Al Arduengo wrote:
Do NOT place a 12V-6V supply in parallel with *rechargeable* NiMH AAs.
The AAs get mighty hot. :-/ Alkalines will be fine. This way I can
still use the AAs if I want but don't have to remove/replace them.

G! Actually I'm a bit surprised you don't have trouble with the
alkalines too, but I don't have much experience with alkaline batteries.

If you hook a power source across a battery, and the battery isn't fully
charged, the battery will draw as much charging current as the power
source can provide. If that current is high enough, it can certainly
cause things to get rather warm. Indeed, it's not impossible to cause
an explosion!

The little wall chargers have enough internal resistance to keep the
charging current under control. A good "stiff" well-regulated supply is
lacking this resistance and can be a problem... (you could try putting
a 6v light bulb in series though in practice I've had trouble getting
the battery to charge fully that way)
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com