I initially had the cut-off start set at 3.8V/cell and cut-off end at 3.7V/cell but in the meantime I've learnt a thing or two:
My Zippy Flightmax Lipos arrived but the weather was bad and I was bored so I decided to see if I could take an objective approach to setting the low voltage cutoff and whether or not I would need different settings for different Lipo packs.
So I had some good clean geeky fun with my Arduino and did a discharge test.
I set up the Arduino to measure the pack voltage over the balance plug while I discharged the Lipo over a 35W halogen bulb. I had the Arduino send a timestamp and the measured value over the serial port to my PC, from where I could import data into Excel.
I had a cell voltage monitor/alarm set up at 3,6V so I could leave the setup alone until stuff started to get interesting.
The 35W bulb drew a current of approx 2,8A. Unfortunately I didn't have anything more powerful, it would have been fun to see the effect of a larger discharge current, especially a current close to the max discharge rating.
My setup:
The results:
Some observations:
- 3,6V per cell will be a good cutoff limit for both packs. Under 3,6V the voltage drops VERY fast. It took both Lipo's about 15 min. to reach 3,0V coming from 3,6V at approx 2,5A, imagine how fast the voltage drops when you're drawing 10 times that current.
- Both curves show a small dent at 4,0V, I don't know what is going on there, it could have been someting in my setup.
- The Multistar takes 13% longer to discharge to 3,6V while it only has 4% more capacity, while the current in both tests was identical. I don't know how accurate my test was but one thing for sure, range with the Zippy's will be shorter.
If I ever decide to repeat this kind of test, I'll put my wattmeter in the circuit as well and maybe even have Arduino monitor and log individual cell voltages, and have it shut off automatically at 3,0V
Now I'm waiting for the weather to clear so I can go out and do a real-life test.
I'm also brooding on the idea of having my Arduino log values from the VESC's over CANbus and store them on an SD card, while also logging accelerometer data, which would give me useful data to analyse after a ride.