Some good info there, Hopi!
I'd like to clarify "memory effect" vs "wear out" as I use these terms.
"Memory effect", to me, means that if you regularly only drain your battery 50%, then the battery will think it has only 50% capacity, and you can no longer drain it further. Your 4 hour battery becomes a 2 hour battery. Repeatedly fully charging and discharging is good for this type as it maintains 100% capacity.
In contrast, "wear out" means that the cells in the battery will slowly die, one by one, and no longer be able to hold a charge. If a battery comprises 5 cells, the most used one (the first cell, lets say) will die first, not being able to be charged any more. At this point, a 5 hour battery now will only last 4 hours no matter what you do. This shifts the most charge/discharge cycles to the second cell, which will then die, too. Now you're down to 3 hours. Likewise, the remaining cells will in turn die (actually somewhat more quickly since they get used more frequently being the only ones left). Eventually the last cell dies and your battery will show something like a red light (meaning it's failed), or it just won't show anything any more. For these, while they don't care how much you charge/discharge them, the more you charge & discharge the faster they wear out, and conversely, discharging them as little as possible greatly extends their lifetime while maintaining near 100% capacity even after years.
The old NiCad (nickel cadmium) batteries were really bad for the memory effect. If you faithfully fully charged and discharged them, they would last quite a while, and keep close to 100% capacity.
The subsequent NiMH (nickel metal hydride) were less susceptible to the memory effect, but still had it. They also showed more evidence of wearing out over time. Better capacity and recharge rates, too.
LI (lithium ion) batteries eliminated the memory effect, but are very susceptible to usage, which is why these I recommend keeping charged by keeping the device on AC as much as convenient.
The latest LI polymer batteries are even better, having no memory effect, and greatly reducing the wear out rate. These ones (used in the eDGe) you don't have to worry about wearing out as much, but if they're kept plugged in, they'll last that much longer. Also, draining these below 5-10% requires a "deep recharge cycle", which I suspect is probably harder on the battery, so it's best not to let the eDGe run out completely.
For reference, look at
http://en.wikipedia.org/wiki/Lithium_ion_battery. Note the right side bar indicates 400-1200 charge/discharge cycles. It also states the following (corresponding to my "wears out" term):
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Charging forms deposits inside the electrolyte that inhibit ion transport. Over time, the cell's capacity diminishes. The increase in internal resistance reduces the cell's ability to deliver current. This problem is more pronounced in high-current applications. The decrease means that older batteries do not charge as much as new ones (charging time required decreases proportionally).
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Lithium ion polymer batteries (
http://en.wikipedia.org/wiki/Lithium...olymer_battery) are better having more than 1000 charge/discharge cycles (but not infinite
) It supports the statement by enTourage that the eDGe batteries will last quite a while at near-full capacity:
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Originally Posted by dontpanic
The battery in the product is a very advanced design and should last more than four years before performance begins to degrade. At that point, the Pocket eDGe may be returned to enTourage systems to have the battery replaced for $85, not including shipping. There should be no concern about the need to replace the battery on the Pocket eDGe.
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The following article gives another detailed comparison of rechargeable batteries with the pros/cons/characteristics:
http://en.wikipedia.org/wiki/Battery_cycle.