Do not worry if you don’t know what this means because we are going to define it here. The confirmed difference in electrical potential between the positive and negative terminals, when there is no load connected, is what we refer to as a battery’s open circuit voltage (OCV).
The main purpose for having this test done is to know the capacity of the battery or its electrical potential.
Another term that can be used to refer to OCV is the electromotive force (emf) of the battery. In this case, the electromotive force refers to the difference in the maximum potential when the circuit is not closed and there is no current. A short circuit is the direct opposite of OCV.
Let us now look at the steps taken to measure a battery’s open circuit voltage.
How to Test Battery Open Circuit Voltage
After acquiring your multimeter, you can now follow these steps.
Step 1: Ensure the battery is not connected to any load. Have all the connections at the terminals disconnected Turn on the headlights for a few minutes before having the terminals disconnected so that you ensure there is no surface charge on the battery. When doing the disconnection, start from the negative terminal first then proceed to the positive terminal.
Step 2: The voltmeter should be set to the DC so have it done then adjust the reading to 20 Volts. To avoid damaging the multimeter and obtaining incorrect results, always use a setting that is higher than the battery’s rated capacity.
Step 3: The multimeter has two probes the red one and the black one. Have the red one connected to the positive terminal of the battery and the black one to the negative terminal of the battery.
Step 4: You can now record your readings.
How are the results from the open circuit voltage measurement interpreted?
After obtaining results, these results are used to understand the battery’s health and this cannot be done without a proper interpretation of what the results mean. A battery that has each cell in good health and is giving an approximate of 2.1 Volts each should give a total voltage of 12.6 Volts or more.
When you have a voltage that is below 12.4 Volts, It means that the health of the battery is reducing or the battery just needs to be recharged. You can use the following results to have a clue on the percentage of the charge on the battery.
12.6v = 100%
12.4v = 75%
12.2v = 50%
12.0v = 25%
11.9v = 0%
You can perform a battery-heavy load test when the voltage gives you a reading of 12.6 Volts or more.
Battery Heavy Load Test
Worry not if you don’t know what a battery-heavy load test is because we got you! Whereas the open circuit voltage test we have talked about above identifies the state of the battery’s charge, it does not tell how much power a battery can give to have the engine cranked.
A battery-heavy load test measures the ability of the battery to provide cranking power. To measure the cranking power the battery Vote Amp Test (VAT) is used and this can only be done on a battery that has more than 75% of its rated capacity.
Below is a guideline on how you can do the heavy load test.
Steps To Perform Heavy Load Test
Step 1: Connect the multimeter with the positive probe from the multimeter going to the positive terminal of the battery and the negative probe from the multimeter going to the negative terminal of the battery
Step 2: Have the load on the battery increased you can do this by turning the load increase control unit up to the point where the ammeter gives a reading that is at least 3 times the rated amp hour or half of the rated cold cranking power (CCA) Increase the battery load.
Step 3: Take the voltmeter reading while maintaining the load for 15 seconds or less.
Step 4: Readings without interpretation are meaningless so have your readings interpreted. When you have a reading of about 9.6 Volts for a battery of 12 volts then it means the battery is okay but when the reading is below 9.5 Volts, it means there is a need for replacement because the battery is faulty.
For best results, it is advisable to keep the electrolyte temperature around the room temperature of 210C (700F).
This is so because the heavy load test is affected greatly by temperature fluctuations in that when the temperature is high the voltages will be low and when temperatures drop, the voltage also drops.
At times it is not possible to maintain the room temperature and in that case, you can use the below table to do proper conversions.
9.6v = 210C (700F)
9.5v = 160C (600F)
9.4v = 100C (500F)
9.3v = 40C (400F)
9.1v = -10C (300F)
8.9v = -70C (200F)
8.7v = -120C (100F)
8.5v = -180C (00F)
Final Thoughts
The battery open circuit voltage is very helpful because it helps to determine the charge levels of the battery. However, for you to know the battery cranking power you will have to do a heavy load test.
When you get to do these two tests together, you will be in a position to know the health of your battery and also be able to plan accordingly in case you need a battery replacement.