Although I disagree with Andy on some of what he has posted about batteries, it is precisely this that he rightly complains of.
The point of greatest stress to cells is as they reach full charge and that is the same for the original charge or subsequent topping up of reducing content. Unplugging the charger to prevent it intermittently topping up very small losses avoids those unnecessary repeats of maximum cell stress.
.[/QUOTE
I think you'll find the latest intelligent chargers cover that aspect. In the case of the latest Bosch Powerpack batteries it specifically states no harm can be done to the cells by keeping it on charge. The battery, if interrogated on dealer equipment, can tell you the precise number of charges and how long, and even keeps a tally of partial charges then converts them to the equivalent of full cycle charges for warranty purposes. The topping up of natural drain is controlled by the battery system and, supposedly, these top up charges are not as aggressive as plugging in initially for charging after use. It seems logical to me, but I'm no battery technician, that if the battery control system KNOWS it's just topping up then that charge is not necessarily going to be the max cut off of 4.2v. Perhaps on the top up charge the cut off is slightly lower; I just don't know. All I know is that Bosch categorically state leaving it on charge does not harm it (and the battery records all use) and they back that up with a decent warranty so I have no worries on that score. Any use I get over the three years guaranteed life is a bonus as far as I'm concerned. It is, quite frankly, an acceptable running cost even at retail price. However there are other options such as having it revelled for a third of the cost of a new one and I get the chance to upgrade with higher capacity cells at the