Right, here's what I know (for what it's worth!)
NICD:
Traditional NiCD batteries still suffer from memory effect unless they are charged in an intelligent charger. By this I mean a charger which can monitor the cells, balance them and charge them equally. As far as I'm aware only 2 main manufacturers use these type of chargers for NiCD. Using these type of chargers you can expect up to 3000 charge cycles, however using a less sophisticated charger will give you around 1000 cycles. They will also work from a temperature range of -20 to +80 degrees. Another problem with NiCD is that they self discharge at a rate of around 25% per month.
NIMH:
NiMH batteries were seen as the holy grail when they were introduced a few years ago because they were marketed as having little or no memory effect. However, while this is in the main correct there are also many down sides to NiMH - hence the reason so many manufactures launched ranges of NiMH cordless tools, and the quietly went back to NiCD almost immediately. The main reason being that no matter what charging system you use for these batteries, the maximum amount of life cycles you can expect from them is around 600-700 cycles. They also suffer from self discharge at a similar rate to NiCD, however they MUST be charged every 6 months or they start to experience breakdown of some of the cells. - 6 months - not a problem everyone thinks, I charge my batteries every couple of days / weeks etc. That's fine, the problem arises when the batteries have been stored for a long period of time. As an example: the machines are manufactured and then shipped to the relevant manufacturers warehouse for storage (say 1 month between rolling off the production line and landing in their warehouse), they are then distributed out to dealers where they sit on the shelf until they are sold. The problem is that so many dealers will buy a job lot - say 50 / 100 / 200 machines, they could quite easily be buying up to a years worth of stock at any one time. Which obviously means that by the time you buy a machine the batteries could already be performing badly. This added to the fact that NiMH batteries need a higher capaticy to offer the same level of performance as NiCD (i.e. a 3Ah NiMH will only give you the same power output as say a 2.2Ah NiCD), and they won't work below -2 degrees means that they probably offer the lowest all round performance of all three battery types.
Li ION
There are basically 3 main advantages to Lithium Ion.
1. They are nearly half the weight of NiCD & NiMH.
2. They offer far better performance than NiCD or NiMH.
3. They offer around twice as many life cycles as NiMH. (approx 1500)
Lithiuim batteries as we all know have been used for a while in things like cameras, mobile phones etc. They are a good power source for low current applications, however using them in power tools is slightly different. Basically there are two cell types for Lithium - 1.1Ah or 1.5Ah. The 1.1Ah batteries can handle up to 30 amps current draw with no problems, howeve the 1.5Ah cells can only handle around 24 amps before they start to overheat and fail. The mindset of having a tool with a high amp hour rating may be true for NiCD & NiMH, but is certainly the opposite for Lithium. With Lithium you really don't want to be going any higher than 2.2Ah or you could run into problems (Don't believe me - look at the spec of the new Bosch 36v SDS drill, this tool only has 2.0Ah batteries, for the reasons stated above).
Lithium batteries also need a discharge protection to prevent cell damage - all manufacturers apart from one use this protection. In addition they have a self discharge rate of around 5% - much less than NiCD & NiMH, and work in the same temperature range as NiCD.
The bottom line is that Lithium Ion will take over battery tools in the next couple of years, no doubt about it. Bosch, Makita, Millwaukee & Metabo already have Lithium tools in their range. Dewalt have a 36v range coming later in the year (already out in the US), and the others will follow suit (with the possible exception of some of the no name brands). Some manufacturers will continue to sell some NiCD & NiMH machines but only be so that they can offer cheaper machines in the interim.
