Al Arduengo wrote in message

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.

You should be nice to your batteries and regulate the charging
current. If the power supply was stout, you could fry a battery, or
even have it explode from the overheating unless it's protected some
way. The simpliest method is probably using light bulbs in series. IE:
if you want to limit charging current to 100ma, place a small bulb
that draws 100ma, in series from the power source to the battery. The
current drawn from small bulbs can vary from about 25 ma for a 12v
"grain of wheat" bulb, to 200-300 or more ma for a larger auto lamp
bulb. IE tailight bulb, marker bulbs, etc..The best charge is a slow
charge. Thats 1/10 the capacity of the battery in mah for current, for
appx 15 hours. IE: if the battery is a 1500mah, you should charge at
150ma for appx 15 hours. If you have a VOM, you can check the current
draw when trying different bulbs. MK