Close

Progress!

A project log for Smart Battery Tester

An intelligent test device that tests batteries by discharging them to measure capacity

adam-oakleyAdam Oakley 12/22/2014 at 03:220 Comments

After months of inactivity I've started working on this project again with some prodding from a friend. We prototyped the constant current load circuit. At first we tested the circuit just with a potentiometer providing the input. It worked fine, so we added an arduino to provide the PWM signal driving that controls the current draw. We also used a few analog inputs on the arduino to monitor the voltage on the current shunt resistor, and the input to the opamps.

We noticed that the voltages we were reading on the arduino were not stable at all, so we changed a couple things to improve that. First, by looking at our opamp input with an oscilloscope we noticed that the RC filter output had about 70mV of ripple. Not a whole lot, but something easily fixed by increasing the 15k resistor to 150k. That improved the readings a bit but didn't entirely fix the problem, so then we changed the code to take 50 samples in a row and average them. Now the readings are fine.

Because we're not using a signal conditioning amplifier, we don't have a whole lot of resolution to read the shunt resistor voltage. The current can be read to a resolution of about 10mA, which should be fine for this. I also decided to change the design of this project somewhat. I'm not so concerned about the charging, so I'm going to use a dedicated charger IC just to top off the battery before we do a load test, and recharge it after we're done. I also want this device to be usable without a PC, so I'll be adding an LCD screen and some controls to allow you to operate it completely PC-free.

Discussions