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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 Ah battery will provide 5 amps for 20 hours. At discharge rates above 5 amps, the
battery will not supply 100 Ah. 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 24 V system), hopefully about 20 hours later, turn off the load and look at the amp-hours
display on your Link 20. The amp hours displayed is your actual battery capacity.
If less than 20 hours passed before the battery voltage fell to 10.5 V you can still
determine the capacity with some arithmetic. For example: Assume a 12 V battery rated at
100 Ah. Apply a 5-amp load. Suppose it only took 10 hours for the voltage to reach
10.5 volts. The Link 20 would display -50 Ah. 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 (3 A) 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 10.2 V. 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.