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There are several important differences. The practical difference between Lithium batteries and Lithium-ion (Li-ion) batteries is that most Lithium batteries are not rechargeable but Li-ion batteries are rechargeable. From a chemical standpoint Lithium batteries use lithium in its pure metallic form. Li-ion batteries use lithium compounds which are much more stable than the elemental lithium used in lithium batteries. A lithium battery should never be recharged while lithium-ion batteries are designed to be recharged hundreds of times.

Lithium ion batteries are not available in standard cells sizes (AA, C and D) like lithium batteries are.

This post was shamelessly ripped from here.

Li-Ion technology is horrible in terms of performance, reliability, safety, and environmental impact.

Don't buy from RadioShack, buy from GreenBatteries. I've shopped with them before, and gotten great deals... haven't seen $2.75 each for NiMH AAs in 12 packs at WalMart. Unfortunately, they don't seem to have the common form factor Li-Ion batteries either, but I've been sastisfied with the NiMH so far.

I've had the same charger for about two years, the thing is great. Plus the optional car charger cord makes this even better (although I had to run a constantly hot fused feed to my cigarette lighter).

One thing about NiMH, they need to go through about 3 dicharge/charge cycles before they come up to full capacity.

The place where I bought mine says to keep your batteries in a sealed bag in the freezer when not in use, they self-discharge about 40% in about 30 days at room temperature.

I've had the same charger for about two years, the thing is great. Plus the optional car charger cord makes this even better (although I had to run a constantly hot fused feed to my cigarette lighter).

One thing about NiMH, they need to go through about 3 dicharge/charge cycles before they come up to full capacity.

The place where I bought mine says to keep your batteries in a sealed bag in the freezer when not in use, they self-discharge about 40% in about 30 days at room temperature.

The problem with most rechargables is that the voltage is less than with alkalines. A single AAA, AA, C, or D battery should be 1.5 volts, but all NiCd's and most NiMH I've seen are 1.2 volts, which means that whatever device you are using has to draw more current (amps) from the battery, and the battery either won't last as long (for low power), or you may pull too much current and have problems (for really high power devices). I don't understand why rechargeables have a lower voltage, and at this point it's probably just because they've always been that way.

Anyways, if you need a good quality battery that lasts a long time, I'd suggest either modifying the device you have to use a known Li+ (Lithium-Ion) battery, or go to your local custom battery shop and have some made in the form factor you need. For low power devices, you can probably get longer life from a charge on a Li+ battery than alkalines, for the same or less weight. My sony minidisc walkman uses a fairly old one, about half the size and weight of a single AA, and it powers the minidisc player for over 10 hours per charge.

Of course, when shopping, look at the mAH (milli-Amp*Hours, a AA is around 2500-2800 for low power, much lower for high-power) of the batteries, but take into account the type of device you are using. High-power devices usually favor Li+ or NiMH over NiCd or Alkaline. This is a good site for battery myths that should help, but they don't cover the (superior) Li+ batteries. If you really need the power, use Li+, nothing beats them for power/size/weight.

Personally, I have used the Maha brand battery and charger and have been happy with them.

Quoting from the article (emphasis mine):

The unit uses a NiMH battery which means that there may be a memory problem so make sure it has a good first charge. Once and awhile it is good to totally drain the battery and recharge it totally, this is called refreshing.

NiMH rechargeable batteries have a negligible memory effect (the correct technical term would be voltage depression). References:

Duracell (HTML)
Duracell (PDF, more exhaustive)
Energizer (search for voltage depression)
Greenbatteries

My experience is that most so-called memory effect problems with NiCd and NiMH rechargeables can be traced back to cheap charger units which overcharge the batteries. Use a microprocessor controlled charger whenever possible, it may be a little more expensive at first but it will save you trouble in the long run.

Interesting. This page suggests that putting NiMH in a freezer prolongs their life. I guess your link is referring to the effect of cold on batteries in use, while mine is referring to batteries not in use.

You clearly have no clue about battery technology. Over the life of a standard 1.5v Alkaline battery, it will actually average 1.2v. Only new alkaline batteries are 1.5v. The volts will drop as soon as it starts being used.

See this FAQ