Battery Testing: Our Team would like to share our downloadable study with your team!

Our team has completed an exhaustive battery testing regiment and in the spirit of FIRST, we would like to share our experience with your team. Teams accrue batteries over time, but which ones are good for competitions? Newer doesn’t always equate to performance.

To better understand battery performance in a competition environment, students researched and implemented four industry standards of battery test. They then tested sixteen of the team’s batteries with three performance benchmarks. The batteries were then ranked from first to worst and a strong correlation was found between not only battery age but the number of charge and discharge cycles a battery goes through. Finally, an engineering report was written so that other FIRST teams would benefit from our findings.

In the 2014 competition season, our robot had a complete failure during a critical qualifying match while on the field. The team did an in-depth analysis of the failure and the cause was determined to be low battery voltage, which reset the robot’s processor and shut all functions down. This was a surprising finding – as we were using a new battery that was checked before being placed in the robot.

This finding provided the incentive to pursue the following questions:

  1. How do we really know how our batteries will perform in a competition?

  2. Are there industry standards for battery testing?

  3. Can we perform regimented testing and rank our batteries?

  4. What are the characteristics of the FIRST lead acid batteries and can we better understand how to measure them?

  5. Is there a way to log battery performance while the robot is actually running?

We answered these challenges in this report, and we hope it can also help your team!

http://the-charge.com/uploads/3/3/5/9/3359086/testing_and_analysis_of_first_robotics_batteries.pdf

Testing and Analysis of FIRST Robotics Batteries.pdf (4.96 MB)


Testing and Analysis of FIRST Robotics Batteries.pdf (4.96 MB)

Bob.
Congratulations to your team on a spectacular paper demonstrating battery life and operational issues in FRC robot batteries. I can suggest that your battery ranking likely points at batteries that have had more charge/discharge cycles than others. This could explain the unexpected ranking of older batteries but also may be influenced by the robot design used in the years you have owned those batteries. Can I suggest that teams that might participate in your study also perform a long term (over several years) data collection of charge/discharge cycles for each battery and if possible some form of classifying current demand by robot. With no formal data, I have observed some robots that will drain a battery in a single match while others with more efficient designs can typically run two or three mtaches without using a fresh battery. Your students should be very proud of the work they have performed.
Al

Al,

Thank you for your kind words. I know that the kids really worked hard to get this report together, it was a great experience for them to do research, testing and analysis.

I agree with your observation on charge and discharge cycles. One of the students on the research team took it upon himself to create a spreadsheet to track our charge and discharge cycles so that in the future we can research this aspect as well.

We’re all trying to learn as much as we can, and if our knowledge is helpful to others, that would be really great.

-Bob

*I enjoyed reading your paper. Nice work.

I have one question, one observation, and one request.

Question:

When doing the high current discharge test, did you allow the load resistor(s) to cool to the same starting temperature before performing each test?

Observation:

Figure4 on page 14 looks strange. The voltage at 30s at 200A is lower than the voltage at 15s at 400A.

Request:

Would you be willing to provide more in-depth detail about the design of your dynamic loader and logger?

I’m very impressed with your paper and findings. I had done a similar set of tests using a West Mountain Radio battery charger. It shows possible weak battery sets, but not to the level you were able to get to.

The programmable dynamic tester is really nice. That in combination with the logger would let teams really look at their active battery loads and determine how close to the edge they are going. With the relaxed motor rules and teams pushing harder on the electrical loads knowing how the battery will react is a great thing to know.

One of the best white papers I’ve seen on CD, thanks for taking the time to do the research!

Dear Ether,

We allowed time for the load resistors to cool between tests. We also had a fan blowing across the resistors during the test.

Figure 4 (from SAE J537) is showing the non-linearity of battery discharge based on load current. This was the reason we tested at two different discharge currents. The curves make sense, as each is a stepped based on the current load, i.e. the larger the current, the lower the output voltage at that current over time, since the internal resistance is making up the rest of the cell voltages.

The team plans to do a “makable” on the battery logger and dynamic loader, hopefully as a summer project once the maelstrom of competition season is over and we get a breather. We specifically kept it out of this report because it would have shifted the focus from strictly analyzing batteries and ranking them.

Thanks again for your feedback, we really appreciate your comments!

-Bob

Yes, the larger the current, the lower the output voltage at that current over time.

But I think you missed the point I was making: For equivalent drained Ah, the higher current drain rate should have a lower voltage.

200A30s = 6000As = 1.66Ah
400A
15s = 6000As = 1.66Ah

So the voltage at 400A at 15s at should be lower than the voltage at 200A at 30s. But it’s the other way around in the figure.

This is just a minor observation and in no way detracts from the impressive accomplishment of the work and the report.
*
*

I see what you mean. These family of curves were referenced from the SAE standard, and our interpretation was strictly that the discharge was non-linear when comparing different load values. Your keen observation certainly goes beyond what we intended from that graph, but I appreciate your feedback. Thanks.

Foster,

Thanks for your comments. Once again, I must applaud the kids for putting the time into this study - and I know they are happy to share that with others.

Bob,
Under ideal conditions, the terminal voltage for the 400 amp load should be approx. 7.6 volts while at 200 amps it should be 9.8 simply due to the drop across the 0.011 ohm internal resistance. Your mileage will vary…

Same here!

I suspect that the high-current-draw tests didn’t actually draw the intended current, largely due to the increased voltage drop across the battery internal resistance, and also partly due to heating, which increases resistance in conductors. If the resistors were selected to represent a 12V battery with no internal resistance, the 200A test would use a 60 mohm resistor, and the 400A test a 30 ohm resistor. According to the attached document, a new fresh battery has a nominal internal resistance of 11 mohm. Assuming that the fully charged battery is actually charged to 14V (for negligible load), the current draw for the 200A test case would be 14V / 71 mohm = 197A, not bad at about 1.5% low. For the 400A test case, it would be 14V / 41 mohm = 341A, or about 15% low. Of course, these numbers are only valid when everything’s still at room temperature. As the resistors and batteries heat, the 400A test will get hotter than the 200A test, increasing the resistance disproportionately, and further decreasing the current ratio.

Again, let me speak through Ether’s words:

I will certainly include a lesson strongly referencing this paper in this summer’s robocamp. The test descriptions are well done, as are the summaries of the research done prior to testing.