Electric and hybrid owners
Electric Car Battery Health Check: The 12V Battery
About 6 minutes to read
Search for an electric car battery check and most results are about the big traction battery: range, degradation and health reports. Those matter, but they are rarely why an electric car suddenly refuses to unlock or switch on. The usual culprit is far smaller. Nearly every electric car and hybrid also carries a conventional 12 volt battery that powers the computers which wake up the high voltage system.
When that 12 volt battery dies, the car can show a dark dash and refuse to go into ready mode even with a full charge. This page explains how to tell whether the 12 volt battery is the problem, what a health check on it involves, and when the traction battery is genuinely the thing to worry about.
Which battery is the problem?
The two batteries fail in very different ways.
| What you see or hear | What it means | What to do |
|---|---|---|
| Car will not unlock, screens dark, charge level unknown | 12 volt battery flat | Jump from the documented 12 volt points, then test the 12 volt battery |
| Screens flicker, warnings appear, car will not enter ready mode | 12 volt battery weak | 12 volt load test. Replace if it fails |
| Car drives normally but range has fallen gradually over years | Normal traction battery ageing | Traction battery health report from a dealer or EV specialist |
| High voltage or drive system warning on the dash | A fault in the high voltage system | Dealer or EV specialist only. Do not attempt anything yourself |
What a 12V health check covers
Resting voltage
With the car asleep, a voltage reading gives a first idea of state of charge. It is only a starting point, because a tired battery can show a reasonable number and still fail as soon as it is asked to work.
Load test
A load test shows whether the 12 volt battery can still hold its voltage while delivering current. This is the part that separates a battery that just needs a charge from one that is finished.
Charging check with the car switched on
In an electric car the 12 volt battery is charged by a converter from the traction battery rather than an alternator. A reading with the car in ready mode shows whether that charging is reaching the 12 volt battery.
Terminals and connections
A corroded or loose terminal can cause the same dark dash as a failed battery. It is quick to check and quick to put right.
Replacement where the test says so
If the 12 volt battery fails its load test, the fix is a like for like replacement, usually a small AGM. It is fitted without touching the high voltage system.
Mistakes that make it worse
- Assuming a full traction charge means the car must be fine, and waiting for a dealer appointment when a 12 volt battery would have fixed it.
- Connecting anything to the orange high voltage cabling. It is never part of a 12 volt jump or test.
- Using a jump pack from an electric car to start a petrol car. Many manufacturers advise against using an electric car as a donor.
- Leaving the car unused for weeks with lots of connected features, which slowly drains the 12 volt battery.
- Replacing the 12 volt battery with the wrong type. Most electric cars use AGM, and a basic flooded battery can fail early.
12V help for electric cars and hybrids
We jumpstart electric cars and hybrids from the documented 12 volt points only and never go near the high voltage system. We can load test the 12 volt battery and replace it on the spot if it has failed. We are open 24 hours from our base in Oldbury. Traction battery health reports and high voltage faults are dealer or specialist work.
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Questions
Q.Do electric cars have a normal 12V battery?
Nearly all of them do. It powers the locks, screens and control units, and wakes the high voltage system. When it dies the car can seem completely dead.
Q.Can you check the main EV battery?
No. We test and replace the 12 volt battery. A traction battery health report needs a dealer or EV specialist with the right equipment.
Q.Is it safe to jump start an electric car?
Yes, from the documented 12 volt points only. We never touch the orange high voltage cabling.
Q.Does the same apply to hybrids?
Yes. Hybrids also have a 12 volt battery that wakes the system, and it fails in the same way.
The short version
If your electric car or hybrid suddenly will not unlock or switch on, suspect the small 12 volt battery before the traction battery. A 12 volt health check is a voltage reading, a load test and a charging check, and a failed battery can be replaced without touching the high voltage system.