Amp gauge on battery charger

[sgs]

I have an old 10 amp Sears battery charger and for a few years now the surface mounted square amp gauge has had its crappy clear plastic cover missing. Popped the case top off and behind the square guage is a 2 inch round hole through the metal face of the charger. I'm thinking...why not just stick in a basic 2-inch round automotive amp gauge?

To you ohms guys...any reason that wouldn't work? An auto gauge reads both + and - and (as I recall) most don't actually read the number of amps.

This original meter only reads amps + from 0 to 20, which to a wrench head sounds odd because its only a 10 amp charger. I assume a 10 amp charger will max out at 10 amps output...no?

Anyway I'm sure somewhere in a box I have an old 2 inch round automotive amp gauge. Is it worth me digging? Will it work. ..keeping in mind all I need is to see if the battery is pulling a charge and once charged...assume it should be in the center null point or at least close to it?

Gordon S.

 

[Bobf]

I'm no electrical engr but a amp meter reads current flow so it is placed in series
with the output. I would just use some short pieces of wire to temporarily put it in series with the output, if all appears well wire it up permanent.

 

[1/2 Cup]

It will work fine, the reason they put in a 20 amp gauge in a charger that only outputs 10 amps is that when you initially switch on the charger you will notice the needle usually will momentarily flick to full scale due to the inrush current then settle at a normal charge rate for that particular battery. It is what is termed "over scale" for a meter.
The over sized meter will cope with this initial in rush of current without damage.

 

[sgs]

Ok...then it's worth digging to find that old gauge I'm sure is in some box. ..somewhere.

Gordon S.

 

[jkwilson]

I'm no electrical engr but a amp meter reads current flow so it is placed in series
with the output. I would just use some short pieces of wire to temporarily put it in series with the output, if all appears well wire it up permanent.

Ammeters generally can't handle current flow on their own so you need to be careful. They are just voltmeters that measure the voltage across a low value resistor that most of the current flows through. If you attach a meter without an internal shunt in line without adding a shunt, it will be destroyed quickly.

 

[aka rotten]

Would it not be as easy just to replace the old plexyglass with new or reg. glass if you have way to cut them?Know i replaced one on my garden tractor with clear window glass i broke one time.[different story]

 

[G_P]

Ammeters generally can't handle current flow on their own so you need to be careful. They are just voltmeters that measure the voltage across a low value resistor that most of the current flows through. If you attach a meter without an internal shunt in line without adding a shunt, it will be destroyed quickly.

I took a very old automotive amp meter and mounted it in a wooden box. It works great with no shunt required. It's a 30-0-30 gauge. I would assume if it needs a shunt it will say so.

 

[sgs]

I woner if an automotive amp gauge has a shunt built into it?

Gordon S.

 

[jkwilson]

I woner if an automotive amp gauge has a shunt built into it?

Gordon S.

I expect so.

 

[sgs]

This replacing an amp gauge thread is sort-a stupid...and its my thread! 35 bucks...just go buy a new charger?

Anyway...suspect one Frankenstein looking part inside the charger is a shunt resistor which leads into my next stupid question...if internally there is already a resistor. ..and one is built into a basic automotive gauge, is it a problem to run 2?

Someone please write just wire the f-ing thing up and see if it works. Screw-it...going to do just that.

Gordon S.

 

[srmofo]

Yes 2 resistors is a problem.to be honest you provided a ****** description, no pics, and don't want to do any leg work to figure out what is actually needed.

So hook it up 1 of 3 things will happen

Nothing (2 resistors)
It works
You let the smoke out of the gauge.

I'm guessing depending on the value of the combined shunts you might still get a little bounce from the needle.

 

[sgs]

Works perfect.

Trust thats a clear enough description.

Gordon S.

 

[Greatbear]

Most of the amp gauges in typical battery chargers are very simple affairs compared to more traditional instrumentation. They rarely rely on a traditional shunt-and-millivolt meter, instead they use a simple magnetic deflection movement. Since the accuracy needed for these applications isn't a high priority, the deflection gauges fits best. The movement consists of a simple heavy gauge bar or wire in close proximity the a small magnet attached to the pointer, with a calibrated (more or less) hairspring to zero the meter when no current flows. When current passes through the bar/wire, a magnetic field proportional to the amount of current forms around the conductor. This deflects the magnet/pointer assembly accordingly. The pointer shows the approximate current on the scale. Lower current meters of this type will often have a coil of heavy wire surrounding the moving magnet to increase the sensitivity of the instrument. Advantages of this style of meter are they're cheap, and there is very little power loss across the meter.

Some people might have used the old school alternator tester that looked like a simple round gauge with no external connections, instead there was a groove in the back of the instrument where you placed the current carrying wire. This is the same type of meter, but with an external "electromagnet."

Most automotive accessory amp gauges work the same way, the two studs on the back of the unit lead to that internal bar and moving magnet arrangement, the difference being the zero point is set midscale, and the direction of the current flow deflects the magnet/pointer in the appropriate direction. In cases where the full scale current is over 100 amps, the meter will often rely on an external current shunt and the meter itself will be a millivoltmeter. This is done to minimize the long runs of heavy cabling through the firewall and to the dash.

 

[Gary S]

Be careful and know before hand just what you are putting in there, and what is really in there.

As stated, amp meters go in series with the circuit to measure amps. But, that makes for a bit of a dangerous situation in a car because it forces you to run very heavy wire from the alternator into the interior and back out to make a series circuit. Because of that, many factory "amp meters" used in cars were not amp meters at all. They were very sensitive differential voltmeters that measured voltage drop between two points in the charging circuit. This way they are wired in parallel and can use very small wires.

I know for a fact that GM used lots of these in their cars. What the other guys used? I don't know, but it would be worth figuring out before the sparks fly.