john west wrote:
There's a lot of information about that if you keep a Laptop battery
charged between only 80% full and only 40% full, it will be servicable
over the years for four times as long.

Other information I find says it is the charging and discharging that
wears the battery out.

There's several things that control the life of rechargeable batteries:

The quality of the cells
The temperature they run at (and available cooling)
The loads they get put to (see temperature)
The smartness of the battery manager in terms of charging, discharging and
balancing.

As always 'it depends', but I'd suggest the battery manager is smarter than
you are are regards maintaining the optimal state of charge to keep the
battery happy. The only thing is that it has no prescience over what you're
going to do in future - eg store the laptop for months unplugged - but
otherwise I'd suggest it's best to leave it to it.

That doesn't apply so much to old laptops (dumb batteries), cheaper laptops
(poorly written management software) or removable batteries (in some cases
the manager might be in the appliance or charger not in the battery -
unlikely with laptops but can apply to eg power tools).

The laptop i have (with a built in battery) is mainly used in one room,
so it is easy to keep it plugged in *all the time* to mains electricity.

Would it be better to prolong the life of the battery to keep it plugged
in all the time or to go for the 80% - 40% option. Thanks.

It'll depend on the battery manager in that particular machine, but one
thing to say is keep the battery cool. If the laptop gets hot, that'll
degrade battery life. Also it's worth using the battery from time to time -
don't leave it on charger forever. But otherwise it's going to be pretty
hard to maintain 40-80% without causing lots of charge/discharge cycles
which will wear the battery.

Theo