Model Forum / Radio Controlled / Air Models / July 2004

Hi Abel, Now I remember you. I just discarded your envelope for the
check you sent for the NiMH batteries.

The battery is still out there and I will do as you suggested. It's
the prudent thing.

Thanks,
Wan

Discharge, puncture and soak in salt water works well.  Discharge them
first, or you will get arcs and sparks when you puncture the bag.

I had some packs that had been damaged badly by running them down too far.
They held less than 1/2 capacity.  It worried me to keep around a damaged
dangerous object, so I used them as test articles.

1) I shorted a pack.  The leads turned red, and the solder came off the
tabs.  Some swelling, but not much.  This is much like what happens when you
short a NiCad pack.
2) I drilled several cells without discharging them.  Some white-hot sparks
came out, but nothing else happened.  It ruined the drill bit.
3) The rest I discharged with a 1K resistor across each 3-cell pack until
there was no voltage across the resistor.  I did have some problem
puncturing the cells, so I turned to a 2x4 and a sharpened cold chisel.

I did not overcharge a pack.  I expect that this would create a fire,
because you would put a lot of energy into it that has no place to go but
into heat.  With the flexible pouch, I don't think an explosion is possible.

I did not heat a pack to 180 deg. F (80 C) as could happen on the front seat
of a car.  The chemistry of the current cells says that the cell will start
to decompose at at 160F or so.

In other words, I think the safety issue is overblown.  There are three
rules I think are important:

1) Charging is risky.  Chargers that automatically check the cell count can
be fooled, and chargers where you set the cell count can be mis-set.  Follow
the charger's instructions faithfully, but assume you can screw up.  Keep an
eye on the process, and don't charge on a flammable table.

2) Keep them in a place below 140 degrees or so.  Cool them off between
flights.  (I use a small 12V fan while field charging.)

3) There is probably some sort of mechanical damage that a crash might cause
that would make a major internal short.  Therefore, a fire can happen after
a crash.  From what I have seen, if you throw the whole thing into a bucket
of water, the cell cannot generate enough heat to make an interesting fire.
Otherwise, observe until the crashed pack is cool to the touch everywhere.

Now, go fly and have fun.  Remember - glow fuel, gasoline and propellors are
dangerous when used by the ignorant or the foolish.

-- Mike Norton

The text before this explains that a significant over-charge risks
destruction of the battery.  It then states that the pictures show what
happens when a cell is significantly overcharged.  I guess it was setup
deliberately.

I'm sure the rest were *not* deliberate though!