Some sound advice from Wired UK:

A phone battery is made up of two layers, one of graphite and the other of lithium cobalt oxide. Energy releases when lithium ions move from the graphite layer to the lithium cobalt oxide layer. Charging up a battery shifts the lithium ions back the other way, preparing the process to start again.

What you this tells us is that the real sweet spot for a battery is a 50 per cent charge – when half of the battery’s moveable lithium ions are in the lithium cobalt oxide layer and the other half in the graphite layer. This equilibrium puts the least amount of strain on the lithium ions that power your battery, and basically extends the number of charge cycles it can take before degrading. Therefore it’s best to keep your your battery between 20 and 80 per cent, and regularly top up with partial charges.

The article also talks about badly engineered apps and the downside of trickle charging - which is what I was doing with the Belkin.