On Wed, 26 Dec 2012 11:27:56 +0000,
lid (Adrian Tuddenham) wrote:

Jeff Liebermann wrote:

[...]
http://www.buchmann.ca/article18-page1.asp

[..]
Overcharge could occur
even though the NiMH battery feels cool to the touch."

Where does the excess energy go if it doesn't finish up as heat in the
battery?

Thermal lag. It takes a while for the battery to become warm to the
touch. If the -deltaV sensor says it's done at a conservative point,
you'll never feel the heat (which is a good thing). With a fast
charge, you could easily overcharge before the internal heating has
time to make it to the surface. We're also not talking about much
heat. 0.5C charge into 1.35V is only about 1.4 watts. Heating
shouldn't be a problem with a slow charge, which might simply radiate
the heat before it gets excessively too hot. Extra credit for
manufacturers that include small fans in their chargers which insures
that the temperature sensors don't work.

However, I've never seen a cold and overcharged battery. With
reasonably fast charging, the batteries I've played with all get warm
or hot as they approach full charge. The battery manufacturers are
making sure that the battery gets warm by playing with the ma-hr
capacity number. For example, a 2300 ma-hr NiMH battery, may achieve
the rated capacity at 0.1C discharge. However at 1C discharge, it's
only good for 2000 ma-hr. Less for higher discharge rates. Old
batteries also lose surface area, and therefore lose capacity. Trying
to charge a moving target like with a fixed timer charger, is just not
going to work. It will overcharge.

Now, if you really want entertainment value, try putting a sealed
zip-lock plastic bag over the charger and give it a nice hefty
overcharge. The bag will inflate slightly with even a modest
overcharge. Where does the gas come from? It's from blowing the vent
seal on the battery and boiling off electrolyte. Note the pressure
curve:
http://www.buchmann.ca/Article18_files/Figure1.gif
The safety vent opens at about 130 psi and stays open forever. The
gas is mostly hydrogen, so you probably make a small bang. You can
sometimes see condensed electrolyte on the bag surface. The potassium
hydroxide can be detected with ph test strips. Do that a few times
and there will be little electrolyte left in the battery. I
"rejuvenated" at few NiCd cells by filing off a corner of the cell,
and using a hypodermic to inject KOH back into the cell. I'm not
certain how well it worked, because there may have been other damage
inside, but it worked well enough to run an HT for a few months.

I guess that's one advantage to 0.1C charging. It will never get hot
enough to vent.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558