On Sun, 24 Jun 2012 11:28:23 -0500, Jim Yanik
wrote:
Jeff Liebermann wrote in
:
If you use NiMH batteries, which tend to self-discharge at a rather high
rate,
the 2300 mAH AA Everready NiMH(not pre-charged) that I bought at WalMart
seem to not have a high discharge rate. I use them for my LED bicycle
lighting system. the NiMH cells came with a "smart charger".
I charge them maybe once a month.
With typical NiMH, I loose about 20% of charge in the first 24 hrs,
and then about 1% per day. I've confirmed this with my own tinkering.
After about 3 months, I typically have a half dead battery. It varies
hugely with temperatu
http://corrosion-doctors.org/Batteries/self-compare.htm
http://en.wikipedia.org/wiki/Low_self-discharge_NiMH_battery
With the LSD (low self discharge) NiMH batteries, I find that it loses
about 10% on the first day, and then very slowly drifts toward about
75% capacity within about 3 months, and then sits there forever.
However, such batteries also have less capacity than typical NiMH
cells, so you end up with about the same capacity after a few months.
The problem with using such batteries is that the clock needs to be
able to operate over a long period of time. It could be designed to
operate with a battery permanently at 50% capacity, but that would be
a waste of money for the battery. Using LiIon would be cheaper and
not waste any capacity.
However, LiIon and LiPo have another problem. They self deteriorate
at full capacity and at high temperatures.
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
See Table 3. Therefore, Lithium batteries aren't going to work for
the clock. I guess one needs to use the same approach as the smart
meter vendors are doing. High capacity, non-rechargeable, seems to be
the best.
http://www.tadiranbat.com/pdf.php?id=metering_international_20_years
they hold a charge a lot longer than the DeWalt 9.6V NiCd packs for my
drill-driver.(and don't have the dendrite problem...) :-)
My Skil something drill and NiCd packs are in my car, which tends to
get rather warm. During the summer, I'll have a nearly dead battery
pack from self-discharge in about a month. During the winter, the
pack will probably last all winter.
I also have some pre-charged,low self-discharge NiMH cells but they are
only 2100 mAH.
I'm not saying that is the case for ALL NiMH cells.....
but they -may- be improving on that self-discharge rate.
Notice how all the specs are at 25C. Reality requires a higher test
temperature.
It also irks me that the NiMH cells in the stores don't list the mAH rating
on the packages.
Not a problem. All the specs are lies, but that's ok, because nobody
really understands them. The capacity tests are usually an
unrealistically low discharge rates resulting in inflated figures.
I'll give Sanyo credit for using C/5 discharge rate, which is more
realistic:
http://www.eneloop.info/fileadmin/EDITORS/ENELOOP/DATA_SHEETS/HR-3UTGA_data_sheet.pdf
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558