Electrical Engineering - Re: Adding voltmeter to car

Author

Re: Adding voltmeter to car - shunt questions

Andy

2006-04-14, 4:21 pm

> "Andy" <nomail@nomail.com> wrote :
>
> Auto voltmeter - also known as the "somebody stole your battery"
> gauge. Because other than a missing battery, it basically tells
> you, too late, what you already know - weak lights, poor start,
> bad battery, etc.

On 13 Apr 2006, hob<dehoberg@comcast.net> wrote:
>
> An in-dash ammeter, on the other hand, tells you before you are
> stranded if you are charging or discharging the battery and how
> much is going in or out, and the amps are if steady or
> fluctuating , all of which can be used as an indicator for the
> condition of the alternator and battery, the connections, and
> switch-off draining.
>
> Don't waste your time putting in a voltmeter.

Hob,

It seems to me that I can just connect a voltmeter across the
battery terminals and (as someone in this thread mention) include
an fuse inline with the hot wire near the battery as a safety
precaution.

I'm not familiar with current shunts (which you mention
elsewhere).

Where can I get a suitable current shunt from for my car and very
roughly what do they cost?

Do I simply install the shunt across the battery terminals and
then place a voltmeter (sic) across the two terminals of the
shunt?

Would I need to calibrate my multimeter to the shunt in order to
measure current values correctly?

Thank you.

[color=darkred]

hob

2006-04-15, 2:21 am

"Andy" <nomail@nomail.com> wrote in message
news:Xns97A5C73F32FA474C1H4@127.0.0.1...
>
>
> On 13 Apr 2006, hob<dehoberg@comcast.net> wrote:
>
>
> Hob,
>
> It seems to me that I can just connect a voltmeter across the
> battery terminals and (as someone in this thread mention) include
> an fuse inline with the hot wire near the battery as a safety
> precaution.
>
> I'm not familiar with current shunts (which you mention
> elsewhere).

Most external ones that I have seen are mounted on the firewall. The cases
were about two-three inches long. They are more used on vehicles with big
alternators.
Connection: Remove cable between alternator post and battery and run to
post on the shunt case which is bolted on the firewall; Alternator post
cabled to the other post on shunt case. Small leads off the two case posts
go to dash meter movement.

Dash mounted ammeters (one in my hand) have internal shunts inside the case
that fits in the dash, and there are posts on the back of the case to
connect heavy wire eyes. That would be the easy way to go for a smaller
vehicle.
Connection: Alternator post cable goes to dash gauge post, other dash gauge
post goes back to battery post. About number six wire, I think.

(Note that some European vehicles used to run the starter lead to the
alternator post and then on to the battery, unlike American cars that ran
separate leads from the battery to starter and alternator - the former had a
lot of problems in the US in cold climates/corrosion since some of the
hundred plus starting amps naturally went through the alternator rather than
all through the starter cables, smoking alternator parts as cars aged. Note
that If you put a regular auto ammeter in line with a starter, you may smoke
the ammeter)

Again - an ammeter is connected in series with the alternator-battery
cable so it reads the smaller charge-discharge amps of the alternator, and
it is not connected so it reads the large starting current.

>
> Where can I get a suitable current shunt from for my car and very
> roughly what do they cost?

The ammeter gauge I last bought was about $10 US. Truck repair places
probably have internals on hand (and probably also external)

For a while, they made dual gauges - top part an ammeter and the bottom part
a voltmeter (or vice versa). Haven't seen those in a while, though

background on the shunts in ammeters-

The shunt is in parallel with a meter movement (a voltmeter, bascially)
having an amp scale on its face, and the "voltmeter" reads the voltage drop
across the shunt. Since the shunt is an accurate resistance, the amps is
interpolated internally for the amps of the "ammeter" face (from ohms law).

The particular shunt and particular ammeter movement are coordinated -
electrical supply houses sell shunts, and you are looking an ammeter with an
external shunt around ,around .001 ohm/ 60-100 amps.

http://www.deltecco.com/resources.htm

>
> Do I simply install the shunt across the battery terminals and
> then place a voltmeter (sic) across the two terminals of the
> shunt?
>

NOT across the terminals. That would short out the battery.

Basically, it is connected in line with one battery cable, with the meter
movement reading ONLY across the shunt.
It is the shunt voltage drop that the meter movement is "calibrated to" so
as to give accurate amp readings

> Would I need to calibrate my multimeter to the shunt in order to
> measure current values correctly?
>

Yes - but, again - in-dash ammeters are all set up - internally shunted -
just connect them in-line.

> Thank you.
>
>
>

Gary Schafer

2006-04-15, 2:21 pm

On Fri, 14 Apr 2006 23:50:21 -0500, "hob" <dehoberg@comcast.net>
wrote:

>
>"Andy" <nomail@nomail.com> wrote in message
>news:Xns97A5C73F32FA474C1H4@127.0.0.1...
>
>Most external ones that I have seen are mounted on the firewall. The cases
>were about two-three inches long. They are more used on vehicles with big
>alternators.
> Connection: Remove cable between alternator post and battery and run to
>post on the shunt case which is bolted on the firewall; Alternator post
>cabled to the other post on shunt case. Small leads off the two case posts
>go to dash meter movement.
>
>Dash mounted ammeters (one in my hand) have internal shunts inside the case
>that fits in the dash, and there are posts on the back of the case to
>connect heavy wire eyes. That would be the easy way to go for a smaller
>vehicle.
> Connection: Alternator post cable goes to dash gauge post, other dash gauge
>post goes back to battery post. About number six wire, I think.
>
>(Note that some European vehicles used to run the starter lead to the
>alternator post and then on to the battery, unlike American cars that ran
>separate leads from the battery to starter and alternator - the former had a
>lot of problems in the US in cold climates/corrosion since some of the
>hundred plus starting amps naturally went through the alternator rather than
>all through the starter cables, smoking alternator parts as cars aged. Note
>that If you put a regular auto ammeter in line with a starter, you may smoke
>the ammeter)
>
> Again - an ammeter is connected in series with the alternator-battery
>cable so it reads the smaller charge-discharge amps of the alternator, and
>it is not connected so it reads the large starting current.
>
>
>The ammeter gauge I last bought was about $10 US. Truck repair places
>probably have internals on hand (and probably also external)
>
>For a while, they made dual gauges - top part an ammeter and the bottom part
>a voltmeter (or vice versa). Haven't seen those in a while, though
>
>background on the shunts in ammeters-
>
>The shunt is in parallel with a meter movement (a voltmeter, bascially)
>having an amp scale on its face, and the "voltmeter" reads the voltage drop
>across the shunt. Since the shunt is an accurate resistance, the amps is
>interpolated internally for the amps of the "ammeter" face (from ohms law).
>
> The particular shunt and particular ammeter movement are coordinated -
>electrical supply houses sell shunts, and you are looking an ammeter with an
>external shunt around ,around .001 ohm/ 60-100 amps.
>
>http://www.deltecco.com/resources.htm
>
>
>NOT across the terminals. That would short out the battery.
>
> Basically, it is connected in line with one battery cable, with the meter
>movement reading ONLY across the shunt.
> It is the shunt voltage drop that the meter movement is "calibrated to" so
>as to give accurate amp readings
>
>
>
>Yes - but, again - in-dash ammeters are all set up - internally shunted -
>just connect them in-line.
>
>
>
>

Hob,

The way you are explaining to hook up the ammeter will only give
indication of charge output from the alternator. It will not show any
discharge from load on the battery.

The ammeter needs to be in the alternator lead but between the point
where the supply to the car's electrical system is taken from and the
battery. In other words, the lead to the cars electrical system will
be on the alternator side of the lead that connects to one side of the
ammeter. The other side of the ammeter would go to the battery.

Sometimes this is difficult to do as often the car's power is taken
from the battery cable where it attaches to the starter. Sometimes it
is taken at a splice / junction in the lead between the alternator and
the battery, much easier to do here.

Regards
Gary

hob

2006-04-16, 5:21 am

"Gary Schafer" <gaschafer@comcast.net> wrote in message
news:2f7242dl9ls9psvbhmpr13v5ee35hjd5d4@4ax.com...
> On Fri, 14 Apr 2006 23:50:21 -0500, "hob" <dehoberg@comcast.net>
> wrote:
>
cases[color=darkred]
big[color=darkred]
posts[color=darkred]
case[color=darkred]
gauge[color=darkred]
had a[color=darkred]
than[color=darkred]
Note[color=darkred]
smoke[color=darkred]
and[color=darkred]
part[color=darkred]
drop[color=darkred]
law).[color=darkred]
an[color=darkred]
meter[color=darkred]
so[color=darkred]
>
> Hob,
>
> The way you are explaining to hook up the ammeter will only give
> indication of charge output from the alternator. It will not show any
> discharge from load on the battery.

I imagine that vehicles may differ, and perhaps I wasn't clear
- in mine, the starter gets a heavy cable off the hot post, and the
alternator gets a lighter cable off the hot post. The vehicle non-starter
power is then taken off that alternator post which feeds the battery. That
puts the ammeter between the alternator and power panel lead, and the
battery.
If there are three feeds off the hot battery post (starter, alternator,
and accessory power panels), you may well need to cut that (third) power
panel feed cable at the battery post, add a crimped on eye on the end, and
move that power panel end over to the alternator post holding the
alternator-to-battery cable.
In other words, move the panel-feed end from the battery post to the
alternator post ( it's the same point electrically)

>
> The ammeter needs to be in the alternator lead but between the point
> where the supply to the car's electrical system is taken from and the
> battery. In other words, the lead to the cars electrical system will
> be on the alternator side of the lead that connects to one side of the
> ammeter. The other side of the ammeter would go to the battery.

Yes, agreed

>
> Sometimes this is difficult to do as often the car's power is taken
> from the battery cable where it attaches to the starter.

While I have personally not seen that panel-connection coming off the
starter, I would think the moving of the panel-connection cable end over to
the alternator post would solve the problem.

Right? Input is always good - you never know...

Sometimes it
> is taken at a splice / junction in the lead between the alternator and
> the battery, much easier to do here.
>
> Regards
> Gary
>

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