On Tue, 31 May 2011 11:27:21 -0700, (Dave Platt)
wrote:

Interesting. Another page in the Sanyo FAQ section (at
http://us.sanyo.com/eneloop/FAQs) says:

Though it is possible to charge an eneloop battery in a "Quick
Charger", it is not recommended. We recommend charging eneloop
batteries in a NiMh charger that is 2 hours or more. Charging eneloop
batteries in a "Quick Charger" can reduce the overall life of the
battery.

Like the alleged increase in NiMH capacity produced by initially
"conditioning" the battery, there's the question of how much does
quick charging a battery reduce its overall life? According to
various documents which I'm too lazy to find, a cell is considered
dead when it reaches 50% of its rated capacity. One text mumbled that
quick charging will reduce the number of charge cycles by 20%. For a
400 to 800 charge cycle battery, that would be not a big deal for
cheapo AA cells, and a major cost issue for expensive radio battery
packs.

My guess(tm) is that internal heating is doing the damage. With a
defacto standard 16r 0.1C charge, there's some excess power going into
heat. For a perfect battery, 10 hrs of this 0.1C is all that's
required to get to 100%. The other 6 hrs of charge is going
somewhere, most likely into heat. Over a 16 hr charge, that's not
going to heat up the battery much. Over a 0.25 hr 4C charge, that's
quite a bit of power heating the battery. Note that the internal
resistance (ESR) of the battery also heats the battery on discharge,
making a rapid discharge risky.

So, why the wide variation in capacity and recommended charging
methods? Because none of them will satisfy everyone. If you ask
marketing, they will offer the highest possible science fiction
capacity, over unrealistic discharge times (20 hr), with the fastest
possible (4C) recharge because that's what consumers want. If you ask
the company legal counsel, they will offer a much lower capacity to
avoid getting sued, and the safest possible recharge method, for the
same reason. Everyone lies, but that's ok because nobody listens.

The problem is that I don't have the time to run a proper test. I
could take one of the brand new Energizer cells, charge at 4C (15
min), discharge at 1C (about 45 min), and repeat as many times as it
takes for the battery capacity to drop 50% from 2300 ma-hr. Without
automation, my guess is that I can do about 4 cycles per day. That
will take 3 months before I kill the battery.

I've read statements to the effect that one reason you would want to
use moderately-fast charging rates (e.g. 1C) rather than the low rates
traditional for most NiCd cells (e.g. 0.1C), is that the NiMH cells
will heat up more abruptly when they reach full charge, and the
charging power starts turning into heat rather than electrochemical
potential.

Sorta. The amount of heat (calories) produced by fast or slow
charging is the same. However, with fast charge, the heat is produced
over a much shorter period, resulting in a much higher cell
temperature. Methinks it's this temperature that kills the cells.

Incidentally, I learned the hard way with NiCd batteries that once the
heat gets to the case and is able to be measured, it's too late. The
internal damage is already done. Internal sensors are required.

Note the magnetic temperature sensor on my battery fixture (that has
fallen off and landed on the spring).
http://802.11junk.com/jeffl/NiMH/cba-II.jpg

The abrupt rise in cell temperature is easier to detect
reliably (via a temperature sensor and/or due to the zero-delta-V
effect) than if you were charging more slowly... and thus a more
reliable "full charge, stop now!" shutoff circuit can be designed, and
avoid overcharging (which I understand NiMH cells don't tolerate at
all well).

Yep. Quick charging will produce a higher cell temperature which is
easier to detect.

During my experiments with NiCd batteries, I've found that I can
charge a single cell at almost any rate (I've done 20C). As long as
the battery is below 100% capacity, there is almost no heating. As
the battery approaches about 95% of capacity, the heating starts, and
rapidly increases. I've quick charged 650 ma-hr NiCd batteries to
almost full charge in about 3 minutes. However, there's a catch. If
I miss the 95% point, and go over, the overcharge will either kill the
battery, or vent boiling caustic electrolyte. It's for this reason
that I suspect really quick chargers are not sold or recommended. I
have not done any such testing with NiMH.

A good decision! It bothers me that a lot of HF and VHF radios "poop
out" at much below 12 volts.

Fishing boats like to run quiet, with the engine off. There are also
sailboats and small boats that only charge the battery at the dock.
(These daze, they have solar chargers). It's not unusual for them to
use crappy batteries, that have a no charge terminal voltage of
10-11VDC. Getting 10 watts output from a 25w bipolar Class-C stage at
10VDC was not easy.

For what it's worth, the Sanyo FAQ page cited above does make
reference to the break-in effect:

Should I store my batteries charged or uncharged?

If stored with charge, it is possible that when you return to use
the battery there may still be some charge left in the battery,
therefore it will enable you to use it right then and there. Also,
by keeping some charge in the battery, it will require you to
"cycle" the battery fewer times until it reaches its peak charge.
If you store them with no charge, you will have to "cycle" the
battery multiple times until it reaches its peak charge.

Note the misuse of the words "peak charge". It should say "peak
capacity". Kinda reads like this was written by marketeering, not
engineering. If they had mentioned that ignoring this sage advice
might result in an initial loss of 10-15% of peak capacity (per my
tests), I suspect that users would probably ignore the problem.
However, because the effect is not innumerated, it looks a major
problem.

So, if I cycle a new NiMH battery with a 4C charge, and a 1C
discharge, how many cycles would you guess I would get before the
battery hits 50% of rated capacity? The winner gets what's left of
the test battery (suitable for recycling) when I'm done.

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