On Mon, 24 Dec 2012 04:32:24 -0800, "William Sommerwerck"
wrote:
In the dark ages, I've charged NiCd and NiMH batteries with a timer.
It's a good way *IF* you know the capacity of the battery and the
state of charge. Capacity is easy as it's in the spec sheet. However,
a given battery can have radically different capacity values at
different charge/discharge rates. To make the numbers look huge,
manufacturers use a slow discharge rate. The battery also has a
charging efficiency, where it takes more coulombs to charge the
battery, than it can deliver. I use 75% for most batteries. Once you
know all that, and the state of charge, you can calculate how long
you need to charge the battery. If the battery gets warm at the end,
you're overcharging and have probably killed the battery.
This is one of those rare cases where I have to disagree with Mr Lieberman.
How warm is warm? "Getting warm" is an indication that charging is complete,
or near-complete.
That's easy. This is Figure 1:
http://www.buchmann.ca/Article18_files/Figure1.gif
from this article:
http://www.buchmann.ca/article18-page1.asp
The graph is for NiCd but NiMH is similar. I've also created my own
temperature curves for my experiments in extremely rapid NiCd
charging. Not that the temperature starts to rise at about 75% of
full charge and has risen 45C (113F) above ambient at 100%. That's
more than "getting warm". That's hot.
Many NiMH manufacturers claim their cells need to be "slammed" to get maximum
charge.
I believe I demonstrated this in the past. See:
http://802.11junk.com/jeffl/NiMH/
It seems that you have to charge cycle a NiMH battery about 3 times
before it will deliver full capacity. However, the difference is
small. For example:
http://802.11junk.com/jeffl/NiMH/Duracelll-NiMH-2050.jpg
shows that the first charge cycle results in a 1180mA-hr capacity,
while the third charge cycle increased it to 1360mA-hr or about 15%
increase. Is that what you mean by "slammed"?
I'm reluctant to do this. However, I've charged NiMH cells in those
"15-minute" chargers, and though the cells got hot, they were not destroyed.
My favorite NiMH charger is a Radio Shack 23-1305 30 minute NiMH
charger. The cells get warm at the end of charge. I haven't bothered
to measure the temperature. Again, if you look at the previous
article showing the temperature graph, the cell should be 113F at full
charge. Hopefully, the charger slows down the rate of charge before
reaching 100% or it probably will get that hot.
(I no longer do this. I have about 20 cells of 2500mAh and higher capacity, so
I never need a quick charge.)
I've noticed that my NiMH battery collection (flashlights, cameras,
GPS, radios, etc) spent much of their time in the charger simply
recovering from self discharge. So, I invested in some Sanyo Eneloop
batteries, which have a much lower self discharge rate. I took some
photos yesterday with a camera that has been bouncing around my car
for about 5 months. That battery indicator showed about 3/4 charge.
Had this been the older batteries, they would have been almost totally
discharged.
The best way to charge batteries is with a charger that lets you set the
charge rate, and watches for the signs that indicate the cell is charged, such
as the MAHA C9000. I generally charge at 0.3C or 0.4C, which is considered on
the low side for NiMH cells. Contrary to what Battery University claims, I've
never had problems with the cells overheating, or the charger failing to halt
at around 1.42 to 1.45 volts. (Note that his gripes are principally directed
at "consumer" chargers.)
http://www.mahaenergy.com/mh-c9000/
Nice charger/analyzer. I dunno the break-in and battery-forming
features. Stressing the battery doesn't seem like a good way to
increase battery life though it might produce rated capacity earlier.
However, it's a much better charger than the common consumer junk
chargers, and would probably be a good charger for the OP.
The C9000 displays the battery voltage, so I can choose to stop charger at
whatever voltage I feel comfortable with. I'm not stupid enough to let any
charger operate without checking it occasionally. If you like, stick a timer
on the charger to shut it off.
Have you tested your battery for rated capacity after one of your
charge cycles? The MH-C9000 will run a discharge mode test and
display battery capacity.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558