Same here...and bike lives outside 12 months of the year.
Optimates are very good at masking the problem, but I'm sure they're not actually doing a battery any good, leaving it plugged in all the time
But your 'kept outside bike' is used regularly, no doubt, so keeping the battery in tip-top charge? Or maybe it has a larger battery than that fitted to an OE 1200GS, which may need a little help from time to time?
Optimates do not mask a problem at all, nor do they do any damage to anything,
no matter how long they are left connected or whatever type of battery. 15 years of use has established that, to my satisfaction at least. Optimates (and similar devices) simply keep a battery that is not being used, topped up. Nothing more, nothing less. They will
not switch into deep recovery mode if the battery does not need it. They do it very well. BuMW's own smart charger does the same, as will near enough any half decent charger / trickle charger. But they will not keep a battery good forever. Just as a battery that is not charged at all, or is only charged on a bike (no matter how good its charging system) will not last indefinately.
If, when the topped up battery is run on the bike and it is not charging from the bike's system - or the battery is the bike's sole electrical system due to another fault - (broken alternator belt, broken alternator or rectifier, say) then the battery will go flat in pretty short order. That will happen, no matter what.
Lots of people blame the bike and the battery. More often than not it is not the problem at all. People always look for the most complicated solution, ignoring the easiest ideas first. More often than not they are simply not riding the bike far enough to get a decent charge and / or are leaving the bike for weeks or months on end, to simply discharge the battery. Flat twins require a lot of umph to get them started. Factor in modern electrical systems which will not light up, unless a half decent current is there (anyone tried to bump start a modern car with a very flat battery?) and the result is a forgone conclusion.
Or, just as likely, they have have imported some extra drain on the battery at rest: Aftermarket alarms and / or umpteen devices wired direct to the battery (because it's easy) and thus permanently live. How often do we see that touted as a good idea?
Factor in irrational fears fears about de-sulphating* and / or canbus burn out and, just like oil, it develops a life of its own.
Their solution? Spend good money on ever bigger batteries,
to mask the problem, more often than not. Then not charge them properly and still wonder why their batteries are going flat.
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*Some people should maybe do a Google search to see how a battery works and what happens when a battery is left to go flat and stay flat for months on end. It really is very simple. You can make a very basic battery from an orange and two different metals, for heaven's sake. If the plates end up shorted together, or so covered with deposits, that no current will flow, no amount of effort will bring it back to life or keep it charged.