stumped by slow starting

[mr burns]

ok i,ve been having a problem with my battery slowly losing power until between one to two weeks i get the clicking sound with dead battery, i thought it was a duff battery so changed it new ones a yr old now and when i charge it the cars fine starts on the button no problem i then thought bad earth so cleaned them all up then onto the alternator checked that bout 13.8 volts at battery when idling and when under load the same ( lights and blower on) then thought something draining but i use it as a daily so the alternator as said i think is ok should be topping battery back up so why am i getting this problem??. Can anyone help please and is there anything else i need or can check, thanks in advance ideas most welcome if to just save my sanity 

[mwep201081]

what distance do you cover in your daily journey?

[mr burns]

approx 15 miles each way, also forgot to mention its a new alternator belt and i,ve checked its not slipping and tight

Thanks

[rich1977]

mines doing the same but think its my alarm and imobiliser draining it as thats the only thing I can find drawing any kind of current when car isnt running, and then with it being winter and only doing short journeys with lights blowers stereo and rear screen heater on the alternator doesnt really get chance to re-charge whats lost

[boxer1968]

last time mine drained my boot light was not making contact properly over time it drained my battery worth a check!!! plus batterys  do take a beating this time of year you could be just unlucky and have a duff cell!!