Home-brew battery measuring station

[Finnster]

I really like the idea, but I do have some concerns with the setup and the dependability of the data. Namely, I don't know how you will credibly report #'s. You can watch in person and record #'s, put them in excel and make graphs. I can do that too w/o doing anything other than making stuff up. You will post it up, and there will be huge controversy of the credibility of the data. The members here will trust your results, but outside of a small sphere, it won't have merit.

I've thought about this for a while and here are some of my thoughts.
Why are you using a purely resistive load? What is the consistency of this test method and durability? How much will the resistors degrade over such harsh treatment? If the results are not very repeatable, ie resistances change slightly over temperature or time due to breakdown, your test will lack necessary sensitivity, and will not be able to make good comparisons done to tests 6mos or 30 cycles previous.
If a resistor blows and needs replacement, the test setup has fundamentally changed and would need some validation of its capability of reproducing earlier results.


Now, I had considered using a mechanical load, basically a dyno. A motor could spin up a set of weights, and the data could be recorded electronically by an ET. By using hall sensors, you can even get rpm data of the spinning weights. Add temp probes if you want too. What you can produce in the end is a graph of all these factors (rpm, amps, volts, temp, etc) on a single chart.
Aside from just peak #'s, you could read in great detail the behavior of the cell, and get create a profile (ie how quickly the cell recovers, or is able to sustain a large transitory load, ie punchiness.) This creates a dynamic test where the resistive load tests are pretty static.

The problem I foresaw in doing this is properly controlling the thr in a reliable way. Your device may take care of this, or there may be some other clever tricks. Also, the device would have to be maintained properly so mechanical friction does not greatly increase and skew results.
However, the ET can be calibrated, which is essential for getting reliable and comparative results, plus if you used a power source like a lab power supply, you could first use it to calibrate the test system and use it as a control to normalize any deviations of the test system (ie higher friction) to produce comparable results. A high quality voltmeter like a Fluke can be used to verify voltages of the test system and can be calibrated by the manufacturer with a verifying certificate.

IE. Use lab supply to provide 7.00 volts/X amps to test system. Voltmeter is attached to monitor. ET reads data. Results of calibration read of ET, resulting voltage should be within X% (say 5% at most.) recalibrate ET as necessary. (has function to do this manually.) For each batch can be done to prove accuracy of test. It can also serve as a reference standard in each batch of tests to help comparability between batches.

Also, you could then use different weights to test diff cells. A 2500 cell may need quite a diff setup than a 5000 cell to achieve a proper load. This would also be a design concern of your system.

another benefit of ETs, raw data files can be posted and independently reviewed. Data can be exported and overlaying graphs created for individ batt comparisons. A clever web programmer could prolly create a web app to o this from drop down menus linked to a result database ( a java app or whatever.) I'm sure it would fit nicely on your website if you could do it. that way its always accessible and data not buried in some thread somewhere with 50pgs of bickering over the results as most of these things on any forum always degenerate into. (We can still have that , but at least the info would also be somewhere easily and quickly assessable to anyone.)