Re: My Battery Monitor

[Lee Hart <leeahart@earthlink.net>]

Darrell N. wrote:
> Pot it in epoxy or silicone potting compound.

That's a good solution. For a hobby project, I'd use something that can
be removed if you need to fix or change it.

> [voltage reference] Select the proper device, with the specifications
> you need.

Yes; there are lots of excellent parts for only slightly more than a
zener or 3-terminal regulator, but with far better specs.
 
> you can build in calibration constants in software (aint it wunnerful)
> to ensure they all read the same regardless of component tolerances.

This works to eliminate initial error, but not drift.

> There are lots of off the shelf voltage regulators which have built
> in load dump and reverse transient protection. Automotive electronics
> is a big market...

Exactly. The automotive market created the demand for more rugged
3-terminal regulators that will survive the auto environment. They are a
much better choice than generic parts.

> Protect all I/O pins that go outside the box with suitable ESD
> protection.

Yes, though it can be a challenge. It adds a lot of parts, and people
are inclined to not believe they are needed. "ICs have great built-in
protection, right?" Wrong!

> Watchdog timers usually are only as good as the software used to run
> them... illegal opcode trap... unused interrupt vectors point to the
> Reset vector... external Low Voltage Inhibit...

All good advice. However, the sad fact is that micros still crash in
ways that watchdog timers won't catch.

> Can't beat a crimp on terminal for reliability.

Yes you can. No connector is more reliable -- so use *NO* connector! :-)

I would have wire leads coming out of a potted module. No connectors any
where near the batteries.

For data, I like the idea of fiber optics. An LED or phototransistor can
be put in the module. Use pieces of inexpensive acrylic fiber optic as
"wire" between the modules and a central controller, several feet away.
Such a system would work even submerged in battery acid.

> Optocouplers are pretty reliable... Get ones rated for use directly
> on AC line applications. They have a list of approval initials (UL,
> CSA, VDE, etc)...

A good idea. UL listed optocouplers have actually been tested to prove
that they meet their isolation claims. Garden variety parts have not.

For example, GE used to use glass chips between their LEDs and
phototransistors on UL listed optocouplers, but plastic in the ones that
were not UL listed.

> Come on, Lee, 10,000 samples? If you couldn't get a decent reading
> after 16 or 32 samples, it was time to throw it all out and start
> again. There had to be some serious design flaw(s) for things to get
> that bad.

You try making stable measurements of battery voltage when it is being
hit with 100+amp charging and discharging transients!

If you are only taking 16 samples, and one of them just happens to hit
at the same instant as a contactor closes, the voltage can be VOLTS
different from the true average.

> The best way, IMHO, is to set up some sort of addressable bus
> architecture, where a master unit polls each slave in turn for it's
> information. Each slave would have it's own unique address, perhaps
> set by DIP switches...

"I want to turn on a light with a switch."
"OK, first we need to pick the microcomputers for the light and switch,
and define the network protocol they use to communicate..."

Do you really need all that complexity? All we're trying to do is
measure a battery's voltage :-)
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen