Re: Battery matching solution?

JB wrote:
I'm trying to match AA battery capacities so they can be used together
in battery packs and I think created a low-tech way to do this. I'd
like to hear more informed opinions.

This would apply to NiCads and NiMh but it should work with lithium-ions
too.

Take all your batteries of the same type and charge them all in parallel
to the same voltage. Then one at a time remove them from the float
voltage and attach them to a fixed size resistor (say 6 ohms) for a
fixed period of time (say 3 hours) then measure their voltage. Well
matched batteries should have similar voltages.

Does this wash?

The only thing I wonder about is the full charge state voltage. Say I'm
doing Ni-Cads which usually top-out around 1.65 volts (where they
experience a voltage drop), and I set the float voltage for the bunch at
1.5 volts. Would the maximum voltage of each cell make a difference
here?

Thanks for your help.
Under the parallel charging scenario you outline you should be
charging your NiCds with a CURRENT appropriate for the total amount of
capacity you have in parallel. I'd suggest the cells be reasonably
well charged before paralleling them for a final finishing charge.

The Li-Ion cells are the ones you need to charge to a specific (and
safe) voltage.

Your discharge regime sounds OK, but in addition to a discharge for a
certain length of time I'd continue that to the desired cutoff voltage
for the cell in question and record the time required to reach that
point. That will require you to be there and be more vigilant, but it
will give a far better figure for cell comparison.

You might want to plot voltage vs time at rather close intervals on at
least one cell just to see how voltage changes over time. Do that and
you'll probably see why I recommend measuring the time to a specific
cutoff voltage.

Lithium Ions are odd.

For the others, It is best to stay with same model cells. They are not
created equal. It is pretty easy to just make the pack and do your testing
with the MAHA, then throw out the cells that fall down first. I have the
Radio Shack/MAHA and sometimes it doesn't detect properly until the pack
gets going.

I have a handful of surplus NiMh 1ah AA cells that I constantly rotate and
they get used in cameras, radios, flashlights and anything else. They were
tab type that I pulled the tabs off and soldered a blob of Silver solder on
the plus to make them fit in holders. Been using them for at least 5 years
now.
====================================================
Nicad and NimH cells not regularly used can be best continously tickle charged at 10 -40 mA depending on size/capacity.
Example : I have a Wahl cordless battery powered soldering iron 2.4 V (2 cells) which is continously trickle charged at 10 mA . When in use the charge rate is switched to 300 mA.

Frank KN6WH / GM0CSZ

.

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