For you be able to know the actual capacity of a battery, a capacity test must be done. A battery capacity test can be very vital in tracking the health life of the battery and also in estimating the remaining life of a battery before you settle on doing a replacement.
The manufacturers always indicate a battery capacity rating before the battery leaves their factories. This is what is referred to as the rated capacity. It is worth noting that when actual measurements are made, these values do tend to be lower compared to the rated capacities which are normal.
The battery also has a discharge table which indicates the discharge current for a specific period down up to the lowest discharge voltage.
How To Measure Battery Capacity
For you to be able to know how much capacity (current x time) which is measured in amperes-hours (Ah) that the battery can have delivered before the terminal voltage is reached, then the battery capacity test has to be done. This measurement assumes that the flow of current remains at a constant rate.
When dealing with lead-acid battery, the test time is approximated to be near the duty cycle of the battery. Most lead-acid batteries have a duty cycle of about 5-8 hours and in that case, this is the timeline used. The end discharge voltage is in most cases 1.75-1.8 Volts per cell which translates to 10.5-10.6 Volts.
For you to get the best of results, you need to use the same testing times in the lifetime of the battery so that accuracy is improved and the results become comparable.
Battery Capacity Test Results
The battery is termed to be good when the results are analyzed and it is vividly evident that the battery reaches the discharge voltage at the times which are specified in the battery
The battery needs a replacement if the test results show the end discharge voltage is reached at 80% or even less of the rated time.
Let’s take for example if a battery is rated at 10 hours yet the end discharge voltage is reached at 8hours, it implies that the battery needs a replacement because it is at 80%
Battery Capacity Test Procedure
Here are the simple steps to follow to perform a battery capacity test
- Find out from the manual whether the battery needs to have an equalizer charge.
- All connections need to be confirmed that they are secure and in the right place. Also confirm that all the readings are correct for instance the resistance readings.
- Have the battery acid specific gravity of each cell measured and recorded
- Have the float charge in each cell measured and recorded
- Have the temperature of each cell measured and recorded then obtain the average temperature. The average temperature can be obtained by totaling all the temperatures recorded then divided by the number of cells in the battery.
- Have the terminal voltage measured and recorded
- Have the battery charger disconnected. This needs to be done in the right manner by first switching off and unplugging the charger from the main power supply then having the battery disconnecting follow.
- Have the load connected and start the discharge process. A load in this case is any electric device that needs a battery to function like for instance a bulb. This load must have a known power rating. You need to adjust the discharge rate according to the battery temperature got.
- The voltage at each cell at the beginning of the discharge test needs to be measured and recorded.
- Have the cell voltage at specified time intervals measured as the testing progresses.
- You need to maintain the battery discharge until the specified end discharge voltage is reached.
- Have the voltage of every cell after the end discharge voltage has been reached measured and recorded. This voltage will help identify the weak cells.
- You can now calculate the battery’s actual capacity.
Importance of Battery Capacity Test
The cell voltage at the end of discharge helps in identifying weak cells. The individual cell voltage measured will be obtained a number of times as the test is in progress.
A cold battery gives less Ah than a warm battery. For this reason, you need to adjust the time and current during the capacity test for the average temperature readings got.
You will note that most battery manufacturers do specify the battery temperature correction factors and also methods. The battery in some cases, may also specify the discharge voltage and it is of more importance in instances where the load has different regulators for the voltage.
It is worth noting that even as the discharge voltage drops there is an increase in current. The load equipment needs to discharge at a constant power rate even though the battery load test procedures are the same.
Final Thoughts
The time taken for capacity test can be shortened to 1hour or 2 hours by increasing the current rate. This is a plus in the sense that less capacity of the battery is drained out and especially in lead-acid batteries where low voltage may destroy the battery.
It is important to have the battery temperature constantly monitored when doing the discharge test at a high current rate. This is because the temperatures tend to rise very fast.
Before doing the load capacity test, it is vital to have the impedance test of the battery done so that the correlation between the two is obtained.