Great post...but one thing I didn't see mentioned...NiCds suffer from a memory effect where unless you are regularly fully discharging/re-charging them, not just topping them up with the charger, they tend to lose their full capacity quickly. NiMh do not have this problem and so you don't need to replace them as much as NiCds.
MaHa makes a great battery pack charger that can handle both NiCds and NiMh's in many voltage/capacity ranges. I use it to fully cycle (i.e. us the discharge function to drain the battery, then automatically re-charge it) on my 1400 mAh NiCd battery in my Marine Radio. And also to charge differnt battery packs around the house.
Check out
http://www.mahaenergy.com/store/viewItem.asp?idProduct=185 for more info.
Mike.
As a flyer of electric powered Radio Controlled aircraft I have a fairly good knowledge of rechargable batteries. There are currently four technologies which would concern us here:
A. NiCd - (nickel cadmium) commonly known as Nicads. These are the most common type of rechargable batteries and available at just about any hardware store.
B. NiMh - Nickel Metalhydride - these have a capacity that is about 150 to 200 percent the capacity of an equivalent sized nicad.
C. Li-ion - Lithium ion - these are a step up again from both NiCds and NiMh
D. Lipo - Lithium Polymer - these are the batteries that the RC world in a state of excitement. They have a capacity perhaps six times the capacity of a NiCd pack of the same weight.
Li-ion and Lipo are the battery technologies that are used in laptops and mobile phones. These batteries also require that EXTREME care be used when charging as Lipos especially can cause serious explosions and fires. There is a lot of energy stored in those little babies when charged. Overcharging will cause the cells to balloon and explode. Mobile phones and laptops have sofisticated circuitry built into the packs to ensure safe charging and discharging. We RC'ers though construct packs with no such circuitry as we require packs of various voltages and capacities which are not used elsewhere. There have been several near misses and a fair bit of burnt property: cars, houses, garages that have been caused by incorrect handling of LiPo's. Unless you can adapt a mobile phone battery to fit in your GPS (and it happens to be the correct voltage and supply the required current) I would say that this technology is currently not suitable for use in a GPS that is not designed for it. I am surprised that Garamin doesn't have batteries similar to mobile phone batteries in their GPS's. It could be due to the current requirements. Lipos are not able to supply the high currents that NiCds are capable of - they are catching up though.
Bottom line for Lithium batteries: DO NOT PLAY WITH LITHIUM BATTERIES UNLESS YOU KNOW WHAT YOU ARE DOING!!
NiMh is probably the best bet for GPS use as you can buy them from many hardware stores of the shelf. They require a different charger to NiCds so make sure you get the correct charger and do not attempt to charge them with a NiCd charger.
NiMh batteries are also much more environmentally friendly than NiCds. Cadmium is a heavy metal which should not be simply chucked out with your usual rubbish. NiCd batteries must be disposed of correctly. NiMh on the other hand are safe enough to chuck out with the rest of your household rubbish.
Rob.
