cut out 955i

I have a 2000 pre facelift 955i and the other day it just cut out on me, I have the problem of poor idle on over run when coming down to a stop but then I had no dash lights or headlights, no starting or any effort,
I left or for a while then tried again and the lights were all really very dim, when I tried to start it clicked and all the lights went out, I jump started it and whilst it ran it ran well, a little lumpy but I checked the battery and it was charging at 14.5v as soon as I pushed the horn it stumbled but carried on, I flicked high beam on and it completely cut out again and made no attempt to start again, so we jumped it again and I rode ot home about 10 miles, put high beam on again and it cut out once more, and yet again the lights were too dim to see, this is all very odd as I have been riding it regularly for the past 2.5 months and have not had a single problem.

Has anyone seen something like this before? Could it be something as simple as a dead cell in the battery?

Cheers everyone xxx

 

Yes bike has sealed battery and it has an upgraded one already, I jump started it of a friend's car and it was perfect charging at 14.5v with the lights on, and high beam killed it which I initially thought was a short, but once I left it running for a while on its own to charge the fan cut in and it stumbled and died, I have since removed the battery and put it on charge over night, when I out it back on it primed the fuel pump, pushed the button and nothing so I took it to a bike shop and it showed that the battery was perfect, until put under load where it went straight to bad, so it's gone in the bin.
It was an MF battery so cheap and poo, and I've ordered an exide 14bs battery hopefully it will cure it, trouble is it won't be here until Tuesday so I shall moss out the entire bank holiday weekend of use (n)