Designing an E-bike power meter

Hi. I'm trying to build arduino project which will measure my e-bike power, remaining battery capacity and speed. While measuring speed is pretty simple, I find it pretty hard to find the best solution for measuring power.

My e-bike has a battery which is a 16S9P pack of lithium-ion cells. Fully charged battery has 67.2V. E-bike pulls up to 50 Amps of current. For measuring current I've decided to use ACS758 module.

What would be the best solution to measure voltage of 50V to 67.2V? Is simple voltage divider a good and safe choice?

Yes and no. The simple voltage divider is safe and easy but it may not give you the resolution you desire.

There are dedicated chips for this, which measure current and voltage together.

It seems like you are well along in this project. I recommend the voltage divider.

One possibility would be to use a 47V zener diode and a 5K resistor across the battery voltage. The voltage across the resistor will be Vbatt - 47V -- something in the 3V ... 10V range. Then you can further use a higher impedance divider to get that into the range of the ADC.

How about a INA219 module with a .005 Ohm 25 Watt shunt and voltage divider to reduce 70 volts to around 23? And a second divider to reduce the 23V to below 5V for Arduino.

As people are implying battery capacity is difficult as the voltage characteristic is such that it’s pretty constant until all the capacity is gone.
You could try just measuring current and calculating Amphours used - that might work better.

Suggest you google the battery discharge characteristic .

gardner:
One possibility would be to use a 47V zener diode and a 5K resistor across the battery voltage. The voltage across the resistor will be Vbatt - 47V -- something in the 3V ... 10V range. Then you can further use a higher impedance divider to get that into the range of the ADC.

Interesting solution - the zener will have an almost constant current across it, so the voltage will be very stable and the resolution is going to be great, because you utilize a large part of the range of your ADC. I kind of like it! This being said, the chemistery of the batteries is complicated and the voltage will fall under load, then it will slowly recover under no load, etc... I think the best way to do this is coloumb counting, where you measure the current drawn and then calculate the used up capacity. There are special opamps (INA180) or even current sensors (INA219) that you can use to convert a value across a shunt resistor to something useful.

thegoodhen:
Interesting solution - the zener will have an almost constant current across it, so the voltage will be very stable and the resolution is going to be great, because you utilize a large part of the range of your ADC. I kind of like it! This being said, the chemistery of the batteries is complicated and the voltage will fall under load, then it will slowly recover under no load, etc... I think the best way to do this is coloumb counting, where you measure the current drawn and then calculate the used up capacity. There are special opamps (INA180) or even current sensors (INA219) that you can use to convert a value across a shunt resistor to something useful.

I'll be measuring battery capacity by counting amp hours since I know the exact total capacity of the battery (31Ah). I looked at INA devices and they are only capable of 3 or so Amps and I need something up to 60A. That's why I'm thinking of using ACS758 current sensor. I would be very thankful for a schematic or diagram of the zener diode solution. I don't want to mess anything since working with lithium-ion batteries is quite dangerous.

Sorry for my english, I hope you understand what I'm trying to tell.
Thanks, Marek.

I have 3 of these in constant use: digital volt ammeter

this looks just as useful: same thing different way

Thanks but I'm not looking for a ready solution. I will be using bluetooth and mobile app instead of LCD.
I just need good and safe way of measuring current (0-60A) and voltage (50 - 67.2V)

lypzor:
(31Ah). I looked at INA devices and they are only capable of 3 or so Amps and I need something up to 60A.

The maximum current depends mostly on the current shunt resistor used. But using some kind of a magnetic sensor (fluxgate/hall effect) will probably be a good option.