CH questioned some of the approx numbers discussed previously.
Here is a more accurate calculation from data-sheet graph numbers:
Warning, long boring engineering stuff:
Here is a link discussing the rate characteristics of Li-phosphate (?Valence
Saphion?) lower voltage, higher rate Li cells:
http://powerelectronics.com/news/battery-li-ion/
also http://www.valence.com/
These Li-phosphate cells can withstand 10-15C continuously and peaks of up to
25C for 30sec. (compared to normal Li-Co: 2 to 3C max) However, because they
are phosphate cells, C capacity is lower, so in absolute terms, for the same
size cell, they have more like 6 to 8C of a conventional cell! (In this case C
is not a very good method of comparison and the vendor wants to make his
batteries look better given the lower Whrs.) Claimed cycle life >1000 at high
rate, this is much better than conventional cells. A more subtle effect
apparently, is impedance does not increase nearly as rapidly over cycle life,
improving longevity for high cut-off voltage electronics.
?Average cell voltage? (usually actually the voltage at C/5 and 50% capacity)
makes high capacity chemistries look worse than they are, because C is so
different.
However, using C/5 std test points comparison :
initial OC voltage @25% @50% @90% discharge
Li-Co : 4.1V 3.9V 3.75V 3.5V
Li-phosphate: 3.4V 3.27V 3.25V 3.2V
NiMh : 1.4V 1.26V 1.25V 1.19V
(From Molicell 18560H data sheet for high resolution Li-Co graphs.
From Valence IFR18560 data sheet for Li-phosphate,
From Sanyo NiMh Handbook for NiMh )
Retail Vendors of CRV-3 replacements quote 3.25V ?average voltage?.
A more accurate Whr calculation of NiMh vs Li CRV-3 at C/5 rates, is then:
For NiMh the nominal capacity is then : 1.25V*2.2Ah*4 = 11Wh
For CRV-3 rechargeable, capacity is then 3.25v*1.3Ah*2 = 8.45Wh
So the Li-ion, look worse but actual users indicates the CRV-3?s are much
better.
For higher peak currents the NimH starts with a much lower initial voltage.(see
below)
This* likely * causes much lower minimum voltage dips and this is why users
report very poor battery life from NiMh.
The nominal C/5 voltage comparison at 50% discharge is then:
6.5V for Li-phosphate CRV-3
5.0V for NiMh
for comparison disposable CRV-3 :
~5.8V for non-rechargeable CRV-3 at 0.22A (@~C/5 for the CRV-3 rechargeable
Li)
~5.4V for non-rechargeable CRV-3 at 1.3A 4sec pulse (from Kodak CRV-3 data)
and :
7.5V external Li-Co pack
This shows the battery voltage drop under peak load can be ~1.5V larger for the
Li-phosphate cell in comparison to NiMh. That is a huge 30% of NiMh's nominal
5V.
This web site hypes CRV3 replacements ($9.95ea) :
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1084
Claiming more camera shots than any NiMh, and more rapid recycle times from
flashes.
A more subtle advantage may be the much lower self discharge rate for Li
compared with NiMh, so they are better , after lying in your bag for a month!
Regards,
Tim Hughes
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