On Mon, 24 Dec 2012 17:20:32 -0800, mike wrote:
+1 on the c9000
Sigh. I've got an itchy mouse finger hovering over the "buy" button.
There are two categories of dendrites.
Dendrites that have already shorted need to be vaporized with
short-duration
high-current. That clears the short and blasts a bigger hole in the
separator.
Causes higher self-discharge and faster dendrite regrowth.
Back in the day when NiCds were expensive and shorted ones were free,
it made a lot of sense for hobby projects.
I've done my share of NiCd cells directly across a 12v car battery for
a few fractions of a second. No explosions, and a rather minimal
success rate. The ones that worked didn't last very long.
The second class is those that have not yet pierced the separator.
For those, I'm a fan of burp charging. Same process you'd use
for plating when you want a smooth surface.
I did some experiments, but I had nowhere enough data to draw a
conclusion. And I concluded that the cost of powering the computer
and the programmable power supply and load probably exceeded the benefit.
Also known as "negative pulse charging" or "reflex charging". There
are commercial chargers that do this.
Pulse-Power:
http://www.fm2way.com/batterycharger/technica.htm
"Negative Pulse Charging Myths and Facts"
http://www.powerdesigners.com/pdf/Tech%20Brief%20Negative%20Pulse%20Charging%20Techn iques%20Myths%20&%20Facts%20-%20Final.pdf
I'm not convinced, but then I haven't tried one of these chargers.
The biggest problem with charging batteries is determining when to
stop.
Yep. Kinda like filling the gas tank with no gas gauge or automatic
pump shutoff.
I had a friend who did radio repair for the city.
Back in the day, police radios were terminated by a thermal switch.
That worked surprisingly well, but only because the battery was
predictably depleted during the shift.
If you stuck a charged radio into the charger, you did some damage.
He had a lot of failed batteries from people who left the radio off
all day and stuck it back in the charger.
I know it well. The Motorola HT600 and similar "brick" radios. As an
added bonus, Motorola may have designed the charger to destroy the
battery packs.
http://www.repeater-builder.com/motorola/genesis/ht600-charger-mod.html
They also have to be modified to work with NiMH packs:
http://www.repeater-builder.com/motorola/genesis/nimh-charger-mod.html
However, my favorite was a Kenwood something HT, that had small a red
plastic window in the bottom of the battery pack. The charger had a
corresponding window with a IR photo transistor in the charger base.
This was suppose to measure the cell temperature and stop charging
when it became warm. Unfortunately, a little dirt on either window
turned the charger into a battery overcharger.
There's so much variability in temperature, thermal time constant,
voltage gradient, etc. that it's best to measure change than
absolute value.
I'm a fan of 0deltaV termination with timer and temperature fail-safe
for NiMH cells.
For NiMH, that's a problem. Trickle charging an NiMH battery makes
the drop in terminal voltage very difficult to see. It's in the low
millivolt region. I think (not sure) that Maha recommends something
like 0.5C charge on their Eneloop just so that deltaV can be detected.
Here's a hint, without detail:
http://www.flickr.com/groups/strobist/discuss/72157625332323687/
"manufacturers recommend 0.5C to 1C to (1A to 2A on eneloop)
to make negative delta voltage detection more "detectable"."
I did some testing of temperature detections on individual NiCd cells
when I was doing my ultra-rapid charging experiments. I stuck 8
thermistors on various parts of the cell, and proceeded to
intentionally overcharge it. The mass of the cell was sufficiently
high that the thermal lag was substantial. That won't make much
difference for a slow charge, but can easily be catastrophic with
ultra-rapid charge, where seconds count. The best sensor location
appeared to be in the middle of the cell. Unfortunately, the plastic
or paper wrap around the battery is a good thermal insulator. I had
to scrape off the coating and connect directly to the metal case.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558