currently using iphone 3gs. i always charge the battery to full before going to bed(1-2am).the next day the % always drop to like 90% (alarm 8am). and after 1-2 hours the battery drop to like 85%, i only check the message once or two times in this period. so is it normal?

Guidelines for prolonging lithium-ion battery life

    * Lithium-ion batteries should never be depleted below their minimum voltage (2.4 to 2.8 V/cell, depending on chemistry). If a lithium-ion battery is stored with too low a charge, there is a risk that the charge will drop below the low-voltage threshold, resulting in an unrecoverable dead battery.[citation needed] Usually this does not instantly damage the battery itself but a charger or device which uses that battery, will refuse to charge a dead battery. The Battery appears to be dead or not existent because the protection circuit disables further discharging and there is zero voltage on the battery terminals.

    * Lithium-ion batteries should be kept cool. Ideally they are stored in a refrigerator.[citation needed]
    * Aging will take its toll much faster at high temperatures.[36]

A lithium-ion battery provides 300-500 discharge/charge cycles. The battery prefers a partial rather than a full discharge. Frequent full discharges should be avoided when possible. Instead, charge the battery more often or use a larger battery. There is no concern of memory when applying unscheduled charges.

Although lithium-ion is memory-free in terms of performance deterioration, batteries with fuel gauges exhibit what engineers refer to as "digital memory". Here is the reason: Short discharges with subsequent recharges do not provide the periodic calibration needed to synchronize the fuel gauge with the battery's state-of-charge. A deliberate full discharge and recharge every 30 charges corrects this problem. Letting the battery run down to the cut-off point in the equipment will do this. If ignored, the fuel gauge will become increasingly less accurate. (Read more in 'Choosing the right battery for portable computing', Part Two.)

Which mean, to keep fully charge battery is recommended to prevent dead battery? Correct me if i'm wrong. Cuz i always plug in suka hati. afraid battery kong. 

BTW, thanks for the info guys.

iPhone's battery life is based on charge cycles rather than the 'charge only when near zero' principle.

If you are seriously concerned about your battery usage, please read this thread from the start. Not only will you get an answer to your question, you will also learn much more.

@ortoklas7
Yes, you can send to Apple/Mac to change new battery.
About the charges, I'm not really sure.
But it takes like 3 days to replace you a brand new battery and when you get back your phone, your phone would be a fresh phone.. Data gone..
Which means they will restore your phone in front of you due to privacy issue like stealing details from your phone or whatever is in your phone.
Of course, they'll ask for your permission first before restore - ing your phone.
Correct me if I'm wrong.

Everytime I'm charging my iphone with my laptop usb most of the times, is that okies? I'm worried that will kill the battery ;/