batterycharge fault citroen xsara picasso?

i have a y reg picasso, i went out to it two days ago and battery was dead wouldnt start. So looked for a reason then realised the lights were turned on therefor turned them off and tried to bump didnt work so i changed battery to a spare i had still didnt work. left for a few hours put a bit more petrol in (just incase) then it started. yay i thought, then it came up with battery charge fault so i just thought the battery was so low. i sat there for a few mins reving engine to charge battery and it just died. i know a little bit about cars so i changed the alternator and belt but still same prob so i bought a new battery replaced thinking thats it it must be it but noooo. it wouldnt start i mean the starter wouldnt make a noise. so my wife helped me bump it and for a few secs everything was fine. I started to drive home and again the battery charge fault came up on the display. HELPPPP please. ive checked the connectors under the bonnet and both plugs to the alternator are connected.