Help please - not starting after reconnecting battery

[Seaside Steve]

My 2003 2.5L S-Type will not start after reconnecting battery. I have been trying a few times over the past week or 2. The car has not been driven for a few weeks. I thought front seats might have been causing battery to drain because I fitted different seats, so today I removed the 4 fuses in the boot (trunk) fuse box for seat motors. This did not seem to have any effect.

Before not starting just now the battery before reconnecting was reading over 15 V. The starter motor does not spin and I get lots of warnings and instrument needles go right up. The same as last weekend as in this video.

After not starting just now I disconnected battery and the voltage is still about 13 V. Despite the warning there is some fuel in the tank as well.

[Seaside Steve]

I have got the car started so the problem is solved. Found out some stuff as follows.

1. My multimeter was reading the voltages wrong because its battery was on the way out.
2. The car battery is pretty much dead.
3. Good news - I got it started after buying one of the fancy lithium battery jump starters

Interestingly, I discovered that all the warnings and the gauges going mad were caused by depressing the accelerator down slowly twice as advised after reconnecting the battery. The exact sequence that showed this was:
- engine started fine with jump starter
- I remembered about the accelerator pedal throttle reset
- turned engine off and switched ignition to position II
- pressed accelerator down slowly twice
- warnings came up and gauges needles shot up to max and car would not start
- then I switched ignition off completely and left it for about a minute
- tried to start it again and it started fine. Note the jump starter was still connected.