# [EVDL] Indachron-based amp-hour meter (Was: Low-Cost Amp-Hour Counter)



## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> 
> > Bruce wrote:
> >> Is it possible to build a low-cost Amp-Hour Counter?
> ...


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## EVDL List (Jul 27, 2007)

>> The cheapest one I know of is an Indachron.
>
> <snip>
> There are other models, but they don't count coulombs directly.
> Incidentally, "resettable" means you can flip the indicator over once
> it's reached full and make it look empty. Like turning over an
> hourglass. We don't need this feature, I think. We'll just run
> current through it backwards to reset it.

I don't think that will work for an EV. Every battery I've ever heard of
requires you to put in MORE Amp ours than you took out. If you don't have
a way to reset the counter at the end of charge, it won't be at zero and
you will quickly accumulate Amp Hours after every charge

> Is there a simple (analog) way to add a crude Peukert's correction
> factor? And how would you calibrate such a thing, and make it ignore
> Peukert's during charging? Is it worth the effort?

Peukert effects charging as well as discharging.

The simplest way to use Peukert is to calculate the Peukert exponent using
data that brackets your EV's average discharge current, then calculate the
Peukert capacity of your pack using that number, then do a running tally
of Peukert Amp Hours drawn from the pack, preferably updated several times
per second.

In other words, you need a computer.

A simpler option is to use a dual movement meter with crossed needles, but
it takes a bit of work to calculate and create the scale behind the
movements.

I believe Mark Brueggemann originally came up with this idea.
You can find directions for building it on page 14 of this document (as
well as some other methods for tracking capacity):
http://www.eaaev.org/CurrentEvents/pdf/2001/CurrentEvents200111.pdf


>
> How would this device compare to Mark Brueggeman's cross-needle SoC
> meter for determining your pack's state of charge? I imagine it would
> be more physically robust than a panel meter. Which approach would
> give you the most accurate indication of when it's time to recharge
> your EV?

I don't think there are any "accurate" methods, they are all basically
'best guess'.

If you want simple, then you really only have two choices:
A volt meter and amp meter and a LOT of experience (seat of your pants
instrumentation)
Or Mark Brueggeman's meter.

Everything else I've ever seen requires a computer.

FWIW Mark's meter is probably every bit as reliable and "accurate" as the
more expensive processor based solutions.

I don't know if it's been posted recently but you can find directions to
building Mark's crossed needle meter on page 14 of this document (as well
as some other methods for tracking capacity):
http://www.eaaev.org/CurrentEvents/pdf/2001/CurrentEvents200111.pdf



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## EVDL List (Jul 27, 2007)

> Peter VanDerWal wrote:
> 
> > Doug Weathers wrote:
> >> We'll just run
> ...


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## EVDL List (Jul 27, 2007)

>> In other words, you need a computer.
>
> More generally, what you need is a device that performs integration
> over time. A digital computer is the currently fashionable choice, but
> analog methods work too. The Indachron does this by virtue of how it
> operates. All we need to do is feed it a corrected value for the
> current and let it integrate the corrected value for us.
>
> Now I'm on shaky ground, though, not being an EE. How do we correct
> the current? It seems like we need a resistance that varies according
> to the shunt voltage in a way that approximates the Peukert exponent of
> your pack. It won't be linear, it will be exponential. Transistors

Doing a peukerts correction based on the instantaneous current is easy. 
The problem is that you have to also keep track of the Peukert's capacity
for the previous second, and the second before that, and the second....

AND the really hard part is that in an EV the peukerts capacity in EACH of
those seconds is going to be different because the current is going to be
different.

You could do this with analog components, but in essence you'd be building
an analog computer (calculating and storing data).

Seems to me that it would be a lot easier to just use a PIC or similar
uprocessor.

> vary resistance in response to voltage, and (for example) the voltage
> of a capacitor decays exponentially, so it seems like something could
> be put together. Just not by me.
>
> It will need to be settable, so you can adjust it to your pack, and so
> you can adjust it as your pack ages.
>
> If I'm actually going to build one of these, I should probably forget
> about Peukert compensation and just see how it works. Unless there's a
> clever circuit that's trivial to implement.
>
>>> How would this device compare to Mark Brueggeman's cross-needle SoC
>>> meter for determining your pack's state of charge? I imagine it would
>>> be more physically robust than a panel meter. Which approach would
>>> give you the most accurate indication of when it's time to recharge
>>> your EV?
>>
>> I don't think there are any "accurate" methods, they are all basically
>> 'best guess'.
>
> I didn't say "accurate", I said "most accurate". Which is better?
>
> Given the accuracy of the standard ICE gas gauge, perhaps "best guess"
> is good enough.
>
>> If you want simple, then you really only have two choices:
>> A volt meter and amp meter and a LOT of experience (seat of your pants
>> instrumentation)
>> Or Mark Brueggeman's meter.
>>
>> Everything else I've ever seen requires a computer.
>
> Myles Twete has an amp-hour meter on his 1921 Milburn. It's not the
> sort of "computer" you're thinking of (digital).
>
> <http://www.evalbum.com/348>
>
>> FWIW Mark's meter is probably every bit as reliable and "accurate" as
>> the
>> more expensive processor based solutions.
>
> Yeah, I like it too. I intend to duplicate it with a grid of LEDs and
> a couple of bar-graph driver chips, to get rid of the panel meter. The
> Indachron intrigues me because it also gets rid of the fragile panel
> meter, and because it's readable when the power is out (like a stopped
> clock), and because it's similarly elegant.
>
> An Indachron would be a dandy addition to the PFC helper box I'm
> mulling over. It could tell me at a glance if the charge cycle
> completed, without having to power up the car or even get in. Peukert
> correction wouldn't be needed for this use. Open the trunk, check the
> Indachron, stow the power cord, close the trunk, get in, drive off.
>
>> I don't know if it's been posted recently but you can find directions
>> to
>> building Mark's crossed needle meter on page 14 of this document (as
>> well
>> as some other methods for tracking capacity):
>> http://www.eaaev.org/CurrentEvents/pdf/2001/CurrentEvents200111.pdf
>
> Thanks, nice document. It's also available on Mark's site:
>
> <http://www.qsl.net/k5lxp/ev/evgauge/evgauge.html>
>
>
> --
> Doug Weathers
> Las Cruces, NM, USA
> http://www.gdunge.com/
>
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> http://lists.sjsu.edu/mailman/listinfo/ev
>


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## EVDL List (Jul 27, 2007)

>>> be more physically robust than a panel meter. Which approach would
>>> give you the most accurate indication of when it's time to recharge
>>> your EV?
>>
>> I don't think there are any "accurate" methods, they are all basically
>> 'best guess'.
>
> I didn't say "accurate", I said "most accurate". Which is better?

Well, that's the point. Under a given set of circumstances one might be
more accurate than the other, but under different circumstances it might
be the other way around.

The dual movement meter will probably be the best with new batteries, IF
you have good data available when you make the faceplate. However it
doesn't adjust for age or temperature, which might affect the readings. 
And it can't adjust for changes in the packm i.e. bypassing a bad battery
etc. Well, it can't easily adjust.

>
> Given the accuracy of the standard ICE gas gauge, perhaps "best guess"
> is good enough.
>
>> If you want simple, then you really only have two choices:
>> A volt meter and amp meter and a LOT of experience (seat of your pants
>> instrumentation)
>> Or Mark Brueggeman's meter.
>>
>> Everything else I've ever seen requires a computer.
>
> Myles Twete has an amp-hour meter on his 1921 Milburn. It's not the
> sort of "computer" you're thinking of (digital).
>
> <http://www.evalbum.com/348>
>
>> FWIW Mark's meter is probably every bit as reliable and "accurate" as
>> the
>> more expensive processor based solutions.

I vaguely remember reading about this, does it take peukert into account?

> Thanks, nice document. It's also available on Mark's site:
>
> <http://www.qsl.net/k5lxp/ev/evgauge/evgauge.html>

Thanks back, I couldn't find his website on a quick search.

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## EVDL List (Jul 27, 2007)

> Doug Weathers wrote:
> > Having heard this from Lee on a few other occasions, I finally stirred
> > myself to look for this [Indachron]. They're made by Curtis. The ones I
> > found cost US $20-$30.
> ...


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