[Milsurplus] More on charging a PRC-68 battery

[Mark]

Not sure if this has been mentioned, but there is another factor to be considered when charging PRC-68 batteries or, indeed, any other cells that are enclosed in cases like the PRC-68 box.  That is heat.

A while back at Dayton, a friend's PRC-68 battery went dead.  It was a rebuild with his own set of NiMH cells.  He had a spare which allowed him to re-battery his 68 and re-join the fray.  But it was early in the day and he was concerned about the backup going dead, so he wanted to recharge the first battery just in case.  Getting stuck without an intercom radio at Dayton is like being adrift in a dinghy in the middle of the Atlantic.

He had an AC car inverter but no charger.  I had my Maha 777+ there so we rigged up a scheme that hooked his inverter to the Maha and the Maha to the battery via clip leads.

The Maha will automatically shut off at end of charge based on the usual delta-V and Delta-I considerations, and also on temperature.  There was no place to put the temp sensor; we figured the charger would quit on the first criteria.  Biiiiiiiiiiiig mistake.

An hour later we came back to find at least two cells that we could see had blown the ends right off, wrecking the pack.  

The Maha charges at about 800ma or so until tail-off.  Obviously, that rate made the temp rise so fast that it outran the charging process.  In that box there was no place for the heat to go, the cells fed on each other heat-wise, and since the temp sensor wasn't in use there was no way for the charger to tell what was up.

A hard way to learn a lesson... which is: when charging cells in enclosed battery boxes like the one used on the PRC-68, keep your charge rate low.  If you can feel any warmth on the outside of the case, chances are you're over-geeing it since it's probably a lot warmer inside!

AA NiMHs are available with around 2700 mAh capacity.  Constant charging at C/10 or about 270mils would NORMALLY be okay in open air.  I would tend to degrade even that to maybe 200ma when the cells are totally enclosed.  Our 800ma experiment was waaaaaaaaaaaay overboard, and ended up bombing...  literally.  

--Mark Francis
  KI0PF

There are people who make things happen,
there are people who watch things happen,
and there are people who wonder what happened.
To be successful you need to be a person 
who makes things happen.
                           --Jim Lovell
                             Astronaut