Hybrid battery failure signs: what to note, and what it is not

What it means

A hybrid has two batteries. The traction battery is the large high-voltage pack that drives the electric motor. The 12-volt battery is an ordinary small battery that wakes the car up, and it is the one most likely to leave you stranded.

When the traction battery loses capacity or is not charging as it should, the car can do less on electricity, so the engine steps in more often. Hybrid traction batteries are generally engineered for a long service life, and how they age varies too much by manufacturer, chemistry and climate to put a figure on it.

Common causes

• Less electric running than before on the same trips
• The engine starting far more often on a route you know, in similar weather
• Fuel economy that has dropped without a change in how you drive
• A charge gauge that swings up and down much faster than it used to
• The hybrid system warning light coming on

What to do now

1. Note the route and the weather when you notice the change, so it can be compared like with like.
2. Describe what changed, not what you think has failed: less electric running, the engine starting more often, economy dropping.
3. On an air-cooled pack, check that the battery cooling vent is clear. A blocked vent stops the pack cooling properly, and that shows up as worse economy.
4. If the car will not go to READY, have the 12-volt battery tested before anyone looks at the traction battery.
5. Have the traction battery and charging system checked by a hybrid-qualified technician.

Traction battery or 12-volt battery?

• The car will not go to READY, the dashboard is dim: very often the 12-volt battery. Have it tested first.
• Less electric running, the engine starting far more often: reduced traction battery capacity or a charging fault.
• Economy has dropped and the battery cooling fan runs louder: the pack may be working harder to stay cool. Check the vent.
• The hybrid system warning light is on: the traction battery, motor or power electronics. Have it diagnosed.

What people get wrong

That a dead hybrid means a dead traction battery. The small 12-volt battery strands far more hybrids than the big one, and it is the first thing to test.

When it has to go to a workshop

Anything involving the traction battery itself is for a technician qualified on hybrid high-voltage systems: opening it, testing it, disconnecting it or removing it. So is the orange high-voltage cabling. What an owner can do is notice the change, keep the cooling vent clear and describe the symptoms accurately.

Read next

• Hybrid won't go to READY? Test the 12-volt battery first
• Hybrid car maintenance: what differs from a petrol car
• Why hybrid brakes can feel different, and when it is normal

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.