Fault code PO300 and battery problem

Question

My car has had engine light on for a while, 2 weeks ago the battery started going flat.
I took it to the garage, I paid for battery check and charge , I was told immobiliser was draining the battery.
Diagnostic test on engine, multiple misfire , he changed the Sparks the situation still showed a problem, he said he checked ignition coil and other bits all fine.
He said cylinder 3 was losing compression and it would be a big job , estimate £400 and that he would fit new time belt within that price, so it's been booked to go in this coming week.

My worry leading to my question is, a few days ago the battery started being flat again every morning, the mechanic said for an extra 35 he would pop out to start it.
I said no thanks ( I had already paid 24 for battery check, and 107 for sparks plugs and the diagnosis of cylinder problem.
I used my breakdown on my insurance, the Rac came out, got battery going but opened the cells and said there's no water in the cells that's ur problem, and you may have ruined the battery by charging it to often with no water, he also said this could have caused low compression .
My problem now is wether to trust the garage as I paid for battery check and it seems they didn't check it.
Could the cylinder problem be a result of battery with no water and could the compression be remidied much cheaper??
Would really appreciate some advice.

Answer 1

The only way to loose the water in the battery is when there is a overcharging condition.If the compression is low. this would be a mechanical condition the route you went for the ignition change and sparkplug are good, but unfortunate its is mechanical