Measuring the Voltage: E-Bike Blog #17
- Tim Ellsworth
- Jul 3, 2017
- 3 min read
Test Battery
A while back while I was thinking about how to make my E bike battery I made a 2 X 3 test battery to experiment with the BMS setup, the test leads and measuring voltages. Test leads are something that I still haven't installed on the E bike battery. The ultimate goal is to have a device that measures and records the voltages of each parallel group in the battery as I ride. I need the test leads to access the voltages. Right now I have several remaining tasks to complete before I get there:
1) Add the test leads to the battery so that they can be easily accessed
2) Find a way to measure the individual parallel groups in the pack
3) Find a way to store that voltage data
First:
Above is the test battery that I made to experiment with voltage readings. The BMS that I installed on it can be found here with the diagram to connect it. The many tiny wires coming off of the battery are my test leads. The idea here was to plug them into a board or unit which housed the voltage testing device. They are soldered to the nickel strips in between the cells to prevent excess heat being dumped into the cells during soldering. One of the solder connections can be seen below.
I ran into several problems here which I am still working on resolving. The first problem is that the leads I bought are too short. They look ok on the test battery but I don't think they will work on the E bike battery. They probably won't reach outside of the battery casing unless I find a way to fit the voltage testing device inside of the battery case. The second is that the female part of the connector which I planned on soldering to a prototype board was not easy to solder to. The connector melted and the pins moved. Also the solder on the back side wanted to stick to the green pcb better than the connections. I don't know if it was my soldering skills or the quality of the board but I might need to come up with a better solution. The melted female connectors and globs of solder can be seen below. The idea was to alternate positive, negative, positive...
Second
I have been finding the voltage testing part of this to be very difficult. I realized quickly that using an Arduino as a voltage measuring device on its own would not work due to the limited number of analog inputs. You can however measure one voltage between 0 and 5V which I discovered here in Paul McWhorter's video series on using the Arduino.
I thought that I would try using a multiplexer for Arduino which would allow me to get up to 16 analog inputs. I tried using this multiplexer, which is pictured above, coupled with this Russian tutorial I found on Youtube. The multiplexer worked great even though I didn't fully understand the workings of it. I was able to measure a voltage between 0 and 5V on each channel but there was a major problem. I could not measure the parallel groups at the same time because that would force me to use the same ground. I would effectively short out the parallel groups back to the negative of the battery for each channel. I ditched the whole idea altogether. I am investigating using an Arduino style voltage and amperage sensor to collect the readings. It is looking like it will have to be the overall voltage if anything.
Third:
To log the data I am looking at using an SD card logger for Arduino which is pictured above and which can be found here on Amazon. I have been testing the module with a GPS module (which I also want to use later on the bike to record the distance, speed, time, location etc...) but I have not had any luck yet. I will experiment with basic examples first to understand how the SD card system works before applying it to logging voltage values (or GPS NMEA sentences). Once I get each system up and running I will post a Fritzing diagram to show how it is all laid out.
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