When charging these units is it detrimental to the battery to charge them before they are totally dead? Is it then not a good idea to "top them off"?
Thanks!

You'll get the best life from LiIon batteries by using them between full and 50% discharge. There is no need to discharge them below 50% as they have no memory effect. And it's best not to leave them sitting at full charge, so charge just before use, use them on repeated rides until they're about 50% discharged, then charge just before the next ride. This is optimum for battery life, but don't get too OCD about it. If you go below 50% or leave it sitting around at full charge there's only a very slight reduction in useful life.

[MENTION=210772]Looigi[/MENTION]'s answer is pretty complete, but I'd like to add to it anyway. NiMH and NiCad batteries were around for a long time, and it became well known that they had a memory effect. It was helpful to discharge them fully as often as possible. LiIon are the opposite. Discharge them fully as little as you can. If you have to run one down every so often, don't worry about it, but don't do it on purpose, at least not frequently.

It seems to me that age is the biggest factor in how long a LiIon battery lasts, whether it is used lightly, heavily, or not at all. They last three years.

Thanks for the replies!