Re: battery charge level indicator

[John Denker <jsd@AV8N.COM>]

fred bucheit wrote:
>   Most electronic equipment today has an LED readout that indicates the
> energy left in the battery. Any idea how accurate these are and on what they
> rely?

There are various ways of skinning that cat.
The simplest ones just look at the voltage.

For a lead-acid battery, voltage is a rather reasonable
indication of state-of-charge.  The sulfuric acid is one
of the principal reactants, and as the battery discharges
the concentration of acid goes down.  This directly affects
the voltage ... Nernst equation and all that.

For the NiCd and NiMH batteries, I imagine the physics is
roughly similar ... but it must be the electrode material
that is being depleted, since the only thing that is being
taken from the electrolyte is H2O and you're not going to
significantly deplete that in normal operation.  The usual
description of equilibrium found in chemistry textbooks
doesn't handle this case too well, because they assume any
solid reactant is never the rate-limiting reactant, so you
have to go back to first princples... big mess.

> Can the type of battery (NiMh or NiCAD) fool them?

Sure,anything that changes the nominal full-charge voltage
will badly fool a checker that only looks at voltage.

Of course there are very, very much more sophisticated
checkers out there also.  These look at lots of variables
including
 -- voltage of course.
 -- temperature of the battery (which markedly affects voltage)
 -- impedance of the battery.
 -- charge history (integral of current)
 -- etc. etc.

Imagine you had a small submarine with a million dollars worth
of battery ... it would be worth your while to take really good
care of the battery.