The car has a small backup battery in the boot. It is used with the "secret button" to let you unlock the doors with the key fob when the main battery is dead. One owner explains the button connects the backup battery to the alarm circuit, so if the main battery is flat you can still unlock with the fob. The thread notes you need the fob and the button together. The pictures carry the Photobucket watermark.
Do you need to change it?
- With the ignition off, press the secret button and listen for the click of a relay somewhere round the front of the car. If you can't hear a click (and it isn't just noise around you), the backup battery is probably flat.
- Most cars are many years old and nobody has changed it. Several owners found theirs had been dead for years. One found his had leaked and exploded and was "sitting in a pool of white powder".
- Check it now and then with a multimeter, as nothing else should use it (one owner's advice).
Where it is
Open the boot and look above the nearside wheel arch for a black plastic box. The battery is underneath it.
Changing it
- Remove the cover. There are three Pozidriv screws: one on top of the box facing you and two that go into the wheel arch. Catch the screws and washers before they roll beneath the boot carpet.
- Undo the terminals one at a time and cover each loose terminal with tape so it can't short out. With the ignition off the wires should be dead. One wire has a red band, which marks the positive. Note which goes where.
- Undo the two nuts (10 mm) that hold the battery clamp, and lift the battery out. The owner found a small sign of leakage on the clamp after about eight years. The photos show rust on the clamp.
- Fit the new battery and refit in reverse, with the right terminals on the right wires. If the replacement is a motorcycle battery supplied dry, follow its instructions for filling. Sealed types are said to be fine mounted on their side.
Is it charged by the car?
The thread disagrees at first. One owner reads the wiring diagram as showing it on a constant supply through a 20 A fuse, so it shouldn't need charging, though he adds that if you let the main battery go flat this battery will probably fail too. Another owner who read the diagrams in 2014 notes the feed goes through fuse FR1 and then a diode (V25), which he thinks stops the backup battery draining into a flat main battery. One owner suggested adding a relay so it only charges while the engine runs. That is a modification idea, and nobody reports doing it.