nimh float voltage
The topics of battery charging can be a bit tricky. You can float charge any battery, but beware if you do it at the wrong voltage as this will dramatically shorten battery life and (in case of Li-ion etc. batteries) can cause even an explosion. And that is real cool to see if you google a little bit, btw. It is very important how you charge a battery as many of it's properties are dependant of its history. That is the way it was charged before and the way it was discharged. Was it exposed to temperature variations etc. Lead acid are the most forgiving of all, but of course the penalty is their charge/voluume and the weight. And health. (OT: does anyone think ROHS is a dumb move? Or is it just me?) Beware: same chemistry batteries can differ much from manufacturer to manufacturer so the best move is to design for a selected battery and let the consumer worry if the appliance explodes killing his neighbours' cat.
If I recall correctly the correct way of charging NiCD and NiMH batteries is constant current untill the voltage at the battery reaches a maximum and then detect a drop in the battery voltage (at the same current!) soon enough and change the regulation to voltage mode and let the battery float at a certain low enough voltage. The algorythm is the same for these two chemistries, except the voltage drop indicating full battery is more pronounced for one of them, don't remember which one... It is interesting to note that often a temperature measurement of the battery is very usefull as some actually drop a few degrees C when full. Temperature of course plays a vital role in the performance and charging of batteries, especially for Li-ion ones. (That's why many of them come with several connection terminals - they are the most susceptive...) The net is full of info about this topic, but be careful, much of those are questionable at best. When I did the design, I got most of my informations from more or less known manufacturers (Panasonic atc.) and I was even lucky enough to get datasheets of the 2 most used batteries on the field. Some papers are avaible on IEEE.
Now for the lead-acid batteries which are used in the field and are trickle charged at too high a voltage, those were replaced every two years for reliability issues. Again I say, the lead-acid ones are the more robust. They work many years if done right. Imagine what happens with some other chemistry batteries.
You never stated the way the batteries are used? Cycle use or stand-by use? Cycle are for example cell phone, laptop batteries. Stand-by are emergency lights. One of the safest way to charge a battery is in the stand by mode. Some manufacturers do not put a limit to the charging current of batteries when charged in st-by mode. Consider this, it is the simplest way.
Charging a battery can take between 4 and 24h, sometimes even more and for some chemistries much less. Keep in mind that a battery gets 70~80% of its charge relatively fast and that the last few percents are the lenghty ones.
Also consider the charge efficency. That is how much charge is transfered compared to the charge stored in the battery. Highly dependant of well, everything, really...
One thing that must be acounted for: if you have a 2Ah battery and you discharge it at say 1A? How long will it last? About 2h. What about if you discharge it at 4A? Much less. Consult datasheets!
Happy charging.