03/03/97
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BATTERY CAPACITY TESTING
Your Link 20 can be used to conduct periodic capacity tests that tell you the actual
amount of energy your batteries can store. A capacity test should start with a battery that
has been properly charged and equalized. The objective is to find the maximum available
capacity.
Many times, deep cycle battery capacity is stated as a 20 hour discharge rate. A
100 A hr battery will provide 5 amps for 20 hours. At discharge rates above 5 amps, the
battery will not supply 100 A hr. For example: If you are drawing 100 amps out of the
battery it will last less than half an hour. Consider the following table:
CAPACITY AT VARIOUS DISCHARGE RATES
(As a percent of 20 hour rate)
Hours to Discharge Capacity (percent of rating)
20 100%
10 84%
5 67%
3 56%
1 47%
To test battery capacity, reset Amp-hours to zero. Then turn on a load that draws
approximately 5% of the expected battery capacity. Measure the current with Amps
display. The load should be constant, such as incandescent lighting. Now put the Link
20 in the Volts display mode. When the battery voltage drops to 10.5 volts (or 21 volts
if you're testing a 24V system), hopefully about 20 hours later, turn off the load and look
at the A hrs display on your Link 20. The A hrs displayed is your actual battery capacity.
If less than 20 hours passed before the battery voltage fell to 10.5V you can still
determine the capacity with some arithmetic. For example: Assume a 12V battery rated
at 100 A hr. Apply a 5 amp load. Suppose it only took 10 hours for the voltage to reach
10.50 volts. The Link 20 would display -50 A hr. This is the 10 hour capacity. Dividing
50 by 84% (10 hour rate) from the table above, you determine that the actual 20 hour
capacity is about 60 Amp-hours. You could repeat the test at 5% of the tested capacity (3A)
to verify the actual capacity.
USING YOUR INVERTER
TO TEST BATTERY CAPACITY
Your inverter might be the type which makes testing battery capacity easy. First,
fully charge (Equalize if necessary) the battery to be tested. Use a load like incandescent
lights running on the inverter whose amperage consumption is 5% or less of the battery
capacity. Many inverters will shut down on low voltage below about 10.2V. When the
inverter shuts down, read the number of Amp-hours that have been consumed from the
battery. If it is not close to the expected number use the procedure outlined above to
estimate the capacity.
CAUTION!
Be sure to completely recharge your battery after a discharge test.
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