Ummm... currently, my on-going investigation has given me the following information:
(1)Nickel Metal Hydride batteries: still has some memory effect but can be restored to full capacity after a couple of full deep cycling (meaning complete drain and then full, slow recharge).
Using multi-stage computer controlled NiMH charger works best in maintaining performance and consistency in these battery types. There are some really terrible cheep battery chargers from Energizer and the likes that are nothing short of old-fashioned NiCd charger (current controlled) and they do not work very well in my case.Maha gets my vote on this one (so is the multi-stage, world voltage computer-controlled Sony 4xAA battery charger)
No 2 brands of NiMH batteries are created the same. There are some really ccrrrapty battery from China that will fail in about 100cycles (that incl. GP rechargeables, which has failed 2 times on my wifey's situation -for Canon S2). I would carefully pick the good ones that are still made in Japan (Energizer AA and Sanyo Eneloop are both Japanese made).
Do not fall for the "bigger is better" mentality when it comes to buying NiMH cells for I have experienced more 2500~2700mA/hr AA battery failures than anything else so far (and yet my Japanese made Panasonic 1800mA/hr, Sanyo Eneloop and Energizer 2200mA/hr have not failed so far).
(2) Li-ion batteries: this is a very complicated subject for the technology isn't fully mature yet, according to the industry.
Li-ion battery may hold the charge better initially and also comes with a much higher capacity vs size(or weight) in the rechargeable battery arena but they do not last very well: typically withing a couple of hundred cycles max. before they become so bad (charge so shallow) that they do not hold/receive charge properly anymore.
Thus the issue of laptop battery cells going bad in 1 yr or so.
Also: charging Li-ion battery requires computer multi-stage control and not just silly tempurature-monitored constant current run-of-da-mill, cheep chinese made miracle charger method. So far, most Japanese makers with extensive rechargeable battery background and technologies (e.g. National/Panasonic- Matsushita Industries, Sony, Sanyo, Toshiba, etc.) have the ability to develop a decent Li-ion battery charger that provides consistent performance.
http://www.batteryuniversity.com/print-partone-12.htm
Most chinese made Li-ion batteries, maybe due to poor manufacturing facilities or lack of technology, they tend to lead to a very short life (far shorter than that of the major manufacturers). Most likely due to ionisation layer contamination (been told that it's more like an intoxication than anything else) as recharge cycles go up. Once these batteries fail to hold their charge, they are considered "dead" and cannot be revived for reuse.
Hope that helps (based on my rather limited understanding on Li-ion battery)
Q.