[Does anyone know if/how the newer NiHM[sic] batteries are affected by memory effects?]
This memory effect thing is mostly an urban myth. Memory only affects one very specific kind of battery: sintered-plate nickel-cadmium designs. Pocket-plate nickel-cadmium batteries are free of the effect, as-well-as, all nickel metal-hydride designs. Nickel metal-hydride batteries share the same positive nickel electrode as its older cousin, but the negative electrode is made from hydrogen-storing metal alloys, such as the lanthanum-based alloys (developed by Philips).
[So how does it affect the battery life, if one frequently takes the notebook off the AC strip, uses it for a short while off the AC, and then plugs it back in (where of course it is being recharged)?]
This has less to do with the battery and more to do with the "smartness" of the charging system. If your charging system forgets that the battery is already charged, and thus overcharges the battery until it detects that the battery is already at full capacity, you will "dry" out the battery. Overcharging causes the electrolyte to disassociate into hydrogen gas at the negative plate and oxygen gas at the positive plate. These gases, especially hydrogen, do not diffuse back through electrolyte separator very easily and can build up enough internal pressure to cause the battery to vent. Many of the newer charging systems maintain a charge/discharge history on the battery and are smart enough to not even attempt to recharge until the battery has been discharged more than 10% or 20%. If your computer employs one of these newer systems you have nothing to worry about. Famous last words.
What is the difference between Ni-Cd, Ni-MH, and Li-ion?
Li-ion (Lithium Ion). This is one of the newest cell types available. It is also the lightest battery type currently available on a commercial basis and can provide more power than the other main cell types. There are no known problems of memory effect with this battery type and it is the easiest battery type to care for. The downside of this battery is that it has the highest engineering costs and therefore the price is usually considerably higher than other cell types.
Ni-MH (Nickel Metal Hydride). This cell type is the most common cell type currently available for laptop computers (although Li-ion is rapidly becoming the most popular). This battery type is relatively cheap to manufacture and therefore tends to be cheaper than Li-ion. This cell type is prone to memory effect, so it is important to take good care of your Ni-MH battery to ensure that you obtain the best runtimes.
Ni-Cd (Nickel Cadmium). This is one of the oldest cell types and is generally only found in older laptops. The main pro for this cell type is its ability to handle higher loads, and therefore is more commonly found in portable power tools or devices that need a lot of power to work efficiently. The main downside of this cell type is that it is notorious for suffering from memory effect, so good care must be taken with this battery type to ensure most effective use of battery.
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