT OF CHARGE
NEEDED
1.265
100%
0%
1.225
75%
25%
1.155
25%
75%
1.120
0%
100%
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:
Example:
Amp hour Rating = Reserve Ca 16
2
NOTE:
The reserve Capacity can be obtained from the
battery specification sheet or the owner’s manual.
To calculate time needed for a charge:
• Find the percent charge needed. (A battery at 50%
charge that will be charged to 100% needs another
50% [.50]).
• Multiply the Amp Hour rating by the charge needed
(.50) and divide by the charger setting (4, 15, or
60 amps).
• Multiply the results by 1.25 and you will have the total
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 of charge
Charger Setting
100 (Ah Rating) x .50 (charge needed) x 1.25 = 3.125 hours
20 (Charger Setting)
100 x .50 x1.25 = 3.125
20
You would need to charge your 100-Ampere hour Battery
for a little more than three hours at the 20-Amp charge
rate using the above example.
