Re: Rechargeable AA Batteries

Mike Rivers <mrivers@xxxxxxxxxxx> wrote in ODV%j.24$%Z1.3@trnddc05:">news:ODV%j.24$%Z1.3@trnddc05:

William Sommerwerck wrote:

Why is everyone some exercised over the "shelf life" of a charged
rechargeable battery? Why is this so important?

This is important for things that we don't use (and recharge) every
day. It's nice to charge the batteries on Sunday and know that by the
time I need it on Friday (or in a week or two) I won't have to
recharge the batteries again before I use it.

I just spotted this thread now, and will try to make some constructive
comments.

1. Contrary to what Mike surmised many posts back, the better
conditioning discharges do _not_ discharge down to anywhere near zero
volts. For both NiCd and NiMH, that is actually quite destructive. A
better choice is something like 1 V, for your choice of 10 ohm load. I
personally ruined over a dozen NiCd cells years ago using multi-day
discharge through 10 ohms targetting under 10 mV final voltage.

2. Comparing voltages of NiMH, NiCd and Alkaline cells is a bit tricky.
Their output impedances are wildly different, and also the shape of
their change in voltage and impedance with state of charge. While it is
true that an Alkaline AA fresh out of the box at a 1 mA discharge rate
has a far higher output voltage than the other two in any state, for
somewhat higher current draws it can easily be break even, especially
later in the discharge cycle. At really high rates (think flash guns
and older digital cameras), the NiCd's actually provide the highest
voltage of the three, with NiMH second, because the very low impedance
dominates. I have one thing which requires enough voltage that NiMH is
life-limited (an older remote thermometer). Otherwise NiMH is a happy
choice for my battery operated things, from thermostats (low draw--one
year) to a GPS receiver (medium draw--15 hours) to a flashlight (the
incandescent kind are generally high draw).

3. Regarding lifetime, reversal is bad, and generally arises from
mismatched sets imprudently run too low. Good cells charged in good
independent-channel chargers not left on the shelf too long after
charging are the key defenses against mismatched sets. Imprudently
running too low defense is a combination of user care and machine
design.

4. "Shelf life" in this thread appears to have been used for two rather
different things--physical age of the battery, and loss of capacity with
time after charging. The second issue is been an historic weak point of
NiMH comparied to Alkaline or even Lithium-ion. It actually got worse,
even much worse, in most brands as the race to high label capacity in AA
cells got some compromises made against it.

5. The relatively new low self-discharge NiMH cells (often marketed with
the label term "precharged"_ (most famously the Eneloops) were a
significant step down in claimed zero-hour capacity, and a great leap
forward in charge retention. I believe that for most users these are
the right type of cell to buy for most applications. In actual
extensive tests in my home on many real samples, these usually had more
deliverable capacity after a week or so from charging, sometimes sooner,
than any of the traditional NiMH, new or old.

6. You might find comparison tests I made on eleven brands of AA NiMh
low self-discharge (and five AAA) interesting. I provided comparison
tests on the same regime with eight conventional NiMH in my stock.
(hint, Eneloops are not the best, but all the types are much better than
the conventionals)

If you are interested, see my posts in the topic:

http://forums.dpreview.com/forums/read.asp?forum=1023&message=26078814

I also have the data in a spread***--for a copy just email me a
request.

I have no relationship with any battery maker or seller other than as a
customer.
.