Alternator fault warning light

Hi guys jumped in the p38 this morning turned the key to start and was welcomed with a alarm on dash saying alternator fault when started her up the warning light on but when she fired the light and alarm all went off. this afternoon checked the alternator terminals all tight and clean, but on the last couple of start ups got the same warning just for about 10 seconds. Ever since ive had the RR have had some niggles on starting I work away from home all week and only use the RR on weekends, every other weekend the battery is down and will not start and ive replaced the battery for the correct amps etc if shes used eveyday no problems. ive noticed if locked she pulls the battery down if unlocked it doesnt WHY IS THIS????. done a quick check and the alterntor is putting a charge in the battery do you think alternator is giving up the ghost?? with an intermitant fault looked in to the price of one and found a site selling them for £166. what amps would you suggest i possibly invest in to replace. sorry to bore you all but some help needed please. RR is a 2.5 bmw 1998
thanks :doh:

groan, if you used the search feature you would find that the alternator is ****ed
(test with multimeter) or corrosion on bcem plug

i get that alternator fault warning very rarely. changed battery as terminals were totally dry and now car starts fine but still had the warning about 3 times in last 3 weeks.
tested and all fine.
dont no why it still saying the fault.

thanks for quick reply guys as a new owner havent come across the search feature and have put meter across terminals when running got 14volts
I would of thought if alternator fault it would show up when driving maybe wrong it dont do it every start up. and like you mikey ive just had new battery fitted.

For the love of god,if alternator is charging correctly then the reason you have alternator fault displayed is BECM. which normally is corroded plug on it not giving correct information.

Make sure your EKA code works, disconnect battery and clean plugs with brass brush (suede brush and electrical contact cleaner)

Check the connections at the BECM, the alternator charge light circuit goes through the BECM which is how it detects that the alternator is working. If the connections are corroded it will indicate a fault. Otherwise it could just be the battery is down. If it goes down when locked it's likely to be the RF receiver waking uf the BECM due to picking up signals from wireless doorbells, WiFi etc. Disconnect the blue lead from the RF receiver to see if it cures the problem.

Another thing could be a faulty under bonnet fuse box. My battery light flashed on and off when you opened the drivers door! Replaced the fuse box due to a burnt fan relay contact and it cured that problem as well!! It was proper fu~ked!! Give it a sniff, if it smells burnt replace it.