Not to hijack this thread, but I use a tender called an optimate 4. It charges, tests, de-sulphates and "restores" batteries. I use it on all my bikes and it works great.
Here's a review of the unit.
OptiMate 4 - webBikeWorld
The optimate series of maintenance chargers seem to be the proffered option on this side of the pond although i only have the optimate 3 i have 1 in each shed secured to the rafters so that the charging cable can be dropped down to the bikes when needed great product and keeps my batteries alive during our long winters
Brian