At what point is a vehicle battery charging or discharging? 12.6v ?

> Or you could have a completely open battery.. or the charging system
> has gone out, but your current drain on the battery is low enough that
> it's not visible on the ammeter..

Sure, but with an open battery, that becomes obvious at start time.
Trust me, BTDT.

Proper scaling is also important, no sense using a 100A meter on a 10A
system.

> Auto manufacturers have gotten away from ammeters and gone to voltmeters..

Yes, but the question is "why".. For example, the oil pressure gage in my
truck is shown as a linear gage, but it is driven by a switch. Too many
people got worried over the normal fluctuations in pressure and generated
service calls under warantee.. So now, that useful information is gone.

Most people can sort of understand a volt meter, but are pretty clueless
about an amp meter.
Also, most auto amp meters had fairly useless scales, like -60 0 +60A when
the normal current flow was on the order of a few amps one way or the other.
Me, I'd use a digital.

> in addition to many other reasons, voltmeters are much easier to wire in.

That dosen't make them the right tool for the job.
How will you see a 1A drain on a battery, with a voltmeter?

>You can wire a voltmeter pretty much anywhere in the
> harness to read voltage.. where do you wire an ammeter to monitor
> current into and out of the battery?

In parallel with the shunt resistor. The shunt is in series with the
battery.
If it's designed into the system, then you can pick it off from the
alternator's current sensing circuitry.

If I had to have one gage, I 'd have an ampmeter.
Of course with a three wire connection, you can have volts and amps, but now
you need two meter faces, or a smarter meter.

"Dave VanHorn" > wrote in message
...

>>You can wire a voltmeter pretty much anywhere in the
>> harness to read voltage.. where do you wire an ammeter to monitor
>> current into and out of the battery?
> In parallel with the shunt resistor. The shunt is in series with the
> battery.
What is the function of this "shunt resistor"? That would imply that
there is -already- a real ammeter in place.
An ammeter wired in parallel with any existing "shunt resistor" would
split the total current and therefore produce a false reading.
To monitor current in and out of the battery, one would have to place an
ammeter in series with the battery.

Laurie

On Wed, 29 Dec 2004 08:30:32 -1000, Laurie > wrote:

>"Dave VanHorn" > wrote in message
...

>>>You can wire a voltmeter pretty much anywhere in the
>>> harness to read voltage.. where do you wire an ammeter to monitor
>>> current into and out of the battery?
>> In parallel with the shunt resistor. The shunt is in series with the
>> battery.
> What is the function of this "shunt resistor"? That would imply that
>there is -already- a real ammeter in place.
> An ammeter wired in parallel with any existing "shunt resistor" would
>split the total current and therefore produce a false reading.
> To monitor current in and out of the battery, one would have to place an
>ammeter in series with the battery.

Any ammeter measuring anything over a few milliamps is going to employ a
shunt resistor either external, or in the meter case.

The shunt resistor generates a voltage drop in proportion to current,
E=IR. The meter itself if based on a magnetic principle (not a digital,
for example), will be a milliampmeter with an internal series resistor
to make it a voltmeter. Combined with the shunt resistor, it'll measure
the current through the resistor.

Dave VanHorn wrote:
> > Or you could have a completely open battery.. or the charging
system
> > has gone out, but your current drain on the battery is low enough
that
> > it's not visible on the ammeter..
>
> Sure, but with an open battery, that becomes obvious at start time.
> Trust me, BTDT.

So no advantage ammeter to voltmeter with an open battery when
starting..<grin>.. unless you consider that you'd see 0.0v with key on,
engine off..

Now, engine running might show voltage dropping below 12v when idling
with some accessories running..

>
> Proper scaling is also important, no sense using a 100A meter on a
10A
> system.

A quick search shows fuel injected bikes running charging systems
starting at 500 watts and running up to 1100 watts..



> > Auto manufacturers have gotten away from ammeters and gone to
voltmeters..
>
> Yes, but the question is "why".. For example, the oil pressure gage
in my
> truck is shown as a linear gage, but it is driven by a switch. Too
many
> people got worried over the normal fluctuations in pressure and
generated
> service calls under warantee.. So now, that useful information is
gone.

Agreed on the OP gauge.. I have one in my truck.
>
> Most people can sort of understand a volt meter, but are pretty
clueless
> about an amp meter.
> Also, most auto amp meters had fairly useless scales, like -60 0 +60A
when
> the normal current flow was on the order of a few amps one way or the
other.
> Me, I'd use a digital.

Digital might have the resolution, but considering that current
automobile alternators are available well over 100 amps and starters
can draw 200+ amps, plus and minus 60 amps might not even be enough..
>
> > in addition to many other reasons, voltmeters are much easier to
wire in.
>
> That dosen't make them the right tool for the job.
> How will you see a 1A drain on a battery, with a voltmeter?

On a meter that you can tell 1A, what overall range does it have, and
how useful would that be in an automotive application?
>
> >You can wire a voltmeter pretty much anywhere in the
> > harness to read voltage.. where do you wire an ammeter to monitor
> > current into and out of the battery?
>
> In parallel with the shunt resistor. The shunt is in series with the

> battery.

Specifically what I'm looking for is the electrical connections for
the charging circuit, starting circuit and battery connection... you've
basically got a charging system lead, a vehicle electrical system lead,
and a starter lead (as well as a battery connection). Most ammeters
setups I've seen don't subject the ammeter to starter draw, which can
run into 200-300 amps or more on a V8 engine.. and if you wire in the
shunt wrong in relation to the charging system lead it won't read
correctly.

> If it's designed into the system, then you can pick it off from the
> alternator's current sensing circuitry.
>
> If I had to have one gage, I 'd have an ampmeter.
> Of course with a three wire connection, you can have volts and amps,
but now
> you need two meter faces, or a smarter meter.

I still vote for the voltmeter.. one situation I've seen is where the
voltage regulator allowed charging voltage to increase over 15v.. all
you'd see on an ammeter is that the battery is charging.. not that it's
overvolting.. If ammeters used had better resolution and could
accurately show 0.5 amp or so, then it wouldn't be so bad.. but as you
mentioned, ammeters used are typically +- 60 amp.

Regards,

Jim

> What is the function of this "shunt resistor"? That would imply that
> there is -already- a real ammeter in place.
> An ammeter wired in parallel with any existing "shunt resistor" would
> split the total current and therefore produce a false reading.
> To monitor current in and out of the battery, one would have to place
> an ammeter in series with the battery.

AArrgghh..

Wouldn't you think it inconvenient, to run two conductors of 00 gage wire up
to your dashboard, and back to the battery?

The shunt resistor is in series with the battery, on the positive side. It
is a VERY small resistance.
0.01 ohms typically, even less in some applications.

Just picking some easy numbers for discussion he

When 100A flows through 0.01 ohms, it results in a voltage of 1.00 V across
the shunt.
The meter at the dash might read full scale with 10mA and have a resistance
of 100 ohms, so at 1V input, it's flowing 10mA.

If properly designed, it's perfectly accurate.

You're right, it does split the current. If you're reading 100A in the
system I described, then actually 100.01A is flowing. Of course you can
calibrate out the .01% error, if you care.

> A quick search shows fuel injected bikes running charging systems
> starting at 500 watts and running up to 1100 watts..

Ok, so about 100A maximum into the battery, likely much less.
I would think that even 10A would be ok, after all, if you are pegged out at
10+A into the battery, either you have a big problem, (shorted cell?) or you
won't see that for more than a few seconds.


> Digital might have the resolution, but considering that current
> automobile alternators are available well over 100 amps and starters
> can draw 200+ amps, plus and minus 60 amps might not even be enough..

Do you need to measure the starting draw?
If so, use a different meter. One high range, and one low.


> On a meter that you can tell 1A, what overall range does it have, and
> how useful would that be in an automotive application?

Well, if everything's working right, your current into the battery should
level off to some rather small value. I was assuming that a motorcycle is
going to have a much smaller battery than a car, so the range would likely
be proportionally smaller.


> Specifically what I'm looking for is the electrical connections for
> the charging circuit, starting circuit and battery connection... you've
> basically got a charging system lead, a vehicle electrical system lead,
> and a starter lead (as well as a battery connection). Most ammeters
> setups I've seen don't subject the ammeter to starter draw, which can
> run into 200-300 amps or more on a V8 engine.. and if you wire in the
> shunt wrong in relation to the charging system lead it won't read
> correctly.

Kind of a large-ish motorcycle then?

Again, it's two different problems.
A 0.001 ohm shunt in the starting lead, would give you 0.1V per 100A.
A 0.01 shunt in the charge lead gives you 1V per 100A.


> I still vote for the voltmeter.. one situation I've seen is where the
> voltage regulator allowed charging voltage to increase over 15v.. all
> you'd see on an ammeter is that the battery is charging.. not that it's
> overvolting..

You would see that it's charging more and longer than it should be.
In a properly designed system, the charge current should taper off to some
small value pretty quickly.

> If ammeters used had better resolution and could
> accurately show 0.5 amp or so, then it wouldn't be so bad.. but as you
> mentioned, ammeters used are typically +- 60 amp.

They do what you design them to do..
It's also possible to use a logarithmic scale, so you can see the difference
from 1A to 2A as easily as from 10A to 20A.

Drop some appropriate shunts in, condition the signals appropriately, and
sort it all out with a microcontroller, if that's your bag. Volts, amps,
glitches, etc..

Dave VanHorn wrote:
> > A quick search shows fuel injected bikes running charging systems
> > starting at 500 watts and running up to 1100 watts..
>
> Ok, so about 100A maximum into the battery, likely much less.
> I would think that even 10A would be ok, after all, if you are pegged
out at
> 10+A into the battery, either you have a big problem, (shorted cell?)
or you
> won't see that for more than a few seconds.

I have an aversion to pegging meters out.. but if you weren't worried
about damaging the meter than a +- 5 amp ammeter would probably be even
better.. as long as the user understands what's going on.. as you
mentioned earlier, someone who doesn't understand might have a problem
with a pegged out meter, or one that has big needle movement..
>
>
> > Digital might have the resolution, but considering that current
> > automobile alternators are available well over 100 amps and
starters
> > can draw 200+ amps, plus and minus 60 amps might not even be
enough..
>
> Do you need to measure the starting draw?
> If so, use a different meter. One high range, and one low.

Starter draw is a useful diagnostic tool.. but my point here is that
the installer needs to know how to wire the shunt in.. if you unhook
the positive cable and wire the shunt directly to the battery you'll be
measuring starter draw.. if they unhook the lead to vehicle electrical
system and wire the shunt there not realizing that the alternator lead
connects directly to the battery clamp then they'll never see battery
charging..

>
>
> > On a meter that you can tell 1A, what overall range does it have,
and
> > how useful would that be in an automotive application?
>
> Well, if everything's working right, your current into the battery
should
> level off to some rather small value. I was assuming that a
motorcycle is
> going to have a much smaller battery than a car, so the range would
likely
> be proportionally smaller.

I would expect it to level off around 1/100 capacity of the battery..
so 1 amp or less. Something analog that can accurately show that is
probably going to be pegged out much of the time.. perhaps digital
would work here.

>
> > Specifically what I'm looking for is the electrical connections for
> > the charging circuit, starting circuit and battery connection...
you've
> > basically got a charging system lead, a vehicle electrical system
lead,
> > and a starter lead (as well as a battery connection). Most ammeters
> > setups I've seen don't subject the ammeter to starter draw, which
can
> > run into 200-300 amps or more on a V8 engine.. and if you wire in
the
> > shunt wrong in relation to the charging system lead it won't read
> > correctly.
>
> Kind of a large-ish motorcycle then?

The OP is talking about electric clothing and aux lighting.. I didn't
think it's a Vespa..

>
> Again, it's two different problems.
> A 0.001 ohm shunt in the starting lead, would give you 0.1V per 100A.
> A 0.01 shunt in the charge lead gives you 1V per 100A.

Or wire it to disregard starter draw.. but you have to know how.
Voltmeter.. one wire to hot, one to ground. If you can avoid hooking to
a 5v reference line or to dashboard lighting circuit you'll probably do
it correctly.


>
> > I still vote for the voltmeter.. one situation I've seen is where
the
> > voltage regulator allowed charging voltage to increase over 15v..
all
> > you'd see on an ammeter is that the battery is charging.. not that
it's
> > overvolting..
>
> You would see that it's charging more and longer than it should be.
> In a properly designed system, the charge current should taper off to
some
> small value pretty quickly.

If the user understands what's going on.. either meter could be used.

>
> > If ammeters used had better resolution and could
> > accurately show 0.5 amp or so, then it wouldn't be so bad.. but as
you
> > mentioned, ammeters used are typically +- 60 amp.
>
> They do what you design them to do..
> It's also possible to use a logarithmic scale, so you can see the
difference
> from 1A to 2A as easily as from 10A to 20A.
>
> Drop some appropriate shunts in, condition the signals appropriately,
and
> sort it all out with a microcontroller, if that's your bag. Volts,
amps,
> glitches, etc..

While that's something I'd be interested in doing, the OP wants to
know when he's not charging his battery.. To err on the side of
caution, if he's seeing 12.8v or less on a voltmeter, he's not charging
the battery. A simple solution would be to wire up a comparator circuit
that turns on an LED when system voltage drops below a specified
value.. wouldn't be that hard at all.

Regards,

Jim

All this discussion thus far, about metering,
seems to indicate that shunt devices (in series)
are the way to measure current. I'd prefer to use
a Hall Effect device clamped around the conductor,
especially for large currents.

Here's a link, but there are MANY more (Google
"Hall Effect"):

<http://www.micronas.com/products/overview/sensors/index.php>


--
-- Jack --
=================================
Everything you see is temporary.
=================================
Clean Up Return Address To Reply
=================================
Poway, California (San Diego Co.)
N 32° 57' W 117° 04'
At 508' Elevation
=================================

For those interested, the motorcycle alternator in question creates 20A
at crusing speed, and only 8A at idle. Thats only a fraction of what
some of the other bikes out there can make. Hence my desire to have
some kind of indicator of charging status. I've also thought about
putting some kind of device on the heated clothing that only allows it
to come on when the voltage is above a certain limit, but this could be
problematic because of the voltage drop when the clothing is on.

I just had an interesting idea. Take a dual-color two-lead LED and
wire it in parallel with the battery wiring, taking into account the
proper voltage and current limiters. Now won't you have a simple light
that will be green when the battery is charging, and red when it is
discharging? Would the regular battery wiring be enough of a 'shunt
resistor' to send enough current up to the LED to light it up?

Even better would be to incorporate this using a hall effect sensor as
a previous poster mentioned.

Whaddya think?

-Ryan

> wrote in message =
oups.com...
> For those interested, the motorcycle alternator in question creates =
20A
> at crusing speed, and only 8A at idle. Thats only a fraction of what
> some of the other bikes out there can make. Hence my desire to have
> some kind of indicator of charging status. I've also thought about
> putting some kind of device on the heated clothing that only allows it
> to come on when the voltage is above a certain limit, but this could =
be
> problematic because of the voltage drop when the clothing is on.
>=20
> I just had an interesting idea. Take a dual-color two-lead LED and
> wire it in parallel with the battery wiring, taking into account the
> proper voltage and current limiters. Now won't you have a simple =
light
> that will be green when the battery is charging, and red when it is
> discharging? Would the regular battery wiring be enough of a 'shunt
> resistor' to send enough current up to the LED to light it up?
>=20
> Even better would be to incorporate this using a hall effect sensor as
> a previous poster mentioned.
>=20
> Whaddya think?
>=20
> -Ryan
>
If I remember right, the Hall voltage is roughly proportional to the =
product
of the current times the temperature.
For measuring currents in an automotive environment,
you'd best plan on some fancy temperature compensation.