How to do a basic battery capacity test using simple tools.
I am about to do a 3 week trip into the Simpson Desert and will capacity test my batteries before the trip to ensure I have enough power to run the fridge, inverter and lighting.
Batteries loose capacity from age and misuse. Although the battery may show a full charge with a voltmeter it may not have its rated capacity in amp hours.
To do this test you need a clock, voltmeter and a 50W 12V globe. We use a simple down light as they are readily available and draw approx 4 amps which is easy to use in our calculations.
Step 1) Fully charge the battery to be tested.
Step 2) Connect the voltmeter to the battery.
Step 3) Connect the globe and make a note of the time and voltage.
Step 4) Look at the battery label and take note of the AH rating. (Example: 100AH) Divide this by 4 and this is how many hours it will take to fully discharge the battery to 11.8V. (Example: 25 Hrs)
Step 5) Once the battery voltage has dropped to 11.8V take note of the time and disconnect the lamp. Do not allow the battery to drop below 11.8V or leave it unattended.
Step 6) You can now work out the actual capacity of your battery by multiplying the hours by 4 amps.
Results: If your battery capacity is less than 65 -
Warnings: The globe will get hot so ensure you place the globe on a heat resistant surface or suspend the globe in the air. Make sure all wiring is safe. Take note of warnings on batteries with regard to sparks and explosive gases. This test is done at your own risk.