Not really enough information. But leaving a fully charged battery connected and leaving sit for 6 months, it would be very possible the battery would be dead during that time duration. All cars with EFI systems have a slight power drain from the battery when the car is off to maintain certain data or settings. Same would be true on newer radios. So this isn't very surprising to have happen.
The part about having a fully charged battery which had enough reserve to do a complete compression test, then disconnecting it for the evening, then the next morning only having 7.9V is a bit more surprising. The battery may have been heavily drained with all those starting events to get compression values on all 4 cylinders. However, if it was still able to turn the engine over the last time, then disconnected, if anything, it should have recovered a bit and with the spark plugs out, it still should have turned the engine over. That said, a battery that was previously drained 100% will not completely rebound. A completely dead battery does damage a battery somewhat. It may be possible your battery now as some plates that have shorted together, that reduces voltage. The long period of being dead may have sulfided plates as well.
I would think you should return your battery for a replacement and try again. I would also highly suggest that when you do sometheing like cause a draw to the degree checking compression pressures will do, you put the battery on a charger immediately - don't leave it at a low charge state for any long period of time. You might want to check the voltage at the battery with the engine running, to make sure you are at 14+V (to make sure the alternator is working properly).
The part about having a fully charged battery which had enough reserve to do a complete compression test, then disconnecting it for the evening, then the next morning only having 7.9V is a bit more surprising. The battery may have been heavily drained with all those starting events to get compression values on all 4 cylinders. However, if it was still able to turn the engine over the last time, then disconnected, if anything, it should have recovered a bit and with the spark plugs out, it still should have turned the engine over. That said, a battery that was previously drained 100% will not completely rebound. A completely dead battery does damage a battery somewhat. It may be possible your battery now as some plates that have shorted together, that reduces voltage. The long period of being dead may have sulfided plates as well.
I would think you should return your battery for a replacement and try again. I would also highly suggest that when you do sometheing like cause a draw to the degree checking compression pressures will do, you put the battery on a charger immediately - don't leave it at a low charge state for any long period of time. You might want to check the voltage at the battery with the engine running, to make sure you are at 14+V (to make sure the alternator is working properly).
