Have you woken up to a dead car battery?
Or to the slow cranking of your engine and wondered whether your battery is still good?
Usually, the go-to method for testing a battery is to measure its voltage with a good quality multimeter.
They are handy, affordable, and easy to use.
Whereas it is a good first test, the car battery voltage does not provide a complete picture of the battery’s capacity.
In this post, I will share with you:
- what voltage you can expect from a good quality battery when your car’s ignition is off
- how to get a more accurate measure of your battery voltage and
- what additional tests you can do to get a more complete picture of your battery’s health status.
Car Battery Voltage reading when Car ignition is Off
A car battery’s voltage can read anywhere from 12.3 – 12.7 VDC when the engine is off depending on when you take the reading.
Immediately after switching off the engine, your multimeter may read a voltage as high as 12.6 -12.9V because of the surface charge.
This voltage can drop to as low as 12.3 -12.4 VDC after the battery has rested for several hours and the surface charge has dissipated.
Accessories like the car alarm, onboard car computer, and others also slowly drain some of the battery’s power.
To obtain a battery voltage reading that reflects the state of charge of the battery, you need to measure the voltage of the battery after it has rested for a while.
The battery should be disconnected from the charging source and any appliances that may be draining it.
For example, the car battery voltage may be as high as 14.3 V with the engine running and as low as 11.9V when accessories are switched ON, hence the need to isolate the battery for an accurate voltage reading.
The recommended resting period maybe a few minutes or several hours depending on the battery technology used.
For example, Odyssey’s thin-plate technology battery should rest for about 4-6 hours for an accurate reading while for some battery technologies, about 5 minutes may be good enough
(Always consult the manual for your battery manufacturer)
Which voltages are too low for your Battery?
An open-circuit voltage of 12.3 V and below (resting voltage – with no loads connected), about 65% of the car battery’s capacity is low and not good for the battery long-term even though your car may be able to start.
At 11.5V (no-load connected), the battery’s state of charge is about 0%.
Car batteries are not meant to be deeply discharged and doing so affects their lifespan.
You may also be interested in: How many Volts a Car Battery needs to Start?
Why your car battery voltages may be low?
A low battery voltage does not necessarily mean that the battery is bad and should be replaced.
It could be that you have loose or poor battery connections that slow down the charging or there is a fault with the alternator.
For more information, check out the guide on Why your batteries may be draining quickly
How to Measure Car Battery Voltage Accurately
Here are a few tips to help you get an accurate voltage reading for your battery that reflects the battery’s state of charge
- Make sure that the multimeter probes are firmly touching the battery posts (not the cables).
- Remove surface charge from the car battery if the car engine has been running and charging the battery.
Surface charge is the slightly raised voltage that a battery has when immediately disconnected from a charger.
To remove the surface charge, you can:
- Switch off the car engine (turn the key to the OFF position) and any other electrical accessories that draw power from the battery.
- disconnect from the charger and wait for the battery to rest for 5 min to 4 hours (depending on the battery technology) before measuring the voltage.
You can also turn the high beam lights for about 10-15 seconds to remove the surface charge.
How to Measure Battery Capacity when Car is off
A battery’s open-circuit voltage (OCV) is only a rough measure of its state of charge.
It does not show a complete picture of the battery’s health – for example, whether it can still supply the cold cranking amps (CCA) that it was designed for, an indication of battery capacity and whether it is due for replacement.
For a complete picture of your battery’s health, always start off with a fully charged battery.
A discharged battery may be flagged as bad and replaced when all it needs is to charge first.
Use a good quality conductance/ impedance tester
Use a good quality battery tester that measures its cold-starting ability.
These are generally easy to use – Always consult your battery tester manual but in general, the steps are:
- make sure you make the correct selection for the battery type – AGM, or otherwise
- typically you input the CCAs of your battery that can be read off the battery label
- connect across the battery terminals and in a few seconds, you have an idea about your battery’s health .
These testers connect a load to the battery for a period usually 15 or 30 seconds and if the voltage drops below a specified level according to the battery type, the battery fails.
You can also have the battery tested at a professional auto shop.
What should a car battery voltage be after sitting overnight?
A good quality car battery’s voltage may go as low as 12.2-12.3 VDC after sitting overnight mainly because of onboard computers, alarms, GPS that continue to drain the battery’s power overnight.
Should a fully charged battery drain lower overnight with no accessories connected then quite likely you have a bad battery.
You may also be interested in: Why your Car Battery Voltage Drops Overnight
The voltage of the car’s battery in good working condition usually ranges from 12.3 to 12.7 VDC.
It is best to take the reading after the surface charge has been removed and the voltage has stabilized for an accurate voltage reading.
Beyond the voltage reading test, you can conduct a conductance/ impedance test using a self-administered battery tester for a more complete picture of the battery health or arrange for one to be done by an auto mechanic.
Hope this helps!