Battery Drain Finder
Find what is flattening the battery when the car sits: measure the draw, pull fuses one at a time, and track it to a circuit.
You need a multimeter that can read DC amps (milliamps). Work with the car locked up as it would normally be left, and change one thing at a time.
How is the battery going flat?
Pick the closest match.
Weeks of sitting
Every car loses some charge on its own. A car that sits for weeks will usually flatten its battery, even with nothing wrong.
- A drain of a few tens of milliamps (clock and radio memory) will flatten a battery in a month or two. If it goes flat sooner, measure the draw to be sure.
- For a car that sits, a small battery maintainer is the cheapest cure. Or disconnect the negative terminal when you park it.
A battery that will not hold a charge
If the car is driven regularly and the battery still goes flat, the battery is worn out, or the car is not charging it.
- Charge the battery fully, then have it load tested (most parts shops do this free). An old battery can read 12 V and still fail under load.
- Check the charging voltage with the engine running. It should be roughly 13.8 to 14.5 V at the battery.
Wiring and help: Charging circuit (2.5i) · Charging circuit (2.7e)
Measure the draw
This tells you how much current something is pulling when the car is off.
- Start with a fully charged battery. A flat one will mislead you.
- Close everything as you normally leave the car: doors, boot and bonnet shut, key out. Wait 10 to 20 minutes, because some timers and memory circuits take a while to settle.
- Set the meter to the 10 A DC range and loosen the negative battery clamp. Connect the meter in series, between the negative post and the clamp, so all the current flows through it. Keep the clamp from touching the post.
- Read the meter. About 50 mA or less is acceptable on a car this age. Hundreds of milliamps will flatten a battery in days.
The draw is normal
Nothing is draining the car, so the problem is the battery or its charging.
- A battery that is old can self-discharge faster than the car's circuits do. Load test it.
- Check that it is being charged properly when you drive.
Quick checks for the usual lights
A light left on by a stuck switch is a very common cause of a high draw.
- Look at the boot light, glove box light, map light and interior lights. A door that does not fully latch, or a worn jamb switch, can leave a light on.
- Look into the footwells and under the dash for lights, and check that nothing is plugged into the cigar lighter socket.
- Unplug anything that was added later, such as an alarm, stereo amplifier, phone charger, dash camera or GPS tracker, and check the draw again.
Wiring and help: Interior lights · Rear marker, licence and boot lights
Pull the fuses one at a time
With the meter still in circuit, find which fuse takes the draw away.
- Pull one fuse at a time and watch the meter. If the reading drops sharply, you have found the circuit.
- Put each fuse back before you pull the next, so you can tell which one made the difference.
- Note that the meter reading may spike each time you refit a fuse. Wait for it to settle.
Wiring and help: Fuse and relay finder
Find what is on that circuit
Look the fuse up in the fuse finder to see everything that is on it, then check each in turn.
- Fuses 21 and 27 carry loads that stay live with the key out, such as the radio memory, the on-board computer, the interior lights, the boot and glove box lights, and central locking. (Fuse 12 is only live with the key on.) A control unit that does not go to sleep, or a stuck relay, shows up here.
- Fuse 28 feeds the cigar lighter and the power antenna. A stuck antenna relay or a short in a plug-in accessory shows up here.
- Unplug the devices on that circuit one at a time and watch the meter. The one that drops the draw is the culprit.
Wiring and help: Fuse and relay finder · Fuse 27 detail · Fuse 21 detail
No fuse changed the draw
Something is draining the battery that is not protected by those fuses.
- Alternator: a failed diode in the alternator can let current leak through it when the car is off. Disconnect the alternator's heavy output wire and check the draw.
- Main relay: a stuck main relay keeps the engine computer and injection powered. Pull it and check.
- Wiring that was added later: aftermarket alarms, stereos and lights are often wired straight to the battery and bypass the fuse box. Trace and unplug them.
- The battery itself: a cell that is failing internally can drain itself. Disconnect the battery and have it tested.
Wiring and help: Main relay (2.5i) · Charging circuit (2.5i)
Most likely cause
Normal self-discharge
Nothing is wrong with the car. Batteries lose charge when left.
- Fit a battery maintainer (trickle charger), or disconnect the negative terminal when you park it for weeks.
- Keep the battery clean and its terminals tight.
Most likely cause
Likely cause: the battery is worn out
It fails under load or will not hold its charge.
- Replace the battery. Clean the cable ends and the tray while it is out.
Most likely cause
Likely cause: the charging system
The battery is not being recharged properly.
- Check the drive belt and the alternator's connections, including the heavy output wire.
- If the charging voltage is still wrong, have the alternator and its regulator tested.
Wiring and help: Charging circuit (2.5i) · Charging circuit (2.7e)
Most likely cause
Likely cause: a light, or an accessory
Something was left on or draws current when it should not.
- Replace a worn door or boot switch, or fix a latch that does not fully close.
- Fit any accessory through a switched circuit, not straight to the battery.
Wiring and help: Interior lights
Most likely cause
Likely cause: a control unit or relay that stays on
A unit that does not go to sleep, or a relay stuck closed, draws current all the time.
- Swap in a known-good relay, or unplug the unit and check the draw drops.
- Check for moisture in the unit and its plug. Water is a common cause on older cars.
Wiring and help: Central locking · On-board computer
Most likely cause
Likely cause: the radio, antenna or an accessory
The radio, an amplifier or a power antenna is drawing current with the car off.
- Check that the radio's switched feed is wired to a switched source and not the constant one.
- Check the power antenna and amplifier turn-on wires.
Wiring and help: Radio install guide · Radio and antenna
Most likely cause
Likely cause: a short or leak in the wiring
Insulation has worn through or a connector is damp, and current is leaking.
- Follow the wiring on that circuit and look for rubbed insulation, damp connectors and brittle wires, especially where the harness passes through a door or the boot lid.
- Repair the wire properly with a crimped or soldered joint and heat-shrink.
Wiring and help: Repairing old harness and connectors
Most likely cause
Likely cause: a failed alternator diode
The alternator leaks current back through its rectifier when the car is off.
- Replace or rebuild the alternator.
Wiring and help: Charging circuit (2.5i)
Most likely cause
Likely cause: a stuck main relay
Its contacts stay closed and keep the engine computer and injection powered.
- Replace the main relay with a good one of the same type.
Wiring and help: Main relay (2.5i) · Relay guide
Most likely cause
Likely cause: added wiring
An alarm, stereo or accessory that someone fitted is drawing current.
- Disconnect the accessory and recheck the draw.
- Re-wire it properly, through a fuse and a switched or relay-controlled supply.
Written from the factory wiring diagrams on this site and general diagnostic practice. Voltages, resistances and pressures are typical figures, so confirm them against your car's manual. Take care with fuel, high voltage and the battery.