Yes, it is normal  for them to "heat" up.

The Ansmann charger recommended by Minolta UK, and distributed by them
now, has a built-in fan and a very cool running charge cycle, and using
Delta-V (a method for reading the battery charge level and adjusting the
charge to a trickle) the batteries do not end up sitting there like a
small electric fire after they are charged.

I've just got the Ansmann - all products - to review and so far I am
impressed. The capacities and charging times are better than anything so
far, with their 2300mAh cells. And their Digi charger has rescued some
old cells I have never been able to get to work - by doing its refresh
cycle a couple of times.

David

Minolta Man <colyng@colyn_NOSPAM_goodson_REMOVE_THIS_.com> was all:

Unfortunately, thanks to marketing that emphasizes the speed of charging over
the longevity of the batteries, that battery manufacturers have no reason to
want your batteries to last as long as possible (the opposite, actually), and
that their chargers are designed to limit the batteries' useful life to six
months to a year or so, it's normal.

If you really NEED to charge your batteries in an hour, it may be worth it to
cut maybe a hundred cycles off the lifetime of your batteries each time you
use a battery burning one-hour charger. You *can* purchase a charger that
will charge almost as fast without heating up the batteries (such as with
negative Delta-V charging technology as mentioned by another poster, as well
as even newer cool-charge technologies), but the price may be a bit higher
and you'd more than likely have to order one online. I've never seen a good
quality battery charger on sale in any large chain store (Circuit City, Wal-
Mart, Best Buy, etc.). Chargers made by mfgrs. such as Maha are usually top
notch and VERY cheap if you don't need ultra-fast charging.

If you can wait 3-4 hours for a quality charger priced about the same as a
junk one-hour charger to do the job, many of which are available from
resellers such as thomasdistributing.com or perhaps the reseller for the
Ansmann unit another poster recommended (-DeltaV is one of the technologies
for ensuring that batteries DO NOT heat up and become damaged while still
ensuring a relatively quick and complete charge), your batteries ought to
last many times longer than if they were charged on the cheap (and I don't
mean their price, their construction; they're horrendously overpriced for
what they do) department-store chargers made by and/or carrying the name of
major battery manufacturers.

If you can wait overnight (10-12 hours) for 4 batteries to charge, or if you
have several sets and can keep one set in the charger at all times, Thomas
Distributing carries a trickle charging 4-battery unit that costs only a few
$USD, which will allow you to simply leave the batteries in until they're
needed, and the batteries should last their FULL rated lifetime. If you're
used to batteries lasting only 6 months to a year (or not even that), a good
charger can extend that out several years or more.

It's sort of annoying that the silver and green Energizer 2300mAh NiMH
batteries available from Wal-Mart are one of the great values for widely-
available, top-performing NiMH batteries right now, but the matching
Energizer one-hour charger is the LAST thing you want to charge them in if
you're concerned about their useful life... Nonetheless, that's the case.

Naturally, some folks don't mind spending a few extra dollars for the speedy
charge and the cheap chargers' availability nearly everywhere you go, but
from the number of websites dedicated to rechargeable batteries, charger
reviews, sales, performance comparisons, etc., there must be an awful lot of
people taking quite an interest, photographers being just one segment of that
population.

Here are a few relevant sites:

http://www.ripvan100.com/
http://www.thomasdistributing.com/
http://www.mahaenergy.com/
http://www.imaging-resource.com/ACCS/BATTS/BATTS.HTM