Actually, the usage of the reader is less that of a laptop, and more that of a cellphone.
On a laptop the battery is best preserved to,when you can, plug it in the wall when using the laptop.
That way the battery remains fully charged, and while using it,you actually use the AC instead of wearing the battery.
Though the same can be applied on the reader, due to it's enormous small powerdraw you most likely not need to plug it in always.
A reader, as with a cellphone, uses way less electricity, and the battery remains charged longer (unless you have these cheap cellphones that only last a day).
On the other hand, I own a Sony PRS-505, and can deplete it's battery in less than 1 hour, by:
- indexing a large amount of files (turn off the power saving mode for this).
- viewing large sized pictures (in my case 4MB 6Mpix photo's from my digital camera).
- opening a book, changing fontsizes to all zoom level, and proceed to the next book to change fontsizes over there,and repeat for a few hundred of books.
- Fast pageswapping through comic books in pdf format(takes little longer)
When doing such tasks I'd also recommend to keep the reader plugged in the wall, if possible.
I myself don't really need to charge the reader, since I upload,and modify plenty of books on the reader's internal memory.
When plugged in a usb port, the reader also charges.
Obviously keeping the reader plugged in at all times will cost you more on electricity (by keeping the AC adaptor plugged in,or your computer running to provide power for the usb port).
When I use it in the evening reading in my bed, I plug it in the wall (AC,not USB) when I see the battery is getting empty.
Usually by the time I finished reading the battery is charged. That way the hour I spend reading, I charged the battery without ballasting it. 2 tasks in one done!
If you can live by such strict rules, I'd recommend yo;but if not then most likely 2 years from now my battery will live 5-10minutes longer than yours on a full charge,so you won't really lose much either way.
Current Li-ion batteries are pretty good.
Generally a normal charge takes less than 1 hour; it takes 2 hours to charge from a completely empty battery on the AC adaptor. Charging it via USB takes upto 4 hours, and you can not simultaneously read books. You can use the SD cardslot and internal memory of the reader to add files or something...
In my opinion it does not really matter if the battery indicator says full,1/2 or 1/4 charged.
The indicator is deceiving, in a way that upon booting the device, the battery meter may indicate that the battery is full, while in real fact it can be 15minutes away from being completely discharged.
Only after using the device a few minutes can you see the real charge.
Seeing this, it is probably safer to charge the battery while the indicator is 1/2 rather than waiting until it is empty.
But I don't really know if 3/4,1/2 or 1/4thof the battery will make much of difference.
The battery used in the reader is a single cell, with no internal hardware to keep track of charging cycles, so I might say it's safer to charge while reading, and while the battery indicates 1/2 charge.
Unplug it when you're not using the device. This saves you energy, and less chance of something going wrong:
I don't know about other products, but sony's products are generally very safe products, and i could not imagine the Sony reader catching fire, but where ever electricity comes to play in a plastic little box, there always is chance of fire.
Be it not the reader, then the adaptor, be it not sporadically, then through a lightning strike on your electricity net... the thought is a bit far fetched, but the possibility is always there.
The fire hazard of electronic devices has dropped drastically compared to electric devices, but it still is not completely nihil.
Another thing,the reader doesn't need to catch fire; but a lightning stroke can permanently damage the reader too,when being plugged in!
Chances are less that this could happen when not being plugged in.
You save the energy of the adaptor being on,using between 0,1 and 1W when not in use.
Do this for all your non-used electronic and electric devices in your household,and you can save about $75 per year on electricity cost! That'd translate in a nice dinner for two, at a fancy restaurant; or the purchase of 3 to 5 books, whichever you prefer.
The Sony reader has a build in protection circuit against over-charging the battery, so you don't need to fear about leaving it plugged in too long.
A general piece of advise is to buy extension cords with a build in switch,that can deflect (block) lightning strikes. And turn off the switch, when not using a device.
It's always greener and safer!