alarm draining battery

my uk spec galant has a cobra alarm is this factory fitted or aftermarket?
the reason i ask is over the last 2 weeks when i try to start the engine it wont. after numerous checks i found it to be the battery was faulty.
it was checked under load and one of the cells was caput.
bought a new one and fitted it thinking problem solved but the next day i went to start it i have the same problem.
my alarm has been going off randomly for nothing also over the last few weeks.
so as a test i have been leaving my alarm off.
this solves the problem.
is this a common problem? can i check and fix this myself? or do i need a new alarm fitted.

the alarm might be going off because of the voltage dropping, it might not be draining it. have you checked if there is anything else draining it?

the reason why it is going off is because of the battery being low voltage

seems i was wrong with the alarm draining the battery.
i didnt use the alarm last night and when i got up it was lazy starting again. took me 5 attempts to get it to start.
1st turn it flatly refused to start
2nd it started then the revs died
3rd nothing again
4th started so i reved to keep it going but it revved high and low then gave up.
last try it started so i left it a few mins to warm up and its been fine all day???

my knowledge of engines is very limited but a freind said give the air filter a clean so i blew some compressed air over it.
he also suggested some spray he has that evaporates leaving no residue to clean the mass airflow sensor so i tried that.
hopefully this sorts it??
any other things i could check or clean?
i changed the spark plugs 4 months ago for ngk platinum with only 3000 miles done since then.
also when the engine is warm i can switch off the engine and it starts fine.
the problem only occurs after its sat for 10 - 12 hrs.