Bluemeanie - don't use your battery tender on your maintenance free battery unless they indicate it is capable of doing so.
barney - never leave any kind of battery on charge unattended, a weak connection can cause a serious fire hazard. If you need to take off for a couple of days, disconnect it and reconnect on your return. I'm sure your insurance deductable must be worth at least 10 times as much as a new battery.
Perhaps I may shed some light here.
Flooded Lead Acid and Maintenance Free batteries have different discharge curves, static voltage and Ah capacities (assuming same case size). Mixing and matching charger / battery types is an expensive mistake.
MF batteries have a higher voltage when at full charge and full discharge than FLA.
This can be advantageous but due to the non linear discharge curve can leave you with a suddenly unexpected discharged battery.
FLA batteries have a lower operating voltage, but can sustain operating voltage much deeper into the discharge curve.
In my experience a properly maintained FLA battery will actually outperform and outlast a MF battery.
In lay terms, Auto - off chargers monitor static and charging voltage as well as charging current to really center in on the internal resistance of the battery. Superposition is used to charge batteries (charger voltage output is higher than static voltage of battery, turning it from power source to consumer). Once the charging voltage and current have reached a preditermined rate, the charger will turn off. Charging rate is usually controlled by SCRs. Altering the firing angle can produce full to nominal charge with no moving parts.
A MF charger will overcharge a FLA battery, and a FLA charger will undercharge a MF battery.
IMHO, batteries should always be seperated from the bikes during storage, kept indoors and charged in a well ventilated area if possible. Keep in mind that alternators are intended to provide a maintenance type charge, not a deep cycle charge.