One of the hidden environmental costs of modern gadgets, such as mobile phones, PDAs, MP3 players and digital cameras are the batteries used. Rechargeable batteries are better than disposable ones, for devices which are used often. But for devices used occasionally, rechargeable batteries can have a short life. Perfectly usable devices are replaced when they just need a new rechargeable battery.
Common nickel metal hydride (NiMH) rechargeable batteries will only last a few years, or about 500 charges. Also the batteries will self discharge even when not used, so for low power devices or ones you only use occasionally you will find the batteries flat when you go to use the gadget.
I bought some "Pre Charged" NiMH batteries (technically known as "Low Self Discharge Batteries") to replace the pack in a mobile phone. These cost a bit more than premium NiMH batteries (and about three times as much as the cheapest). They are claimed to hold a usable charge for a year and to be rechargeable twice a s many times. So far they are greatly extending the life of the phone. Currently these batteries are only available in AAA and AA sizes.
It should be noted that rechargeable batteries should only be replaced with equivalent types (same chemistry and size). The NiMH ones are reasonably compatible with different brands of chargers. However, Lithium batteries used in laptops require very specialist charging. You should buy a battery which is specially made for your device.
Many rechargeable devices which have custom battery packs can be replaced with third party packs, even after no longer available from the original maker. Also your old batter can be "repacked": the plastic case is prosed apart and new cells inserted. I repacked an old Ericsson phone battery myself, but this is something which should be left to specialists.
First I made sure that I had a spare battery pack in case I broke the pack. Then I checked the voltage and polarity of the pack to make sure I would make the new one the right way. The I prised the plastic case to find it had four extended NiMH AAA cells. I bought four AAA precharged cells. These were slightly shorter than the existing cells. I the had to solder the cells to make the battery and attach the terminals on the case and check the voltage and polarity with a meter.
At this point I made the major mistake of using my hot glue gun to stick the cells in the case. I then found the cover would not fit due to excess glue. I then reheated the glue and tried again. At this point I got a little desperate and so first stood on the pack to close it and then applied a hammer a few times (do not try this at home). Eventually the case closed and the battery worked. At least I found the batteries are reasonably robust. ;-)
Repacking battery packs for cordless tools is a lot easier, as they are much larger. These packs commonly use sub-C cells (as fat as a C size battery, but shorter). The sub-C cells are commonly available with solder tags attached for making up into battery packs. However, it may be worth replacing the sub-C cells, with the smaller precharged AA cells. In theory this will result in a loss of 40% capacity, as sub-C cells are available with 3300 mAh, whereas the AA cells 2000 mAh. However, the cheaper sub-C cells can be as low as 2400 mAh and are of questionable quality. For about the same cost, high quality AA cells will start out with 17% less capacity, but retain their charge better and outlast the cheaper batteries.
See: Pre Charged NiMH batteries