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Discussion Starter · #1 ·
Hello everybody,

I have been working to add a volt meter and ammeter to my car for a while.
I think they would look cool if they were digital. I've already created a
copper box to put them in. The meters are something like:

http://cgi.ebay.com/3-DC-500A-Blue-LCD-Digital-AMP-Panel-Meter-Shunt-/260546
536918?pt=LH_DefaultDomain_0&hash=item3ca9c8a5d6

(Item #260546536918) if the URL doesn't come through correctly.

Having already blown one ammeter (similar to the one above but different
supplier), I've been looking back at what I did wrong and I think I
understand it. These are fairly cheap, but I'd rather not blow another!

Situation is:

Positive battery terminal -> manual disconnect -> ammeter shunt -> main
contactor (w/ precharge resistor) -> controller.

The manual disconnect is normally connected except when servicing the
vehicle.

Power to the ammeter is controlled by the ignition switch.

As soon as I started to connect the manual switch (with the ignition off),
my ammeter blew.

The supplier of this new meter recommends that the shunt be placed on the
negative side of the battery. Obviously, I have it on the positive side of
the battery. I think the problem is that the meter inputs (connected to the
shunt) see a DC offset of 150V and while it might be able to take this while
powered up, it can't take it without power.

Does this sound like a reasonable theory?

Are there any other options besides figuring out how to move my shunt to the
negative side of the cells? Because of the configuration in the car, that
will be difficult.

I have a feeling that this problem would also go away if I had an analog
meter. That's starting to look better. Comments on whether that would
work better with a 150V input all the time?

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Discussion Starter · #2 ·
An analog meter will definitely work with the shunt any where in the
circuit. Just remember that you will be bringing pack voltage into the cab
with your shunt on the positive side. I'm glad I put my shunt in the
negative side because when I installed a Cycle Analyst it required the shunt
to be on the negative side.

About blowing the meter, I'm wondering if you over volted it. Did you have a
version with a power supply that would work from your pack voltage?

Mike Nickerson <[email protected]>wrote:

> Hello everybody,
>
> I have been working to add a volt meter and ammeter to my car for a while.
> I think they would look cool if they were digital. I've already created a
> copper box to put them in. The meters are something like:
>
>
> http://cgi.ebay.com/3-DC-500A-Blue-LCD-Digital-AMP-Panel-Meter-Shunt-/260546
> 536918?pt=LH_DefaultDomain_0&hash=item3ca9c8a5d6
>
> (Item #260546536918) if the URL doesn't come through correctly.
>
> Having already blown one ammeter (similar to the one above but different
> supplier), I've been looking back at what I did wrong and I think I
> understand it. These are fairly cheap, but I'd rather not blow another!
>
> Situation is:
>
> Positive battery terminal -> manual disconnect -> ammeter shunt -> main
> contactor (w/ precharge resistor) -> controller.
>
> The manual disconnect is normally connected except when servicing the
> vehicle.
>
> Power to the ammeter is controlled by the ignition switch.
>
> As soon as I started to connect the manual switch (with the ignition off),
> my ammeter blew.
>
> The supplier of this new meter recommends that the shunt be placed on the
> negative side of the battery. Obviously, I have it on the positive side of
> the battery. I think the problem is that the meter inputs (connected to
> the
> shunt) see a DC offset of 150V and while it might be able to take this
> while
> powered up, it can't take it without power.
>
> Does this sound like a reasonable theory?
>
> Are there any other options besides figuring out how to move my shunt to
> the
> negative side of the cells? Because of the configuration in the car, that
> will be difficult.
>
> I have a feeling that this problem would also go away if I had an analog
> meter. That's starting to look better. Comments on whether that would
> work better with a 150V input all the time?
>
> _______________________________________________
> | REPLYING: address your message to [email protected] only.
> | Multiple-address or CCed messages may be rejected.
> | UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
> | OTHER HELP: http://evdl.org/help/
> | OPTIONS: http://lists.sjsu.edu/mailman/listinfo/ev
>



--
David D. Nelson
http://evalbum.com/1328
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Discussion Starter · #3 ·
I admit I've never tried these cheap digital meters in an EV, but that's
because I have some real doubts about how well they'll work there.

For one thing, they don't have much if any noise shielding or rejection
circuitry. The electronic hash from a high power controller, and maybe the
DC:DC converter, is liable to play hob with them.

I'm also concerned about their ability to withstand the extremes of
temperature and humidity, especially the hellish summer heat in a closed
vehicle. Most of them are chip on board design, which is usually only good
for a few years' worth of service under the best of conditions. Add the
stress of a vehicle environment and ... well, let's say that I wouldn't bury
them inside the instrument panel where they're hard to get to.

David Roden
EVDL Administrator
http://www.evdl.org/


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Discussion Starter · #4 ·
I've always seen ammeters installed in the negative side... the reason being
is that both signal wires are very close to ground potential then (this is
always assuming a negative grounded system, and that the power supply to the
meter also uses negative as the constant reference when dropping the
voltage). Asking them to float at 150 volts and 149.925 volts all the time
doesn't sound good to me. With an analogue meter, as long as the isolation
between the meter and the case is sufficient, it works, but you still have
some tiny high voltage wires going through the car. With a digital, it can
probably only take about 15 volts input before blowing (it looks like it
runs on 5 volts after the power supply?... so maybe only 5 volts... that's a
common input high end as well). You need to make sure that it never sees
more than that on the signal lines... which means putting it on the negative
line of the batteries.

Z

EVDL Administrator <[email protected]>wrote:

> I admit I've never tried these cheap digital meters in an EV, but that's
> because I have some real doubts about how well they'll work there.
>
> For one thing, they don't have much if any noise shielding or rejection
> circuitry. The electronic hash from a high power controller, and maybe the
> DC:DC converter, is liable to play hob with them.
>
> I'm also concerned about their ability to withstand the extremes of
> temperature and humidity, especially the hellish summer heat in a closed
> vehicle. Most of them are chip on board design, which is usually only good
> for a few years' worth of service under the best of conditions. Add the
> stress of a vehicle environment and ... well, let's say that I wouldn't
> bury
> them inside the instrument panel where they're hard to get to.
>
> David Roden
> EVDL Administrator
> http://www.evdl.org/
>
>
> _______________________________________________
> | REPLYING: address your message to [email protected] only.
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Discussion Starter · #5 ·
Hello Mike,

One thing I did when I install all my instruments circuits, is that when I
turn off the ignition switch, all circuits to the dash instruments to the
dash is switch off except for the E-meter. I do not like standing voltages
24 hours a day on these instruments.

The E-meter which is a digital meter, has the main battery voltage and
ampere display comes off a shunt on the negative side of the main battery as
recommended by the manufacturer. Each sense wires that go to this digital
meter are shielded, then pair twisted, bundle in a jacket, a shield over
that jacket and another jacket.

The internal wire shields are floating, not connected to any ground and the
external cable shield is only chassis grounded at the meter side.

This meter does not use the full voltage of the main battery pack, it
proportional the voltage to a lower safe voltage.

I also have a analog battery amp and volt meter that is design for a EV by
GE. The shunt is on the positive side of the main battery circuit. Each
sense wire from the amp meter is connected to a 1 amp Busman Limitron fuse
at the shunt.

This shunt is directly attach to a battery safety contactor which is then
connects to a Busman Limitron 400 amp fuse and than to the battery. The
sense wires are shield twisted pair in a double jacket cable that is kept
separate from the other cables.

I also have another motor amp analog amp meter that is connected to a shunt
between the motor controller and motor on the positive side. The sense
screws have a clip on fuse holder for a 1 amp Busman Limitron fuse which the
sense wires have no shield and are twisted together in a wire loom tube.

The sense wires if shorted together will not blow the fuse, but if one is
shorted to one of the volt meter wires, then it will blow the fuse.

If not fuse and it did shorted, you could burn this wire all the way back to
the battery and your EV.

Roland

----- Original Message -----
From: "Mike Nickerson" <[email protected]>
To: "'Electric Vehicle Discussion List'" <[email protected]>
Sent: Thursday, March 17, 2011 10:41 PM
Subject: [EVDL] Digital Ammeter Installation


> Hello everybody,
>
> I have been working to add a volt meter and ammeter to my car for a while.
> I think they would look cool if they were digital. I've already created a
> copper box to put them in. The meters are something like:
>
> http://cgi.ebay.com/3-DC-500A-Blue-LCD-Digital-AMP-Panel-Meter-Shunt-/260546
> 536918?pt=LH_DefaultDomain_0&hash=item3ca9c8a5d6
>
> (Item #260546536918) if the URL doesn't come through correctly.
>
> Having already blown one ammeter (similar to the one above but different
> supplier), I've been looking back at what I did wrong and I think I
> understand it. These are fairly cheap, but I'd rather not blow another!
>
> Situation is:
>
> Positive battery terminal -> manual disconnect -> ammeter shunt -> main
> contactor (w/ precharge resistor) -> controller.
>
> The manual disconnect is normally connected except when servicing the
> vehicle.
>
> Power to the ammeter is controlled by the ignition switch.
>
> As soon as I started to connect the manual switch (with the ignition off),
> my ammeter blew.
>
> The supplier of this new meter recommends that the shunt be placed on the
> negative side of the battery. Obviously, I have it on the positive side
> of
> the battery. I think the problem is that the meter inputs (connected to
> the
> shunt) see a DC offset of 150V and while it might be able to take this
> while
> powered up, it can't take it without power.
>
> Does this sound like a reasonable theory?
>
> Are there any other options besides figuring out how to move my shunt to
> the
> negative side of the cells? Because of the configuration in the car, that
> will be difficult.
>
> I have a feeling that this problem would also go away if I had an analog
> meter. That's starting to look better. Comments on whether that would
> work better with a 150V input all the time?
>
> _______________________________________________
> | REPLYING: address your message to [email protected] only.
> | Multiple-address or CCed messages may be rejected.
> | UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
> | OTHER HELP: http://evdl.org/help/
> | OPTIONS: http://lists.sjsu.edu/mailman/listinfo/ev
>

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Discussion Starter · #6 ·
Mike Nickerson <[email protected]>wrote:

> Hello everybody,
>
> I have been working to add a volt meter and ammeter to my car for a while.
> I think they would look cool if they were digital. I've already created a
> copper box to put them in. The meters are something like:
>
>
> http://cgi.ebay.com/3-DC-500A-Blue-LCD-Digital-AMP-Panel-Meter-Shunt-/260546
> 536918?pt=LH_DefaultDomain_0&hash=item3ca9c8a5d6
>
>
I was able to get this cheap ebay panel meter to work:
http://cgi.ebay.com/Universal-Digital-LED-Volt-Current-voltage-panel-meter-/230384246160?pt=LH_DefaultDomain_0&hash=item35a3f89d90#ht_1357wt_1101

<http://cgi.ebay.com/Universal-Digital-LED-Volt-Current-voltage-panel-meter-/230384246160?pt=LH_DefaultDomain_0&hash=item35a3f89d90#ht_1357wt_1101>
from
this guy:
http://stores.ebay.com/ColdfusionX-Electronics?_trksid=p4340.l2563

My shunt is on the high side and I *think* i had to power the meter with a
small dc/dc converter to isolate it from the regular 12v system. It was
either this meter or the voltmeter that ended up requiring it...

It actually worked very well in the AZ heat, but something in the system was
giving weird readings when it was cold, like it was going in and out.
Could've been a loose connection or something somewhere else... It's going
on 2 years and 10k miles.

Another data point,
Joe
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Discussion Starter · #7 ·
From Mike Nickerson:
> I have been working to add a volt meter and ammeter to my car for
> a while. The meters are something like [eBay item #260546536918]:

This is basically the same meter that sells for under $10 from many
sources. They have added the backlight LED, and changed the input
scaling resistors for +/-75mv full scale. The main cost adder is the
special 75mv/500a shunt they supply.

The meter uses the old Intersil ICL7106 3-1/2 digit LCD meter chip. It
is widely used in very low cost digital multimeters. This chip has
several limitations that the designer has to work around:

- Input voltage range for full accuracy is +/-200mv. Below this,
accuracy suffers unless they added an input amplifier. My
guess is that they don't.

- The chip's internal reference is fair, but not very stable with
temperature. It is only intended for indoor use, so you will see
error at high and low temperatures. A better external reference
could be used, but I'll bet they didn't.

- The chip is designed to be powered by a 9v battery. Battery
power is extremely well isolated and pure DC (no noise). The
analog input voltages have to be about halfway between the battery's
+ and - terminal's voltages.

>> Having already blown one ammeter (similar to the one above but different
>> supplier), I've been looking back at what I did wrong and I think I
>> understand it. These are fairly cheap, but I'd rather not blow another!

My guess is that you powered the meter with a power source that was not
well enough isolated. Even 100 megohms of DC leakage, or 100pf of
capacitance between the input and output of this power supply will cause
problems.

Let's say you used a typical switchmode "wall wart" AC/DC power supply
to power the meter. These supplies usually have a capacitor to ground of
about 4700pF for RFI filtering. Your pack voltage bounces up/down tens
of volts relative to ground as the controller operates. This 4700uF
capacitor couples this AC noise right to the meter's power. 10v will
drive it nuts from noise. 20v will kill it!

These supplies also often have a resistor to ground of something like 1
meg, to discharge its big capacitors when you unplug it so the input
pins don't have high voltage. This resistance tries to pull the meter's
power to ground, causing errors.

The vendor suggests putting the shunt in the negative leg of the supply.
But this only helps if the EV pack has its negative side grounded. In
most EVs, the propulsion pack is floating, and neither side is grounded.

Look for a power supply that specifies its input-output leakage current
and capacitance. It should be something like over 1000 megohms, and less
than 1pf.

Or, use a battery to power it. Or, a small solar panel and a light bulb. :)

> I have a feeling that this problem would also go away if I had an analog
> meter.

Sure; it requires no external power.

--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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Discussion Starter · #8 ·
Mike Nickerson wrote:
> Thanks for the circuit. I think that may be exactly what I need.
> Great solution to the problem!
>
> - It looks like the capacitors are only across the shunt voltage
> so they don't need to be rated for high voltage.

Correct. If you're using this circuit for a shunt, the voltage is very
low, so the capacitors work at less than 1v. It's a good application for
these low-voltage supercapacitors normally used for memory backup.

The capacitance depends on how much current your meter draws. For
example, suppose it draws 1 ma, and you will be switching the relay at 1
cycle per second. Let's say you allow a 1% droop in voltage (meter gets
49.5mv when shunt is at 50mv). Then the capacitor needed is

C = IT/V = 0.001a x 1s / 0.00005v = 2 farads

If you're using a digital meter, their input current is usually much
lower (since they are getting their power elsewhere). They could use
about 1/100th the capacitance. A normal electrolytic, or even a high
capacitance ceramic or film cap could be used.

If you're measuring high voltage, then the meter will have far higher
resistance, and the voltage sag to cause given percent error is far
larger. This allows the capacitor to have far less capacitance, but it
will of course need a higher voltage rating. I wouldn't use an
electrolytic in this case, due to their high leakage current.

> - Do you use something like a 555 timer chip to switch the reed relay?
> Something simpler than that?

A 555, or just about any oscillator is fine. A minimal parts solution
would be a flasher LED, driving the base of a transistor to switch the coil.

The Edison solution is to wire the relay like a buzzer, so its own
contacts switch the coil on/off. The rate of on/off switching was slowed
by adding a copper slug (big shorted turn) to the relay coil.

Hint: Reed relays have excellent life -- billions of cycles at low
current. At a 1 cycle per second rate, and if only enable when driving,
they will last forever. Their contacts are sealed in glass tubes, so
they are essentially silent in operation. You can find reeds with
voltage ratings from 100v to 1000v, so they are an good choice here.
Just be sure to include R1, to keep the worst-case peak current in the
reed within its ratings.

You can make your own reed relays. It's easy to buy the capsules (little
glass tubes with the wires on each end). Put two SPST capsules in any
12v solenoid coil. To make the Edisonian self-oscillating type, add one
more more reed and use its normally-closed contact to switch the coil.
Put the reeds inside a thick copper tube, and put the coil over it. The
copper drastically slows down the pickup and release time. :)
--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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