In article ,
Jeff Liebermann wrote:
Amazing. I've run tests on some 2200 ma-hr NiMH AA cells using my
CBA-II
http://www.westmountainradio.com/product_info.php?products_id=cba3
The results are not very impressive. If I discharge at rediculously
low rates, such as 0.01C, I might be able to squeeze 2200 ma-hr out of
a cell. I tested one at about twice the estimated self discharge
rate, and calculated about 5000 ma-hr out of the cell. I think it
took about a week to finish the test. However, when I ran it at a
more realistic load of 2C, I was barely able to squeeze out 1000
ma-hr.
Most NiMH/NiCd battery manufacturers seem to rate their cells at a
discharge rate of 0.05C (20 hour rate) - that would be around 100 mA
for those AA cells. Sanyo's charts for their "Eneloop" show
about a 10% reduction in useful capacity at a discharge rate of 0.5C
(1000 mA).
If you're actually pulling over 4 amps from them (2C discharge rate)
I'd expect a significant further loss of capacity.
For comparison, the same Sanyo charts seem to show that an alkaline
battery loses about 75% of its useful capacity when you go from 100 mA
rates to 1000 mA rates.
I gather that the loss of useful capacity is due in part to the series
resistance of the battery, and in part due to inefficiencies in the
chemistry under high rates of discharge.
This was also for a brand new cell, as repeated
charge/discharge cycles would also decrease the cell capacity.
Had these cells been "formed" properly prior to your test (via several
slow-charge / slow-discharge cycles)? There seems to be a general
agreement that NiMH cells don't achieve their maximum storage capacity
unless they've gone through several cycles like this, and that many
manufacturers don't take the time to do this during the manufacturing
process. I've seen some indication of this in my own testing (using a
smart-charger which can cycle the batteries and report the capacities
for each cycle).
--
Dave Platt AE6EO
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