Battery went flat on the Focus. Stuck jump leads on it for 20 minutes with other car at 3,000 rpm. Now can't get the Focus immobiliser to switch off. Tried disconnecting the battery positive feed and reconnecting, no change. Strange sound coming from down by the fuse box in the passenger side footwell when the ignition is switched on, like an underwater buzzing sound? When the key is turned, nothing happens at all. Just this buzzing sound. How do I get the immobiliser to switch off?

Do you have any equipment to measure the battery voltage? Do your lights look bright? 

Battery was totally dead Micro, the earth lead on the battery wasn't tight after the garage fitting a new clutch on the 22nd. Car hasn't been used since December 24th and the charge had simply drained away as it wasn't being driven. Even the stereo wouldn't fire up today.Does a battery get flatter? lol

Bought a jump starter from Maplins and got it running about an hour ago, after waiting 6 hours for the booster pack to charge up. Been out for a quick 30 mile blast to charge the battery up and it's starting as normal now. Alternator is charging correctly and the battery is showing 13.8v now on a borrowed test meter. Just the engine check light to switch off and it should be fully back to normal, hopefully. We'll see if it starts tomorrow, or is that later today? lol Key fob issue seems to have fixed itself once the car battery had a decent charge in it and I've stuck a new battery in the fob too, just incase. Noise from down by the fuse box is gone, so I take it that's the immobiliser which makes the 'drowning a cat' noise.

On the plus side, my zetec Escort van which hasn't turned a wheel since the end of November fired straight up. Modern cars suck. lol

15 hours ago, raggamuffin said:

Battery was totally dead Micro, the earth lead on the battery wasn't tight after the garage fitting a new clutch on the 22nd. Car hasn't been used since December 24th and the charge had simply drained away as it wasn't being driven. Even the stereo wouldn't fire up today.Does a battery get flatter? lol

Bought a jump starter from Maplins and got it running about an hour ago, after waiting 6 hours for the booster pack to charge up. Been out for a quick 30 mile blast to charge the battery up and it's starting as normal now. Alternator is charging correctly and the battery is showing 13.8v now on a borrowed test meter. Just the engine check light to switch off and it should be fully back to normal, hopefully. We'll see if it starts tomorrow, or is that later today? lol Key fob issue seems to have fixed itself once the car battery had a decent charge in it and I've stuck a new battery in the fob too, just incase. Noise from down by the fuse box is gone, so I take it that's the immobiliser which makes the 'drowning a cat' noise.

On the plus side, my zetec Escort van which hasn't turned a wheel since the end of November fired straight up. Modern cars suck. lol

Just be aware that high capacity batteries such as car/van batteries don't like being fully discharged, it does permanent damage to their chemical make up.  Keep an eye on the starting behaviour for signs the battery is on it's *****, I played battery roulette with the original Motorcraft battery on my 12 year old 1.8 TDCi, it eventually totally crapped out on a January "morning" when it was -3'C....it was 9 years old so it was entirely my fault!