How to correctly measure the voltage of a fully charged car battery at rest
If you want to know the voltage of a car battery without load, you will have to take into account several aspects to get the right parameters. First, the most correct value is given at an air temperature of +18 degrees. Secondly, for greater accuracy, it is necessary to measure the voltage 12 hours after the engine has been switched off. Thirdly, rely on the battery manufacturer's information – this is indicated either on the battery itself or in the accompanying documents – to understand what optimal parameters are specified by the factory itself.
As for tools that can help you find out how many volts a car battery has, there are several options available to the driver. Modern cars make it possible to retrieve this information via the on-board computer. If an anti-theft alarm system is installed in the car, such security systems often allow the battery voltage to be monitored via a key fob or mobile application. However, such tools give an error, and to a lesser extent.
To get more accurate readings, you'll need to use either a multimeter or a load fork. The voltage of a charged battery, measured with a multimeter, should be 12.5 V (if it is below 12 V, there is a high probability that you will not be able to start the engine without help). When connecting the load plug (measurement time is usually 10 seconds, sometimes twice as long), a value of 12.6 V is considered normal.
If you don't want to or can't wait 12 hours after your last ride, wait at least a few hours after turning off the engine. The voltage value obtained under such conditions is more accurate than the one displayed by the multimeter immediately after stopping.
How to measure the voltage of a car battery under load
As far as tools are concerned, the on-board computer or data from the alarm system can also help here. Or use a multimeter to get more accurate battery voltage data.
To find out the battery voltage under load, you need to:
- Start the car engine.
- Let the engine run for 15-20 minutes;
- connect a multimeter;
- Switch on electrical consumers such as lights or heating.
How long should the battery be displayed under load ? Normally, this value is between 13.5 and 14 V when the engine is running, but without serious power consumers. What voltage should the battery be when it is on? When the low beam (halogen) is on, the value decreases by 0.1–0.2 V, then it should decrease by the same amount when the interior heating, heated windshield, multimedia system, etc. are turned on.
Table and graph on the dependence of the gel battery voltage on the degree of discharge.
To check the state of charge based on the voltage, disconnect everything from the positive terminal of the battery and wait 10 minutes before measuring the voltage.
The following table shows the approximate dependence of the open-circuit voltage of a gel battery on the degree of its discharge.
| Gel/AGM battery voltage, volts | State of charge | Degree of discharge |
|---|---|---|
| 13.0 | 100% | 0% |
| 12.8 | 90 % | 10 % |
| 12.6 | 80 % | 20 % |
| 12.4 | 70 % | 30% |
| 12.2 | 60 % | 40 % |
| 12.0 | 50 % | 50 % |
| 11.8 | 40 % | 60 % |
| 11.5 | 30% | 70 % |
| 11.2 | 20 % | 80 % |
| 10.9 | 10 % | 90 % |
| 10.5 | 0% | 100% |
The following graphic shows the actual discharge behavior of a gel battery with a capacity of 200 Ah. The discharge took place with a constant power of 1300 watts (the discharge current gradually increased from 110 to 130 amperes as the voltage decreased).
As can be seen from this graph, at the beginning, when a strong load is connected, the voltage drops by 1 volt (at a discharge current of 110 amps), and at the end, when the load is disconnected, the voltage increases by about 2 volts. These voltage changes are associated with the voltage drop at the battery's internal resistance, connecting cables, and fuse that occurs when the load is connected. For this reason, it is impossible to accurately estimate the degree of discharge of the battery to which the load is connected based on the voltage alone.
Battery Charge Level Table 24v
| Voltage | Battery Charging |
|---|---|
| 25.4 V | 100 % |
| 25.1 V | 90 % |
| 24.9 V | 80 % |
| 24.7 V | 70 % |
| 24.5 V | 60 % |
| 24.4 V | 50 % |
| 24.2 V | 40 % |
| 24.0 V | 30% |
| 23.9 V | 20 % |
| 23.8 V | up to 10 % |
Immediately after charging the battery, its voltage may be 0.5V above normal – this is not scary. Therefore, it is acceptable if there is 13.2 V or 25.9 V for a 24 volt battery after disconnecting the charger. If you measure the voltage a few hours after disconnecting the charge, you will see the normal battery voltage.
What voltage is considered normal for a car battery?
Battery voltage depends on a number of factors. For example, if we want to know the voltage of a car battery without load or if we are interested in how much the battery should produce under load .
If we are talking about a 12-volt battery without load, then values in the range of 12.5-12.7 V are the norm. And with a 24-volt battery, the normal range is already 25.2 to 25.4 V.
If we measure the voltage under load, i.e. with the engine running and additional power consumers, this parameter will be predictably higher. For a 12 V battery, the normal voltage in this case is between 13.5 and 14.5 V. For a 24 V car battery under load, values up to 29 V are acceptable.
It is important to remember that you need to drive at least 100 km to charge the battery on the road – only in such conditions can the generator provide charging. Short trips around the city with frequent stops and subsequent engine starts will not help to recharge the battery, and in sub-zero temperatures, such a rhythm of movement will only harm the battery.
What is the voltage of a fully discharged gel/AGM battery?
The voltage of a fully discharged AGM or gel battery is 10.5 volts.
Before checking the voltage, you should disconnect everything from the positive terminal of the battery and wait 10 minutes. It is not recommended to leave lead-acid batteries of any kind in a discharged state. To extend the service life, it is recommended to charge immediately after a full or partial discharge.
So very briefly
- For a 12 volt battery, the normal voltage is 12.6 V.
- You can measure the voltage at rest at least 2 hours after stopping the engine, under load – 15–20 minutes after starting the engine.
- A high battery voltage is normal, but a voltage that is too low can indicate technical problems with the vehicle's battery or electrical system.
- Do not let the voltage drop below 10.5 V, otherwise an irreversible sulfation process will begin.
- A drop in battery voltage in cold weather is normal.
- Regularly measure the battery voltage, monitor the condition of the generator and the on-board power supply.