I had an idea about measuring current draw

[norcalwelder]

If you had a resister that could handle the wattage, couldn't you just stick a resister in between on the leads on your battery pack and measure the voltage loss across it and figure the current out that way? If you don't have a amp meter that is...

[CHELLIE]

Originally Posted by norcalwelder

If you had a resister that could handle the wattage, couldn't you just stick a resister in between on the leads on your battery pack and measure the voltage loss across it and figure the current out that way? If you don't have a amp meter that is...

I dont think that will work, Reason being, you are restricting the current/Volts/Amps. going to your esc and motor, you are loosing current/Volts/Amps/ in the form of heat, you wont get a Accurate reading, a wattmeter that uses a shunt is the best way to get a accurate reading, hyperon makes a nice one, A Watts UP Meter is a good Wattmeter too, A Wattmeter is your best friend, it will save your pocket book, dont skimp here Take care and have fun, Chellie

http://www.wattflyer.com/forums/showthread.php?t=60884

http://compare.ebay.com/like/3103195...Types&var=sbar

[JetPlaneFlyer]

Accross the leads (from positive lead to negative) would not work, that could only measure battery voltage, not amps..
If you could find a resistor rated for adequate watts then it could work if you connected it in series in the positive or negative lead, but unless the actual resistance of the resistor was tiny the presence of the resistor would in itself drastically reduce the amps and so the measurement would be meaningless.

Steve

[Larry3215]

Actually, thats the exact way many current meters work - including most RC Wattmeters.

Instead of a normal resistor, they use a precsion shunt , with a precise known resistance that is very low, in-line with the positive or negative lead. Typically in the .01 ohm (cheap models) to .001 ohm or lower (better models) range depending on the quality of the meter. The meter then measures the voltage drop across the shunt and displays the resulting current.

So for a 100 amp meter the math works out like this.

.001 ohms x 1 amp = .001 volts
.001ohms x 100 amps = .1 volts

You would need an accurate volt meter that could read from .001 volts up to .1 volts.

Few hobby grade meters can come close to that.

You also need a shunt that has a very precisely known resistance or you wont know what your current is. Again, very few modelers have meters capable of measuring that low a resistance accurately.

The shunt also needs to be able to handle the max current without significant heating. That means special materials beyond just a hunk of coper wire. if the shunt gets hot, the resistance will go up, messing up the measurements.

Some of the higher end models use a Hal Efect device which eliminates the shunt. It measures the magnetic field generated around the wire inside the meter and gives the current that way.

By the way, you need the super low resistance or you will have too much power loss and heating in the meter. If your using for example a .1 ohm resistor to measure 100 amps, you will get a loss of 10 watts and a lower current reading than the model would have without the resistor.

Much more importantly, the resistor in this case would make the reading off even more because it would be dropping the voltage to the motor by 10 volts - which is huge.

[kyleservicetech]

Originally Posted by norcalwelder

If you had a resister that could handle the wattage, couldn't you just stick a resister in between on the leads on your battery pack and measure the voltage loss across it and figure the current out that way? If you don't have a amp meter that is...

Yes you can. It takes a high current shunt for this purpose, such as those by www.digikey.com, per below:
http://search.digikey.com/scripts/Dk...me=811-1094-ND

I've got several of these units, they are extremely accurate, but require you to cut into the wiring between the battery and the ESC.

A far easier way is to pick up one of those multifunction units such as Astroflights wattmeter, or one of those AC and DC clamp on ammeters such as Sears Craftsman #82369. (Note that most clamp on ammeters are AC only, and not useable for our DC powered models)
http://www.sears.com/shc/s/p_10153_1...psid=21x596999 Nice thing about the Craftsman meter, you simply clamp the jaws of the meter around one of the battery leads to measure current. Can't get much easier than that. Even used it to measure the current on my little back yard flyer, that pulled 2.8 Amps on a 2 cell 180 Mah Lipo.

I've got both, and both work well. In fact today I used my Craftsman meter to measure the current pulled by a club members electric jet, 42 Amps on a 3S Lipo.