1. The camera is a Canon EOS 350D (described in my original question!)
2. The camera is 8 years old (also mentioned in my original question!)
3. The main battery is also 8 years old and the reserve battery 6 years old. The reserve battery has only been installed twice. When I charge the batteries I usually only charge until the green light appears on the charger. Do you advocate a longer charge?
4. The camera has nor received any severe shocks nor has it been used in any extremes of climate or conditions.
5. As far as I can see, the battery terminals are clean.