What Voltage Should My Car Battery Be? A UK Voltage Chart

If you have put a meter across your battery and are not sure what the number means, this UK guide is for you. Here is what voltage a healthy car battery should read, a simple state-of-charge chart, what it should show with the engine running, and — importantly — why voltage alone cannot confirm a battery is actually healthy.

What a healthy battery reads

A rested 12V battery (engine off for a few hours) should read about 12.6–12.8V when fully charged. Around 12.4V is roughly three-quarters charged; 12.2V is about half; 12.0V or below is flat and needs charging. You want a plain-English answer, not a spec sheet, so here it is as a chart.

Car battery voltage chart (state of charge)

• 12.6–12.8V — 100% charged, healthy.
• 12.4V — about 75%.
• 12.2V — about 50%; recharge soon.
• 12.0V — about 25%; charge now.
• 11.9V or below — effectively flat.

With the engine running

Start the car and the alternator should raise the reading to roughly 13.7–14.7V. Much lower and the alternator may not be charging properly; much higher and the regulator could be at fault. This is why a charging-system check matters as much as the battery itself.

Why voltage alone is not enough

Here is the catch. A battery can show a perfect 12.7V and still fail the instant it has to turn the starter, because voltage does not measure capacity. If a car only does a short weekly run to the shops — the kind of use that quietly wears a battery down — a surface voltage reading is especially misleading. To know whether a battery is genuinely healthy you need to measure its cranking ability (CCA) and state of health, which is what a conductance test does in seconds. It is the difference between “the number looks fine” and “it will start the car next week.”

How to check properly

Use a meter for a quick voltage snapshot, then a battery tester for the health verdict — tested CCA, state of health and the alternator's output. For a rarely-driven car, that combination is the reassurance you want before winter. If you would like help choosing a tester, see our car battery tester UK buyer's guide.

Beyond voltage: the real health check

The AUTOOL BT460 is a 12V and 24V car battery and charging-system tester built for cars, motorbikes, vans and 24V trucks. It uses conductance testing to measure battery health across a 30–2000 CCA range and supports the CCA, IEC, EN, DIN, JIS and GB standards, so you can match the test to the figure printed on the battery label. Results appear on a colour TFT screen with a clear health verdict, and a built-in printer produces a paper report on the spot. Reverse-polarity protection means a wrong-way clip will not damage the tool or the vehicle. It also checks cranking and alternator charging, so you see the whole starting-and-charging picture rather than resting voltage alone. It is £58.99 with free UK delivery, dispatched the same day from a UK warehouse, backed by a 12-month warranty, 30-day no-quibble returns and UK-based support, and is rated 4.8 out of 5 from 200 reviews.

Frequently asked questions

What voltage should a car battery be when fully charged?

A rested 12V car battery should read about 12.6 to 12.8 volts with the engine off. Around 12.4V is roughly three-quarters charged, 12.2V about half, and 12.0V or below is effectively flat and needs charging.

What should the battery voltage be with the engine running?

With the engine running the alternator should raise the reading to roughly 13.7 to 14.7 volts. A reading well below that suggests the alternator may not be charging properly, while a reading well above it can point to a regulator fault.

Can a car battery show good voltage but still be bad?

Yes. Voltage shows the state of charge, not capacity, so a battery can read a healthy 12.7V and still fail under the load of starting the engine. To confirm a battery is genuinely healthy you need to measure its cranking ability and state of health with a battery tester.

How do I check if my car battery is healthy at home?

Take a voltage reading with a meter for a quick snapshot, then use a battery tester to measure tested CCA and state of health and to check the alternator's charging output. That combination tells you not just the charge level but whether the battery will reliably start the car.