subject: The battery knowledge:The difference between NiMH and NiCD batteries [print this page] The battery knowledge:The difference between NiMH and NiCD batteries
This is one that tends to confuse people, so I decided to address it early on.
A few years ago, before Lithium-ion batteries became prevalent, most power tools used either a Nickel-metal hydride battery (NiMH) or a Nickel-cadmium battery(NiCd). There are still many power tools (and many other devices) that use one of these two batteries. And the characteristics of these batteries differ enough that it's useful to know their differences.
The biggest difference between a NiMH and NiCd is that NiCd batteries have a memory affect. If you consistently drain a NiCd battery to 50% and recharge, then the battery will "remember" that point and will suffer a voltage drop there in subsequent cycles. You can avoid this by completely discharging the batter every so often. Note: you don't need to do this every cycle. Once every 5 to 10 cycles is enough.
One point in favor of NiCd's is they have a lower self-discharge rate. This is how much the batter depletes itself when not in use. NiMH's are about 30%/month and NiCd's are about 20%/month. NiCd's also have a more constant voltage of about 1.2V per cell. NiMH's start around 1.4V, quickly drop to 1.25V, then drop to .8 to 1.0V around 20% capacity. For comparison, an alkaline battery starts at 1.5V and slowly falls to 1V over the life of the battery.
NiCd's don't perform well when they get hot, so if you have one you want to keep it cool. But NiCd's can have a higher discharge rate, meaning you can draw a lot of power from them very quickly (though NiMH's perform fairly well too).
We will probably see NiCd's fade from use. The cadmium is bad for the environment and the cost of NiMH batteries are falling. So for most uses, the NiMH will be the economical choice. If you need a battery with low self-discharge lithium-ion will be the way to go.
