Flat car battery: what to do

Leads, a booster or a call: all three work.

The answer in 2 lines. A flat battery can usually be jump started with leads from another car or with a portable jump starter (booster). Follow your car's handbook exactly: many modern cars have special jump points under the bonnet. Switch everything off first, no smoking or flames nearby, and never touch a battery that is swollen, cracked or leaking. If you are unsure, or the car is a hybrid or electric, call your breakdown cover. Afterwards, drive for a while and have the battery and alternator tested.

The first 3 steps

Switch everything off: headlights, radio, heater, phone chargers.
Open the handbook at the section on jump starting ("starting with a booster battery" or similar) and find the connection points.
No flames, cigarettes or sparks near the battery.

Stop: danger

Battery swollen, cracked or leaking, or frozen: don't touch it or try to jump start. Call your breakdown cover.
Smell of rotten eggs near the battery: it may be overheating. Keep away and call for help.
Broken down on a motorway or busy road: get out on the side away from traffic, behind the barrier if there is one, and call your breakdown cover. In a live lane or with no hard shoulder: call 999.

The signs

The engine turns slowly or you only hear clicking.
Dashboard and interior lights are dim or don't come on.
Lights left on overnight, a car unused for weeks, lots of short journeys in winter: the battery can't recharge.
Battery warning light on while driving: often a charging problem (alternator); have it checked soon.
Not sure it's the battery? See our guide on battery or starter motor.

Jump starting with leads or a booster

Every car has its own connection points and order. Follow your handbook exactly. The RAC's step-by-step guide and the AA's guide to using jump leads show the usual method:

Both batteries must be the same voltage (12 V for most cars). The cars must not touch each other.
Usually: red lead to the positive (+) of the flat battery, then to the positive of the good battery; black lead to the negative (−) of the good battery, then to an unpainted metal earth point on the car with the flat battery, away from the battery. Never connect red to black.
Start the working car, wait a few minutes, then start yours. Remove the leads in the reverse order.
Choosing a jump starter or leads: for leads, thick cables (the RAC and AA advise against thin, cheap ones) long enough to reach between two cars; for a portable jump starter, check it is rated for your engine size and type (diesel engines need more power), and keep it charged.
Hybrid or electric car: don't do anything without the handbook; many cannot be used to jump another car. Call your breakdown cover if in doubt.

Calling for help

Breakdown cover (RAC, AA, Green Flag, or cover included with your car insurance, bank account or new car): check whether it includes "home start". Basic cover often only applies when you are a certain distance from home.
No cover? Most breakdown companies will still come out if you join on the spot, at a higher price.
On a motorway: use the free emergency phones, or call your breakdown cover with your location (the blue marker posts at the roadside show it).

What not to do

Bump starting (push starting) an automatic or a modern car with stop-start: it is not possible or not advised. Check the handbook.
Switching the engine off straight after a jump start: drive for at least 20 to 30 minutes so the battery can recharge a little.
Throwing an old battery in the bin.

After the jump start

Have the battery and alternator tested: many garages and battery shops do it free or cheaply.
If the battery goes flat again, it probably needs replacing. Cars with stop-start often need a specific type (AGM or EFB): check the handbook.
A new battery that fails early: under the Consumer Rights Act 2015 you can usually reject it for a refund within 30 days, then ask for a repair or replacement.

The old battery: never in the bin

Car batteries contain lead and acid and must be recycled. Under the Waste Batteries and Accumulators Regulations 2009, producers of car batteries must arrange free take-back of old ones; in practice, many shops and garages that sell or fit car batteries will take your old one, and most council household waste recycling centres accept them (ask before you go). To find the nearest place, enter your postcode in the Recycle Now car batteries locator.

Good to know

Cold weather and long periods without driving are the main enemies of car batteries: on a car you rarely use, a smart battery charger ("trickle charger") keeps it topped up.
Save your breakdown cover number in your phone now.

In Sheffield, Hannah left her interior light on overnight. In the morning the car just clicks. She finds the jump points in the handbook but isn't confident, so she calls the home start service included in her breakdown cover. The patrol starts the car with a booster, tests the battery and tells her it is fine, just drained.

A made-up story, for illustration.

The figures (latest published)

12 V: the voltage of most ordinary car batteries (check your handbook).
Breakdown companies such as the RAC regularly report flat batteries as one of the most common reasons for winter call-outs, especially in early January; they publish campaigns rather than official statistics, so we don't quote a figure.

Useful numbers (UK)

999: if you are stuck in a live lane, on a motorway without a hard shoulder, or there is smoke or fire.
Your breakdown cover: number on your policy, card or app.
On a motorway: the free emergency phones on the hard shoulder and in emergency areas connect you to National Highways.

Bobby

Sources, verification, confidence

Verified on 1 November 2026, for England. Confidence: high for the safety steps and the general jump start method (RAC, AA), but your handbook always comes first; high that car batteries must not go in the bin and can be returned free of charge; medium for which shop or garage will take one back, which varies; medium for breakdown cover details, which vary by provider. We found no official UK statistics on flat batteries, so we don't quote one. This guide describes common cases, not a diagnosis.