I was talking about maintaining a battery which keeps its ability to hold a full charge. ie. By only charging the battery to ~90%, you can discharge/recharge it many more times, and still get close to the original life out of it, rather than if you charged it to 100% every time.
If that's based on what "some bloke in the pub told me", I strongly counsel you to check if it applies to the chemistry you're using. This is not my field, but I know that best practices are not portable between e.g. NiMH and Li chemistries.
Heat does seem to reduce a batteries operational life, and putting the last 20% of charge in a battery causes it to get hotter than the first 80%, so it seems reasonable that not trying to push the last 20% of charge into it would increase its life. This wouldn't be true for batteries that suffered a "memory effect", but lithium batteries (in pretty much every laptop) don't, or at least don't in the chemical sense like the NiCd batteries did.
I'm sure google will find some useful studies on this, as well as stuff that people heard from some bloke in the pub :)
This page http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batt... has a few tables showing charge level vs discharge cycles before battery death. In particular: 4.30 (110% charged) 150-250 cycles 4.20 (100% charged) 300-500 cycles 4.10 (90% charged) 600-1000 cycles 4.00 (70% charged) 1200-2000 cycles 3.92 (50% charged) 2400-4000 cycles So I guess 80% is the sweet spot of battery life vs battery usefulness... only ever charging to 50% would give you 8x the battery life, but then each cycle is only giving you half the value and you risk discharging deeply more often which is even worse (see other tables on the link). James