# Measuring High DC Voltage and Amperage

Hey Guys!

I’m working on a project that runs off 58.8V (fully charged) and 225A. This consists of 2 battery packs in parallel with 4 lead acid batteries in series for each pack. The problem I’m facing is that the motor I’m running runs off 10,800 Watts (the motor could go over this value but for a very limited amount of time aka < 10s).

My thought process was that I would log current/voltage and control the throttle using Arduino so I could keep the work exerted by the motor at 10,800 Watts or less.

So if my voltage is at 58.8 and the current being pulled was at 225A the total power would be 13,280 Watts. I would then have the Arduino change the throttle to 81% which should theoretically set the power to the motor at 10,756.

If the voltage dropped, then the throttle would increase.

The challenge here is getting the Arduino the read the proper voltage values! I can get current just fine using a shunt and using Ohms law to calculate current. Getting the voltage however is proving to be difficult. I read online and many suggested to use a voltage divider but the amount of amps that I’m pushing through quickly burns out any resistors that I put in the circuit (As I expected considering that I’m using 00 gauge wire).

I see on Ebay devices that output 300V and 300A values on an LCD screen and just have no clue how they are doing it! There has to be a way of getting the Arduino to read the voltage! heck my multimeter can read the voltage just fine.

Also, the lead acid battery values are based on CCA… and the motor will only run for several seconds at a time.

You only need tiny currents to read voltage, a voltage divider with a 2k and 22k resistor would give you a 12 to 1 attenuation and draw less than 1 milliAmp at 60 volts. Can you post a drawing of your circuit? BTW how are you "throttling" the motor?

outsider: You only need tiny currents to read voltage, a voltage divider with a 2k and 22k resistor would give you a 12 to 1 attenuation and draw less than 1 milliAmp at 60 volts. Can you post a drawing of your circuit? BTW how are you "throttling" the motor?

I have attached my Arduino to the ESC which controls the motor. The Arduino acts as the transmitter and passes throttle to the ESC through PWM. I can control this no problem and I can go into it more if you want to see exactly how I have it hooked up :)

For the purpose of simplicity, the top section of the circuit is the ESC and Motor.

According to falstad this should work... but when I connect it, the 22k resistor blows up every time.

What is connected to the center point at the bottom? you should be able to read 1/12 the motor voltage across the 2k but the Arduino GND would have to be connected to the left end of the 2k, be careful here.

Really need a more detailed drawing, how the batteries are connected to ESC and motor, how the Arduino is connected, how the shunt is connected AND a link to the ESC.

outsider:
What is connected to the center point at the bottom? you should be able to read 1/12 the motor voltage across the 2k but the Arduino GND would have to be connected to the left end of the 2k, be careful here.

I cleaned up the image a little. The thing at the center is the battery. Using their circuit simulator, you have to specify a source of voltage and a source of current separately.

As of now, I haven’t connected the Arduino yet but I definitely know that it needs to be connected at the 2k mark. Just want to make sure I can get the right voltage first before I start frying out boards!

Cleaned up image:

Well the way your drawing shows you would be reading the battery voltage, not the motor, is that what you want? Also I should have said the 22k should be at least a 1/2 watt, 1 watt would be better, still need to see the ESC docs.

outsider: Well the way your drawing shows you would be reading the battery voltage, not the motor, is that what you want? Also I should have said the 22k should be at least a 1/2 watt, 1 watt would be better, still need to see the ESC docs.

Reading the battery voltage is exactly what I want!

The reason for this is because the ESC doesn't control voltage to the motor. It sends it the same voltage it's given to it. The motor spins by pulses.

So if my battery pack is at 60V. the ESC sends 60V pulses to the motor (It took me a while to understand this as I always thought the motor spins by it's given voltage).

If I know the Voltage in my battery pack and I can read the Current using a shunt; I should have a good representation of how much work the motor is doing.

Honestly, I would prefer to install a voltage regulator that just sends a constant 48V to the ESC, but my googling didn't yield any promising results due to the amount of current I'm working with.

Back to the original issue though.. my 22k lights up like a christmas light and dies. I know this SHOULD work lol I don't know why it isn't.. I checked my cables and they all match the diagram so I'm really at a loss.

Ah, I should have known brushless motor when you said ESC, any way back to the voltage divider, here’s a very rough drawing:

outsider:
Ah, I should have known brushless motor when you said ESC, any way back to the voltage divider, here’s a very rough drawing:

Thank you! I’ll try to hook this up at somepoint tonight and see if any issues come up