Ammeter/ Volt meter adjustment

TIP

Purchase a 9 volt non-rechargeable LITHIUM battery. Never use it except to verify voltage. Keep it in a refrigerator. Find someone with an oscilloscope or a 4-1/2 digit FLUKE or 5-1/2 digit BENCH multimeter, and read the voltage, to third decimal, on the battery. This is your calibration/verification tool. Put it in a tight zip-loc to keep it dry. The battery will stay valid for many many years.

I seriously doubt that you meter as in the shunt voltmeter can be used to measure the shunt voltage.

You need a GOOD digital voltmeter that has a 3 decimal digit resolution since you need to measure 0.027V.

doughere wrote:

Gjac wrote:

doughere wrote:
Your shunt is a resistor. Your ammeter is actually a voltmeter reading the drop across your shunt. If you know the shunt's value (usually expressed as Amp to millivolt drop) you can read the voltage across the shunt and calculate the Amperage. My shunt is 50 Amp to 50 mV; so if the volt reading across the shunt is 35 mV, my meter should read 35 Amps. The most common shunts are designed for 50 or 100 mV at max current reading.

Doug

Doug if I understand your post if I take a voltage reading between the two large nuts across the shunt(as shown in BFL's post)and trust my voltmeter I could adjust the amp pot to match that reading. Is that what you are saying?

You say your shunt is 200 Amps for 75 mV. If you measure the voltage across the shunt with a GOOD voltmeter, you can calculate the current through the shunt. Divide the voltage (in mV) across your shunt by 75 mV, multiply that by 200 and you have your amperage.

If you measured 27 mV across your shunt, divide by 75 you get .36; multiply by 200, you get 72 Amps.

I don't know how your meter is powered, but you may not be able to read the voltage from your shunt, if you are using power from your battery system to run the meter. My meter had to have an isolated voltage converter to read correctly.

Doug

My meter is powered from the chassis battery. Using your example should I be able to adjust the amp pot to read 72 amps if I read a 27 mv drop across the shunt?

One thing that puzzled me for some months after I installed a trimetric was low amps from my solar. What I finally discovered was my generator start cables were wired directly to the battery. I didn't think it mattered because I wasn't interested in the small amount of power occasionally consumed to start the generator. What I didn't know at the time was the negative to the generator was also frame grounded at the generator. This created a bypass path around the shunt. The fix was to lift the generator negative at the battery and all was well. CHECK YOUR GENERATOR DC NEGATIVE!

It's one- way. To read the other way you have to switch the two little wires on the shunt. That was what my A-B switch idea was for.

Does your meter read amps that can flow in either direction such as you would want for a battery? Looking at the above link it appears that it only measures amps in one direction such as you want when measuring amps used by lights.

When measuring the amps from a battery what's important to most of us is both in and out (or + and -) amps to the battery indicating battery charge or battery discharge.

For example the charger shows 15A and there is a 5A house draw for lights, etc. 5A from the charger will supply the house and 10A will charge the battery and the shunt will show 10A. This is with the charger connected to frame ground and the positive post just like the house loads.

When however you connect the portable charger to both battery posts you bypass the shunt and hence the shunt does not show the charger amps. The shunt does show the the house loads.

Gjac wrote:

BFL13 wrote:
He may be right.

The Trimetric does not really show "net amps" but only the amps in or out of the battery. If there is a load running while you charge, the load will get its amps from the charger as long as its voltage is higher than that of the battery. Same with solar charging.

So the amps out of the charger or controller can be higher than the amps into the battery, which is what the meter shows, so you need to disconnect all loads to be sure both amps should be the same.

In that case, my worry about one-way amps is wrong, does not apply.

So what you are saying I can't use my B&D to adjust the ammeter? Not sure what you mean by disconnect all loads to be sure both amps should be the same.

Yes you can. Just take off the chassis ground wire so there is no load from the rig drawing, and clamp the charger's neg to the outer end of the shunt. Now the only amps through the shunt are from the charger and they all go to the battery, so you can compare the two display values, and make the meter's read the same as the charger's.

That meter can be powered from the battery itself or from a separate power source as an option. See the wiring diagrams in the earlier link.

Gjac wrote:

doughere wrote:
Your shunt is a resistor. Your ammeter is actually a voltmeter reading the drop across your shunt. If you know the shunt's value (usually expressed as Amp to millivolt drop) you can read the voltage across the shunt and calculate the Amperage. My shunt is 50 Amp to 50 mV; so if the volt reading across the shunt is 35 mV, my meter should read 35 Amps. The most common shunts are designed for 50 or 100 mV at max current reading.

Doug

Doug if I understand your post if I take a voltage reading between the two large nuts across the shunt(as shown in BFL's post)and trust my voltmeter I could adjust the amp pot to match that reading. Is that what you are saying?

You say your shunt is 200 Amps for 75 mV. If you measure the voltage across the shunt with a GOOD voltmeter, you can calculate the current through the shunt. Divide the voltage (in mV) across your shunt by 75 mV, multiply that by 200 and you have your amperage.

If you measured 27 mV across your shunt, divide by 75 you get .36; multiply by 200, you get 72 Amps.

I don't know how your meter is powered, but you may not be able to read the voltage from your shunt, if you are using power from your battery system to run the meter. My meter had to have an isolated voltage converter to read correctly.

Doug

BFL13 wrote:
He may be right.

The Trimetric does not really show "net amps" but only the amps in or out of the battery. If there is a load running while you charge, the load will get its amps from the charger as long as its voltage is higher than that of the battery. Same with solar charging.

So the amps out of the charger or controller can be higher than the amps into the battery, which is what the meter shows, so you need to disconnect all loads to be sure both amps should be the same.

In that case, my worry about one-way amps is wrong, does not apply.

So what you are saying I can't use my B&D to adjust the ammeter? Not sure what you mean by disconnect all loads to be sure both amps should be the same.