On Fri, 02 Oct 2009 14:03:46 +0800, who where wrote:
http://www.batteryuniversity.com/parttwo-34.htm
You've rolled a number of points into one. I was referring
speciically to their 20% p.a. loss of capacity claim, which is a
crock.
Guilty as charged and you're correct, but not for commodity Li-Ion
laptop batteries as the OP apparently is using. According to:
http://www.batteryuniversity.com/parttwo-34.htm
Temp 40% storage charge 100% storage charge
0C 96% after 1 yr 94% after 1 yr
25C 96% after 1 yr 80% after 1 yr
40C 85% after 1 yr 65% after 1 yr
60C 75% after 1 yr 60% after 3 months
Kinda looks like about 20%/yr loss with 100% charge at room temp. My
abbreviated test showed a 40% loss with 100% charge at room temp in 6
months. I'll agree with the Wikipedia numbers until someone specified
a specific chemistry and testing method (that I can perform with my
West Mtn Radio CBA II tester).
However, you are correct that there are new and improved chemistries
that do not have anywhere near the self discharge and
self-deterioration rate of commodity laptop batteries. Eagle Picher
makes Lithium-CFX batteries, that claim a self discharge rate of less
than 1%/year at room temperature.
http://www.edn.com/article/CA6672104.html
These batteries are made to operate at body temperature (37C) and must
therefore not suffer from self-deterioration at elevated storage
temperatures.
If it is a laptop pack recent enough to care about, it will contain a
pack protection module which will preclude discharge beyond a LVCO
point, typically 3.0v, and will also preclude excessive discharge
current. Nothing unsafe about a DYI discharge on that pack.
True. The battery pack has enough electronics inside to protect
itself from excessive discharge. Under Windoze, one can set an alarm
and a shutdown point based on battery capacity. The default threshold
is about 10%. I've never tested this to determine if it works.
Presumably, if you don't want to run the battery down to below perhaps
25% capacity, setting a shutdown threshold below this point is wasted
effort. Similarly, if there's a protection threshold inside the
battery pack, it's certainly not labeled or easily determined.
If it is a single cell from a cellphone or similar, different story.
these tend to have minimal inbuilt protection (if any) and rely
heavily on the host device for the customary protective functions.
Yep. Model airplanes and helicopters are even worse. All the
protection electronics is in the external battery charger. It
protects against fatal overcharge, but does nothing for excessive
discharge.
No protection means all bets are off.
Yep.
No need. I've seen numbers on these before, and Evgenij Barsukov has
posted comment on this previously in sci.chem.electrochem.battery.
(...)
I never suggested that everything you posted is wrong, far from it.
But several points were amiss and they attracted specific comment.
No problem. However, I'll stand on the Wikipedia 20%/year loss at
100% charge at room temperature for commodity laptop batteries. My
results were even worse. I'll concede that there are new chemistries
that offer substantial improvements in self-discharge and
self-deterioration, but I haven't seen any in laptops.
I have seen numerous manufacturer-derived articles recommending 50-60%
SOC and cool/cold but not freezing as the optimum storage condition
for maximised life.
Same here. That's also my recommended storage condition.
I see nothing at all wrong with this approach
*if* maximum life is the sole objective. If OTOH the user wants to be
able to pull the cell/battery out of storage and into service without
an intervening warm-up or recharge, maybe a higher SOC is warranted.
Like many things relating to Li-XX cells, it is a tradeoff.
Well, of course. I mentioned (twice) that one should let the battery
warm to room temperature before using. I don't know what will happen
if the battery is either charged or discharge at near freezing
temperatures, but it probably will not do anything useful.
The extra life obtained by reducing the EOC voltage is well documented
and well worth it in laptop applications BUT the end user doesn't get
to choose. The manufacturer is out to deliver (well, promise) the
maximum discharge runtime he can, and he doesn't give a rats how long
the pack lasts in service.
The user can set the Windoze low battery warning to trip at a much
higher level than the ridiculously low default value of 10%. That
will prevent excessive discharge.
I have a pack from an olde Acernote Lite 370 series dated 9637, so it
is just over 13 years old. It is stored with, but not *in*, the
machine. About once every year or so I pull that out and run it until
the machine shuts down, then recharge it to 100% (sic). It delivers
about 1.3 hours, compared to 2.5+ when new. That is stored at 100%
and room temperature (32S/116E).
The Acernote Light 370 was delivered with NiMH batteries, but later
LiIon batteries were made available.
Why do you discharge the battery before charging? As I understand it,
LiIon doesn't have a memory problem.
I also have several test packs of 18650 cells left over from a project
about five years ago when I designed a commercial Li-XX charger. They
were shelved at 4.20v and currently all are above 3.9v. I haven't
bothered to measure their storage capacity because I have no reason,
but I can assure you that they wouldn't show that sort of cell voltage
if they had lost 20% of original capacity per year.
Agreed. 3.92v is the highest voltage that a LiIon-Cobalt cells will
deliver. You did something right because my 6 month experiment showed
deterioration in both room temperature batteries.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558