Flat battery: finding a drain and testing the charging system

Many owners report a battery that goes flat after a few days or a week of not driving. The threads agree there is usually a small continuous drain (the alarm system and ECU stay live), and that an old battery, short or infrequent drives and a weak alternator make it worse. Here is what was tried.

Step 1: check the battery and charging voltages

One owner gave this chart for a multimeter across the battery terminals with everything switched off. It is a rule of thumb from one owner, not a manufacturer figure:

• Cranking test. Turn the ignition on and try to start. If the voltage falls below about 9 V, either the battery is worn out or there is too much resistance, so check the connections and earths, then the starter.
• Charging test, engine running. The same owner says 14.1 to 14.6 V is healthy. Below 14 V probably means the alternator is going, and above 14.9 V means the regulator has failed and the battery will be cooking (he knew a car that read 16.5 V and the battery boiled). Other owners quote about 13.5 to 14.5 V as acceptable, and 12 V off with about 14 to 14.5 V running.
• A scan tool reading is only a second opinion. One owner advises taking a proper multimeter reading as well.
• Battery light. It should go out once the engine runs. One owner says it stays on if the alternator isn't producing, because the alternator's voltage is what keeps it off.
• Simple lights test (one owner): at tickover in the dark with everything electrical on, raise the engine speed. The lights should visibly brighten if the alternator is doing something. Another owner says that fits what he sees.
• Is the battery itself bad? Give it a decent charge and leave it. If it is worn it will drop off quickly (one owner). A tyre and battery centre can clip a tester across the terminals, and a hydrometer can show if a cell is dead on a wet battery. One chain tested a customer's 18-month battery as "unstable" and replaced it under guarantee.

Step 2: measure the drain

1. Put a multimeter on its amps range in series between the battery positive and the positive lead. Don't try to start the car like this. Leave it a couple of minutes to settle.
2. One owner says a healthy car shouldn't draw more than about 50 mA (0.05 A). If it is higher, pull fuses one at a time until the reading drops.
3. Fuse 8 feeds several things: the ECU, immobiliser, alarm, diagnostic socket and instrument panel (the same owner). One owner saw about 0.5 A on his NA at that fuse, and a smaller drain from the radio memory circuit. Another says the load doesn't always return when the fuse is put back. See Relays and fuses (NA).
4. Another owner found his drain by checking the fuse box circuits with a meter and then using a clamp meter on the main cables, which led to the alternator.

Usual suspects

• The alarm siren. Two owners fixed their drain by changing the siren. The siren has small soldered-in batteries that can fail and then demand constant charge. One says that unless the siren plug is pulled from the back of the siren (under the passenger side headlight) it is the first thing to try, and he went through three car batteries before working it out. Another disconnected the siren after a battery swap and says the car has been fine since. A new siren, or replacing the cells inside, were both mentioned.
• An OBD dongle left plugged in. A foreign device in the OBD port can keep communicating with the ECU, and the general consensus in 2020 was that the alarm system somewhere is the main leak, even when not armed (worse when armed).
• The radio. One owner asked whether the car had a Sony radio, implying a known issue. Nothing further was said.
• A failing alternator. Two owners say theirs contributed. See below and Alternator testing and replacement.
• An old battery. One owner went from a battery flat after a week to starting first go after three months, just by changing it. Another owner says his battery was fine for 6 to 8 years of daily driving, then needed replacing six or seven times after mileage dropped from 8,000 to 10,000 miles a year to about 1,000. His conclusion: the problem is the small drain against infrequent use.
• Infrequent use. Several say the car has a small leak that is easily overcome with regular driving. The remedy is a smart charger, see Jump starting and battery chargers.
• A battery kill switch is a quick fix suggested in the thread, but one owner says it cures the symptom and not the problem, and another points out it isn't an option if you have the optional central locking fitted.
• An owner with a long-term drain says the alarm draws a lot of power and is tempting not to set, but he always sets it for insurance.

Turns over slowly, or clicks but won't crank

A 2010 case: the car clicked from the starter solenoid and wouldn't crank, there was no fuel pump prime whine, but a charger said the battery was full. It bump-started perfectly (which rules out fuelling, battery and immobiliser, one owner said). Things checked:

• Battery 12.8 V and connections clean, so look elsewhere.
• Earths. One owner says clean the earth bolts, including the area they bolt to, with emery paper, because a tight bolt can still have corrosion under it. Another says there is a connector on the nearside bodywork by the wheel, reachable from underneath, which was corroded on his car.
• The yellow relay in the boot (the starter relay, K24 in the diagram the owner posted). Another owner with the same symptoms had a garage find a loose wire at the back of that relay, which you reach with the rear wheel arch liner out. Changing the relay itself didn't fix his.
• Result: the owner found loose cabling at the starter.
• One owner suggested swapping the relay with one from another car, and the hot-water trick for cleaning terminals (pour boiling water over the terminal). Neither is tested in the thread.