If you search for the battery by its voltage and amp hour rating, which
you can calculate from your load requirement; you can find it by Google
search. Given what you told us, you need at least 4.2 AH rating, so
lets' say a 5 or 7 amp gel cell, as I know 7AH gel cells are common in
alarm applications. You don't want to pull a cell down all the way, so
figure you will only pull 80 per cent of the available current and time.
Enter in Google "12Volts DC, 7 AH" in quotes, and see if it leads you
to a specification sheet. Or simply look up "Gel Cells" and read the
manufacturers catalog data sheets and notes on max load you can pull
from a given size.
Always pick a size larger than your exact requirement for best life.
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