Things were going well: the Five was behaving itself, starting on the button and driving superbly for a 21-year-old. Until that is, I left it unattended for a couple of days. When I picked it up originally, the previous owner had needed to jump start it using his wife's car. This should have sounded the alarm bells, but it was cold and wet and I wanted to go home.

So in I came one morning last week to find that the driver's door would only unlock on the key. Of course, a quarter turn of the ignition revealed the battery was completely dead. No matter, I thought, I'll just jump it back to life.

Shortly after, it was back with us, and after a further 20 minutes at idle I got behind the wheel to run it up and down the road a few times and get some charge in the battery. However, a quick test of the headlights killed the engine instantly, and although it would turn over and fire by itself, the request for extra power for the lights resulted in the same thing.

I'd been assured by the previous owner that the Five had been treated to a new battery just two months back, but what greeted me as I lifted up the rear seat bench to access it reduced me to swearing and the foetal position. 

A previous battery had clearly given up the ghost and spewed its discharge into the floor plan. Said acidic discharge had then eaten away at the paint and soundproofing, leaving behind what looked like a lethal tiramisu with a car battery sat on top.

Cautious that there may now be a wider electrical issue, I called the AA into action. The Bimmer's alternator was working just fine, and the battery was charging, albeit slowly, but one reason for it draining so quickly was that the boot lights were staying on even with the car locked, so out came the bulbs. 

Even so, it was clear this new battery wasn't big enough to do the job, and even after a trickle charge it wasn't up to powering the engine and headlights at the same time. A quick check of Euro Car Parts' website revealed there was a more powerful version of the same brand available - and with 50% off, too - three miles away. Half an hour later the new one was in place and I had headlights!

The next job was to clean up the mess, which required a trip to Tesco, oddly. I got some very funny looks at the till, but rubber gloves, baking soda and wet wipes purchase later, I was ready to tackle the problem.

It wasn't a pretty job: the acid had got everywhere, but crucially the surrounding wiring looked fine. With it cleaned up, I placed the new battery on top. The next step is to buy some rust-inhibiting primer and give it two or three coats of that, followed by a rust-inhibiting paint.

Another lesson learned, then: always check the battery when buying a used car. Thankfully, a very firm push on the floor pan hasn't revealed any signs of weakness in the metal, but if I'd left it for much longer, things could have been a lot worse. 

I now have a working car again, even when it's left on its own for a few days. I'll be back next week to update you on how the primer and paint work out. There's a first time for everything!

Part one