For current measurement, I still rely on my trusty old Fluke,
which isn't cheap, but nowadays you can probably get very cheap
DMMs (digital multimeters), perhaps even at radio schlock.
As a matter of fact, here's one that lists for $6.95:
http://tinyurl.com/a9lk9
With any reasonable digital meter, you can just take a VOLTAGE
reading (after the cell has cooled to room temperature) for a
fairly accurate way to match cells. Although you're not
measuring current, if all cells have gone through identical
charging and cooled for the same length of time, then they
should have identical initial voltages. Technically, of course,
the entire discharge curve should be matched, but that's a bit
excessive.
You could also read current through a small resistor, or even a
momentary dead short across the battery in current mode, but I
wouldn't recommend that unless the meter has a peak-reading
(latching) mode so you can touch the terminals VERY briefly. And
cheapo meters may have very little current handling capacity.
However, with AAA NiMH batteries, the internal resistance is
fairly high, so current dumping isn't all that risky.
Please note that some kinds of batteries, especially NiCAD and
Lithium, can dump current tremendously fast (almost no internal
resistance), and could even explode if you do something like
this when fully charged. For safety, or if you're not sure of
the battery type, don't do the dead-short test! Use a 10k
resistor, or such. You're really only after a relative
measurement anyway...
One advantage of rating the cells by measuring their voltage
(which should correlate to energy density in a given
form-factor) is that it's non-invasive. Current measurements,
unless done with a large limiting resistor, will be depleting
the cell as you measure it, perhaps leading to misleading
results.
Four 12V packs in series sounds very reasonable (and
convenient).
ac
Allen Cobb
http://acobb.com
http://shakespeare.acobb.com
http://timbreproductions.com
-----Original Message-----
From:
Behalf Of Rob
Danielson
Sent: Saturday, November 26, 2005 4:55 PM
To:
Subject: RE: [Nature Recordists] DIY 5X9v bat phantom supply
At 4:08 PM -0600 11/26/05, Allen Cobb wrote:
>FYI - Most 9V batteries I've dismantled contain 6 cells that
are
>close (about 2mm shorter) to AAA cells.
Of course! Duh,..!$#! So maybe I wasn't too far off target with
the
2/3's size AA NiMH's. Mike says we'd need to measure the current
of
each cell to match them. Got a link for something cheap to
measure
current? After that, I only feel confident about charging NiMh
with a
MAHA charger so that could mean four, 12 volt (4 X 10 X 1.2)
battery
packs. Mike or Allen, see any problem with using 4 packs
serially?
Rob D.
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