re trouble starting

hi guys.
had a problem for a few weeks know re flat battery. if i use every day no problem. about 2 weeks before xmas put it into local 4x4 garage, not main stealers. garage has a good reputation. he checked battery & alternator. says every thing alright on this front. he said that computor under front seat had something wrong with it & would have to be taken out & sent away. as when in sleep mode draining battery? i am not saying he is pullig a fast one but has anybody heard of this problem before. i have disconnected rf reciever under back parcel shelf in summer & this seemed to work until the cold weather. went to pick my daughter up from work the other night left side lights on for 20 min battery flat. any advice on this problem would be appreciated. am in witney nr oxford if anybody has a comp to check out for me. willing to pay?:doh:

Three possiblities, Battery doesn't like the cold, quite a common problem. Batteries don't work as well when cold. Alternator not charging properly. Easy to do a rough check with a volt meter. Check battery voltage before you start engine, should be around 12.5 volts, start engine, voltage should rise to 14.5 volts or there abouts. Lastly BECM waking up. Even though you have disconnected the aerial from the RF receiver, it may be that in the cold it is malfuctioning and waking up the BECM.

As Data has said really. But NOTHING finds out a dicky battery better than frost.