Car Battery Testing & Voltage - How Many Volts is a Car Battery?
The car battery provides energy not only for cranking the engine, but it also powers a broad range of devices that may be turned on when the engine is off, even when the car is locked, like GPS units, alarms, and other security systems, multimedia devices, lights, etc.
While car batteries feature a nominal voltage of 12 volts, in real life, this voltage can be anywhere from just a few volts (or even 0 volts!) to almost or even slightly over 15 volts. The exact battery voltage depends on the engine being turned On or Off and the battery's charge condition, age, load, temperature, etc.
Car Battery Types and Chemistries
Lead-acid batteries are the most common car batteries, but they also differ and can be wet/flooded batteries, enhanced wet/flooded batteries, Absorbent Glass Mat (AGM) batteries, enhanced AGM batteries, Gel-Cell batteries, batteries with pure lead plates, batteries with calcium plates (calcium-lead plates), etc.
All these battery types have slightly different voltages when they are fully charged, semi-charged, or fully discharged, have slightly different temperature correction/compensation coefficient, etc.
When the engine is turned ON, the alternator charges the battery and its voltage should be in the 13.7 to 14.7 volts range. If the battery voltage is below 13.6-13.7 volts, the battery is being recharged by the alternator - again, this is with the engine being turned ON and the battery is being charged.
Lead-acid battery charging voltage-temperature compensation/correction coefficient varies from -3mV/°C/cell to -5mV/°C/cell, with the average value being -4mV/°C/cell.
However, if possible, it is highly recommended to try to find the charging voltage-temperature compensation coefficient of the battery You own, or even better, a voltage-temperature chart or graph.
Why? Well, for example, Odyssey AGM batteries feature a temperature coefficient of -2 mV/°C/cell (-24 mV/°C for 12V battery) and is not linear - even charging voltage-temperature compensation coefficient changes its value with a temperature ...
But, as long as the battery voltage is above 13.7 volts when the engine is ON, the battery is fully charged. If it is lower than that, something has discharged the battery - maybe lights were left ON for several hours, or the car was not used for weeks, etc.
When the engine is turned OFF, one may check the battery voltage even without disconnecting the battery from the car, since the drain current of the electronics is very low and it doesn't influence the voltage measurement.
Note: be sure that the lights are OFF and any fan, audio, multimedia is also turned OFF.
The following chart lists the battery state of charge for standard flooded/wet and standard AGM batteries measured at 77°F/25°C:
|Type||100% SoC||75% SoC||50% SoC||25% SoC||0% SoC|
|Flooded/Wet||12.65 volts||12.45 volts||12.24 volts||12.06 volts||11.89 volts|
|AGM||12.80 volts||12.60 volts||12.40 volts||12.00 volts||11.80 volts|
Note: these are just 'average' values for 'standard' flooded and AGM batteries. Other batteries may have slightly different values, for example, Odyssey AGM batteries' SoC/voltage relation: 12.84 volts or higher - 100% SoC, 12.50 volts - 75% SoC, 12.18 volts - 50% SoC, 11.88 volts - 25% SoC, etc.
The easiest and the simplest way of measuring the battery voltage is by using a digital multimeter, which can be found in most hardware stores and online shops.
However, when measuring the voltage of the battery at lower or higher temperatures, one should make a temperature compensation, regardless if the battery is being charged or not.
When the battery is not being charged (the engine is OFF), the battery voltage is much more dependant on the battery SoC than on the temperature. For example, the following list shows the voltage-temperature SoC dependency of the standard flooded and AGM battery in the 0 - 120°F (-17.8 - 49°C) temperature range:
- standard flooded battery: 100% SoC 12.52-12.66 volts, 75% SoC 12.32-12.46 volts, 50% SoC 12.11-12.25 volts, 25% SoC 11.93-12.07 volts, 0% SoC 11.76-11.90 volts,
- standard AGM battery: 100% SoC 12.67-12.81 volts, 75% SoC 12.47-12.61 volts, 50% SoC 12.27-12.41 volts, 25% SoC 11.87-12.01 volts, 0% SoC 11.67-11.81 volts.
As one can see, unless you are freezing in the winter cold or cooking in the summer heat, it is safe to assume values @ 77°F (25°C) as more or less accurate and required values when checking the battery's SoC with the digital multimeter - unless the battery is at 100% SoC, either something drained the battery by accident or the battery is being drained constantly and neither condition is a good nor recommended condition for the car battery.
Battery CCA Testing
Another important battery test is testing the actual CCA rating of the battery.
Note: there are several standards that define Cold Cranking Amps (CCA), CA (Cranking Amps), MCA (Marine Cranking Amps) and even HCA (Hot Cranking Amps), but the most common ones are:
CCA (Cold Cranking Amps) is the maximum current that a new, fully charged, 12V battery can deliver for 30 seconds, with the voltage NOT dropping below 7.2V, at 0°F (-18°C).
CA (Cranking Amps) or MCA (Marine Cranking Amps) is the maximum current that a new, fully charged, 12V battery can deliver for 30 seconds, with the voltage NOT dropping below 7.2V at 32°F (0°C).
HCA (Hot Cranking Amps) is the maximum current that a new, fully charged, 12V battery can deliver for 30 seconds, with the voltage NOT dropping below 7.2V, at 80°F (27°C).
Of course, these tests are very demanding for every battery since they push the battery to its limits.
So, in order to test the actual CCA rating of the battery, one may use digital load battery testers, which are fairly cheap and often provide rather accurate results, without overstressing the battery.
Their use is simple - connect them to the battery, fill in required data like the CCA rating of the new battery and similar, and let the device measure/calculate the actual CCA rating of the battery at the present condition.
Another solution is carbon-pile battery load testers which test the battery by discharging them with a certain current for a limited amount of time until the voltage drops at a certain level.
Since true CCA/MCA/HCA tests are rather stressful to the batteries, the actual battery load test should be done at 'just' a half of its CCA rating, for 15 seconds and the battery should keep the voltage of at least 9.6 volts.
Before the test, the battery should be fully charged and the test should be done at room temperatures (70°F/21°C or slightly more).
Note: Carbon-pile battery load testers are true load testers - such tests should be done only after reading and following all the safety and handling instructions of the battery, battery tester, and the car itself. Personally, if You want to make a true battery load-test, go to the certified car shop and let them do the testing.
Long Story Short: As long as the car battery voltage is in the 13.7 - 14.7 volts range while the engine is ON, the battery is fully charged. When the engine is OFF, and the voltage is 12.6 or higher, the battery is fully charged.
If the battery is not fully charged, charge it by using the car more often, but also check the parasite loads of the car - better safe than sorry.
Since a fully charged battery doesn't always mean that the battery is able to provide the required current, it is recommended to make a battery load test from time to time.
Digital battery load testers can easily determine the approximate CCA/MCA/PHCA rating of the battery without stressing the battery, but the true load test can be done with, for example, carbon pile battery load testers and can determine the battery's true CCA/MCA/HCA ratings.
Note: it is highly recommended to do the true battery load tests at the certified car shop... Better safe than sorry, once again...