stair lift problem with charging?

Does anybody know how to fix a Sabre Minivator model 1000 stair lift? It keeps bleeping both at the top of the stairs and the bottom. I have replaced the battery, cleaned the contacts top and bottom both with emery paper and with electrical spray. The contacts definitely are working, and the bleeping subsides for a time, then starts again within a few hours. The bleeping indicates it is not charging the battery, there is a green light that flashes on and off, then either goes out totally or sometimes is a solid green light. I have checked the electrical output with a voltage meter and it is intermittent. The stair lift is in permanent use by two 84 year olds (my in-laws) and was installed brand new in Nov 2005. Does anybody know of any other possible causes of this fault? Also does anybody know if I remove the chair from the runner to investigate other electrical contacts (which are not easy to access with the chair in-situ) how easy is it to fit back onto the cogs? Does it require a special tool? I have a good knowledge of moving parts, electrics etc. as I am an engineer by trade, but do not want to attempt to repair a piece of equipment that is so valuable to two vulnerable people in case it is not straight forward. Does anybody know if there is a website where I can access drawings of the complete workings of this model? I would appreciate any tips whatsoever. Many thanks in anticipation of a reply.