Measure battery condition (safety critical unit)?

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I'm maybe going to build a battery powered embedded unit. It would be
necessary to determine the battery's condition at power on to alert the user
that he must change the battery.



The unit must know that it can activate a relay at least once during power
on. That is the purpose of the battery test.



The unit is going to have a microcontroller.



Any ideas of how to make that test? Maby measure the battery voltage when
the unit applies a resistive load to it. And then determine the V and the Ri
of the battery.



Should I use two batteries so I can measure one at a time at some interval?



It's for a safety critical unit.





Re: Measure battery condition (safety critical unit)?

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That is the usual way of doing it. It may not work very well for a
device that's going to be stored for a long time with the battery
disconnected; you need to choose the right battery chemistry for the
application. (Sounds like you have to solve some of the same problems
as an automatic [parachute] activation device; take a look at how the
pros do it in that application)

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"critical" voltage should be well above the brownout voltage for the
circuit, so you can simply turn on the load and see how far Vbat dips.


Re: Measure battery condition (safety critical unit)?
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Have you considered adding a capacitor (maybe an ultracap)?  If it's a
short term draw then the cap may allow you to operate with a much lower
peak power draw from the battery.

Robert

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Re: Measure battery condition (safety critical unit)?

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If your system is really safety critical you should consider a complete
re-think so that you do not need an active event to achieve a safe state.
Obviously 'safety critical' covers a multitude of possibilities but if there
is a significant risk of causing death or serious injury I would not advise
you to rely on a complex loop involving a battery test and a decision by
your software.

Michael Kellett

www.mkesc.co.uk



Re: Measure battery condition (safety critical unit)?


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For the application I described, it is SOP for the unit to do a self-
test when the user powers it up (before they step in the plane) and
report any problems at that time. It doesn't do the test immediately
before the critical event; it does the test when you indicate you will
soon be relying on it.


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