People often assume that riding the bike will keep the battery fully charged. It doesn't, charging systems replace the charge used to start the engine but if the battery was low to begin with it doesn't get fully recharged just by riding it. At this time of year batteries have a hard life, cold doesn't do them any favours. On small engined bikes with smaller batteries the charging system often runs the current used by lights etc but doesn't put a lot back into the battery so they run most of the time only partially charged. A multimeter is a cheap tool and one that is essential. Check the battery voltage after it's been left overnight. You're looking for over 12.5v. Anything 12.3 or under and the battery is almost discharged. A battery below 12v won't take a charge usually. You can 'prime' them by charging at very low input current - around 500Ma or less. A plug in gel cell charger does the trick nicely. Once back above 12v they will then take a conventional charge. What you say about batteries is very interesting and you clearly know what you are talking about. Could you post a couple of examples of the "Gel cell charger" you use or the spec for it. Cheers Ian