Michael Ross via EV wrote:
I think it is very valuable knowing that a Li ion cell can hold its charge
indefinitely. Particularly compared to the incumbent competitor - lead
acid cells. There is a least an implication that the the loses may be of
no consequence after design and manufacturing improvements. There is no
possibility of achieving this ever with lead acid.
Surprisingly, that's not entirely the case.
The self-discharge rate of a battery is usually traded off with many
other parameters; capacity, life, peak current, etc. For most lead-acid
batteries, the market considers factors other than self-discharge to be
a *lot* more important.
But, there are exceptions. I just pulled some Gates "Cyclon"
spiral-wound lead-acid gel cell off the shelf. They were purchased in
Nov 1994, and used for 5 years in electric lawn mower. My notes say the
last time they were charged was on Nov 14, 2000. At that time, they
still had about 1.5ah of capacity (out of their original 2.5ah). Today,
4 out of 13 *still* have charge in them (1.9v or more open circuit).
One reason for this is that Gates used *pure* virgin lead; not a lead
alloy. Almost all other lead-acid batteries alloy the lead with
antimony, calcium, cobalt, or just the random mix of junk left over in
scrap lead. Or, someone will have watered the battery with tap water,
which adds its own unknown impurities. All this "stuff" worsens the
self-discharge characteristics. But that doesn't matter to most
customers; they will be recharging the battery often enough so it never
runs dead.
If you care, Gates was bought out by GE, who sold out to Hawker, who is
now owned by Enersys. So you can still buy these Cyclons today! But they
have an insignificant market, because (almost) no one cares about
self-discharge.
On Fri, Jun 19, 2015 at 8:11 PM, Cor van de Water via EV<[email protected]>
wrote:
David,
Why do all manufacturers of Li-Ion batteries specify the amount of
self-discharge
and why do people like me who monitor Li-Ion batteries over time, see and
measure
the self-discharge?
I understand that you say "theoretical" the pure Li-Ion cell may not have a
self discharge mechanism and thus the actual present self-discharge
appears to be
a spurious effect, possibly caused by contamination, so the amount of
self-discharge
is a quality indicator of the (lack of) contamination in the cell...
However, saying that the ideal (theoretical) cell does not have any
self-discharge is as valuable
as saying that the ideal (theoretical) connection does not have any
resistance.
In practice however you better take that resistance into account when
sizing your wiring and connections.
Similar to taking self-discharge (and especially the difference in
self-discharge between cells) into account
when designing a battery pack.
Cor van de Water
Chief Scientist
Proxim Wireless
office +1 408 383 7626 Skype: cor_van_de_water
XoIP +31 87 784 1130 private: cvandewater.info
www.proxim.com
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-----Original Message-----
From: EV [mailto:[email protected]] On Behalf Of David Nelson via
EV
Sent: Friday, June 19, 2015 4:37 PM
To: Bill Dube; Electric Vehicle Discussion List
Subject: Re: [EVDL] "Zero self-discharge" (was: Bicycle battery)
Bill,
I have dug through many scientific papers on Li batteries and how they
work and I haven't been able to find anything about a theoretical
self-discharge mechanism. I have found some explicitly talking about the
lack of a charge shuttle mechanism like in lead-acid batteries, however.
One paper I read has a section devoted to figuring out a charge shuttle
mechanism to add to a Li cell. It isn't a paper for the faint of heart,
however. It is "Nonaqueous Liquid Electrolytes for Lithium-Based
Rechargeable Batteries" by Kang Xu published in the Chemical Review, 2004,
Vol. 104, No. 10, pp. 4303-4417 and published on the web 09/16/2004.
While I'm sure I don't get it 100% of the time I try to make sure I say,
"LiFePO4 cells have no theoretical self discharge mechanism."
This, according to all I have been able to find on the cells, is true.
Also, the most ardent anti-BMS people I have found have in fact done a lot
of searching of the scientific literature and came to the same conclusion.
It was not just something they made up to support their anti-BMS position.
If you have other literature showing what the self-discharge mechanism is
please share it.
On Fri, Jun 19, 2015 at 11:46 AM, Bill Dube via EV<[email protected]>
wrote:
This "zero self-discharge" myth seems to be a common thread with the
"anti-BMS" crowd. I don't know where this misinformation comes from.
All cells have self-discharge. It is just a matter of degree.
Also, the self-discharge varies from cell to cell. That's why you need a
BMS.
These are well-documented facts for all those that care to
investigate, either through the scientific literature or by simply
systematically testing cells for themselves (as Lee Hart has done.)
I strongly suspect the "zero self-discharge" myth comes from
the same unscientific source that the anti-BMS myth has sprung from.
Bill D.
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--
David D. Nelson
--
The greatest pleasure in life is to create something that wasn't
there before. -- Roy Spence
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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