Discharge Testing

Load testing a battery after it has been charged can be a valuable tool in determining a batteries state of health. Load test  each battery using a carbon pile tester and record the voltage at 15 seconds. This is the voltage reading at 15 seconds while discharging the battery at 3.5 times the 20HR rating (350 amps on a 100AH battery). It must maintain a temperature compensated 1.75 volts per cell when commissioned and may move to1.6 volts per cell with age. The key is to watch that the voltage does not continue to drop but “holds the load” steady at or above the minimum voltages during the test. Load testing should just form part of your routine when analyzing or routinely checking a batteries state of health and end of life status. Please see "When Installing or commissioning NEW batteries" in this Servicing, Testing and Installing Batteries section for more information.

Different types of batteries use test procedures that test different end of life criteria. For example an electric vehicle or standard deep cycle product would be considered to be at its end of life when it was not able to deliver 50% of its rated capacity. A golf cart battery bank would not be determined to be at its end of life until it was not able to maintain at least 1.75 volts per cell during 40 minutes of discharge at 75 amperes.

THE FOLLOWING INFORMATION IS GIVEN FOR REFERENCE ONLY. 

Although not industry standard testing, the following steps are recommended by on of the worlds leading manufacturers of high quality flooded deep cycle type batteries for determining battery health and end of life criteria with simple constant current discharge equipment:

1. Check to make sure battery is charged
2. Double check that the temperature at and around the battery is above 24˚C  (75˚F) and below 32˚C (90˚F). If it is not do not proceed with the test.
3. Connect discharger
4. Record discharge current rate
5. Record run time in minutes until the battery voltage drops to 1.75 volts per cell.(10.5 volts for a 12 volt battery, 5.25 volts for a 6 volt battery)
6. Record the batteries voltage while still under load (individual voltages if testing a bank)
7. Correct the run time minutes for the recorded temperature at the end of the test using the formula shown below.
8. If the temperature corrected discharge minutes are greater than 50% of the batteries rated capacity at the discharge current then the battery is operational.
9. If the temperature corrected discharge minutes are less than 50% of the batteries rated capacity at the discharge current then the battery should be removed from service.
10. If you are testing a bank of batteries and the banks temperature corrected discharge minutes are less than 50% of the rated capacity, the batteries with a voltage variance of 0.05V lower than the highest voltage should be removed from service.

Steps to temperature correct a discharge reading: 

Minutes of "temperature corrected" run time will equal the recorded minutes x ((1-0.009 x (Temperature in Celcius - 27)) .