I wonder how can one prolong the life of a digital camera battery. Some times I would go about 2~3 months without using my camera. When I try to pick up the camera again, I found battery no longer has any power left.
Should I, before the 2~3 months idle time, fully charge my battery and let it drain naturally, or should I leave it as it was after a normal use (say 25% battery left) and let it drain naturally during the 2 months ~ 3 months idle time?
Short:
Turn camera OFF when not used.
If that makes no difference, remove battery from camera. Just opening the battery door so contact is broken should be enough. (ie camera will not turn on.
If using in standby look to see if any optional features make a difference (GPS, Bluetooth, WiFi, ...)
If battery discharges completely from full in 3 months when out of camera ask for a warranty replacement.
Longer:
A modern Lithium Ion battery should retain the large majority of its charge over 3 months. If it is draining in-camera then the camera is probably draining it. If it is draining out of camera and not connected to anything else then the battery is probably faulty.
If the camera is turned off it should not cause any battery power drain. This cam be checked - see below.
If left in standby mode some cameras shut down to very low levels of power drain and others don't. Internal features like GPS or bluetooth or WiFi can be active continually during standby (depending on brand and model and features equipped) and can be responsible for substantial drain in standby mode. I have had a significant number of Minolta & Sony DSLRs, all of which except my current one use minimal power in standby. My A77 has GPS which will drain a battery completely in about a day - As I value rapid response time I disable GPS unless I specially require it.
Lithium Ion batteries (used in most modern cameras including the Sony NEX series) have two stages of charging from "empty".
They initially charge at constant current ("CC") (set by the charger to meet allowed max battery current) until a maximum allowed voltage is reached
And then charge at constant voltage ("CV") (with battery chemistry controlled decreasing current) until some end point current is reached.
Maximum battery longevity (total lifetime before you need to but a new one in normal use) is achieved if you stop the charging at the CC to CV changover point. This is probably what Mattdm was referring to when he said "40% charge" but it's actually typically in the 70% - 80% charge range. BUT it is at about 40% of charging TIME which may be where the 40% came from. Because ...
Max charge rate in CC is usually at the 1 hour rate - if you charged at this rate for 1 hour the battery would have received a full current ration BUT the nature of the chemical reactions does not allow you to do this safely. Instead you charge at CC until Vmax is reached and this happens to be for about say 75% of the 1 hour rate - or typically about 45 minutes. THEN the charger enters CV mode and current tails off and it takes about 2 to 3 times as long to add the extra 25% or so to get full capacity. The actual time varies both due to manufacturing differences and because the actual cutoff current as a % of CC max varies by design. If cutoff current is low the battery gets more charge, it takes longer AND the battery cycle life goes down. If current is cut off at say 50% of CC max the end capacity is reduced by perhaps 5% to 10% BUT cycle life goes up.
You could do experiments to find out what your charger does but a safe approximation is to charge for say slightly under one hour from empty. If your charger usually takes 3 hours for a full charge then maybe 1 hour from empty. If it takes two hours then maybe 50 minutes.
Checking power drain:
For the excessively enthused :-)
Charge battery fully.
Place in camera set to standby.
Leave for long enough to be useful - perhaps 1 week.
Check loss of capacity.
Camera may have battery state meter.
Charge to full and note time taken. This is a guide only (as above)
Charge fully again and repeat step 2. for each of
Compare
Storing the battery in a refrigerator ABOVE FREEZING POINT (0C / 32F) will probably increase lifetime and slow self-discharge rate BUT this should not be necessary.c If doing this, allow to warm to say at least above 10 C before use.
After-market batteries are usually MUCH cheaper than originals.
Most but not all are close but not up to capacity of originals.
Batteries marked as above original capacity never are (in my experience) and are usually modestly below,
and those marked as being MUCH above original capacity have greater chances of being shonky-junk.
Cycle lifetimes may be lower BUT overall value for money is usually much better.
When buying after market batteries only buy from sellers whjo offer a warranty (at least 6 months, preferably one year). Keep the paperwork and box and mark the battery indelibly so it can be identified. This all is because a small proportion of after market batteries are junk and fail rapidly or have low capacity or capacity fades rapidly. Being able to return them if faulty makes the experience painless enough to be worthwhile.
With lithium batteries, you get the longest overall lifetime if the batteries are stored at about 40% charge and kept cool. If leaving them in the camera is actively draining te battery, and you're expecting a period of disuse, you may want to discharge to as close to that level as you can guess an then stick the battery in the fridge.
