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'[EE] NiMH batteries'
2005\06\02@010625 by Ed Browne

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Would NiMH batteries benefit from a "gas gauge"?

I was told that measuring the charge flow on NiMH would not work if the
batteries experience extremes in temperature.  Nevertheless, the same person
asked me to measure the current out, temperature and voltage on the cells --
doesn't seem useful if the first statement is correct.

These batteries will be used outdoors probably in the 40C range and exposed
to potentially hotter (conducted) temperatures.  The batteries are rated to
65C.  There are 9 stacks to measure so there are 27 measurements; the 9
current measurements are differential.  There is no specification of the
resolution or measurement range (9.6V nominal 8.8V minimum and 5.1V
considered dead), and, as always, there is no space, it must run forever,
and be built yesterday.

I'm thinking that a smart battery health system could be implemented inside
the batteries rather than taking all the measurements out to an acquisition
system.
Perhaps something like this: http://focus.ti.com/lit/ds/symlink/bq20z80.pdf

Anyone having any experience in this area willing to comment?  suggestions?

2005\06\02@013238 by Dave VanHorn

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At 12:09 AM 6/2/2005, Ed Browne wrote:
>Would NiMH batteries benefit from a "gas gauge"?
>
>I was told that measuring the charge flow on NiMH would not work if the
>batteries experience extremes in temperature.  Nevertheless, the same person
>asked me to measure the current out, temperature and voltage on the cells --
>doesn't seem useful if the first statement is correct.

You have a bunch of issues to solve, starting with assuring that your
cells will actually tolerate those temperatures.
Charging most NIMH above 40C isn't recommended, but I know that Sanyo
HR-AUC cells can be done at 50C, per Sanyo.
I have a bit of experience here, might be better offlist though.

2005\06\02@031111 by Russell McMahon

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> You have a bunch of issues to solve, starting with assuring that
> your cells will actually tolerate those temperatures.
> Charging most NIMH above 40C isn't recommended, but I know that
> Sanyo HR-AUC cells can be done at 50C, per Sanyo.
> I have a bit of experience here, might be better offlist though.

Sounds like excellent educational onlist stuff if you are happy to
share.


       RM

2005\06\02@103926 by Ed Browne

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Either way is fine - on-list or off.

I forgot to add another couple of bits of information.  The batteries are
quite powerful so because they will be used in Zone 1 application, they are
enclosed in a custom-made flameproof enclosure.  Actual current draw is a
fraction of the capable current and is intentionally limited to a little
more than 1 amp.  As such, changes in battery impedance caused by gas
formation should be minimized.

{Original Message removed}

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