Re: AA battery tests

On Tue, 28 Feb 2006 18:07:22 +1100, "Phil Allison" <philallison@xxxxxxxxxx>
wrote:


"Tim Wescott

For high current draw capability from a given volume NiCd is probably
best, although NiMH seems to be catching up.


** If you check the Sanyo site - they quote 12 millohms Ri for their KR-800
AA cell.

That's 12 milliohms measured at 1000 Hz, with the cell half discharged. I'm
not sure just how that translates into DC current with the cell fully charged,
so I decided to measure a couple of cells.

I happen to have 4 GE/Sanyo NiCad high capacity (1000 mAh) cells, and 4 Sanyo
NiMh 2500 mAh cells. I connected a couple of 3 inch long pieces of heavy copper
strap to a 500 amp 50 mv shunt, positioned so that I could short a AA cell
through the shunt. All the cells were freshly charged.

One of the NiCad cells pinched tightly between the copper strips gave an
initial current of 550 amps!! The current fell off fairly rapidly and the
cell's positive terminal heated up equally rapidly. I was loath to leave it
connected for more than about 5 seconds. After 5 seconds the current was down
to about 300 amps and still decreasing. Removing the short, the positive
terminal of the cell was quite hot. After about 30 seconds the entire cell was
quite warm. It took that long for the internal heat to diffuse to the outside
case.

Next, one of the NiMh cells was put to the test. The initial current was
about 380 amps, and in a couple of seconds decreased to about 350 amps, where it
remained constant for 7 seconds, when I disconnected it. Its positive terminal
wasn't as hot as the NiCad had been, and after a wait of 30 seconds, the entire
cell did not heat up near as much as the NiCad had.

The other 3 cells of each type gave performance similar to the first.

This test convinces me that modern Sanyo high capacity NiMh cells would give
better performance than NiCads.


Equates to 100 amps short cct current !!!

NiMHs, go weep.



Of greater concern is the fact that you need to maintain 4V with four
batteries -- the rule of thumb for NiCd, NiMH and dry cells are for a 0.9V
per cell "discharged" point; you'll be throwing capacity away with your 4V
threshold.


** Most cell makers say (plus my own tests indicate) that a Ni-Cd cell is
pretty much discharged at 1.1 volts. But you often see 0.9 volts *per cell*
quoted as a voltage not to go below when discharging a pack of Ni-Cd cells.





......... Phil



.

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