I have been recording the dates when I have charged my 650 since I got it in late September.
I have recharged it when the battery meter showed 2 bars or 50% charge used/remaining.
For the first three months I have been powering off the reader between reading sessions, for the lasy two weeks I have been putting it on standby between reading sessions.
Here are the dates it has been charged and the number of days between the charges:
Charge dates and days between charges:
Powering off/shut down reader between sessions.
(date format DD/MM/YYYY)
23/09/2010 - first charge, full charge overnight on date of purchase.
26/10/2010 - 34 days since last charge.
28/11/2010 - 34 days since last charge.
19/12/2010 - 21 days since last charge.
On standby between reading sessions.
31/12/2010 - 12 days since last charge.
11/01/2011 - 12 days since last charge.
25/01/2011 - 13 days since last charge.
Averaging 3 - 5 hours reading/usage per day.
So it would seem that the battery lasts longer between charges when powering off/shutting down between sessions rather than putting it in standby.
Over the lifetime of the ereader's lithium ion battery (3-4 years) the less it is charged the longer it should last. Lithium ion batteries usually have a 'lifetime' of 300 - 400 charges, so the less often it is charged the longer it should last ie 12 -14 charges a year (approx. monthly charges) with powering off the reader betweens sessions as opposed to fortnightly charge, 26 - 30 charges a year, when putting the ereader on standy between reading sessions.
I will be continuing to put it on standby for the rest of January to see what battery usage is like in standby mode.
Of course all this may be moot, who knows what ereader devices will be available in 3-4 years time.