CBA III Settings for 12V FRC battery testing

[iambujo]

Hello all - I am giving our CBA III a try for the first time. I have seen varying plots of battery curves, and I am looking for the recommended settings for our battery in the KOP. Based on the 17.2Ah/6Cell/12V settings and 7.5 A discharge rate (recommendation in other threads), the CBA software recommends not setting the Cutoff Voltage below 10.5V. Is that because of the battery type - Lead Acid?

Posted plots seem to show lower cutoff voltages, but the CBA III SW warns against it. Should I leave it at 10.5V or is there a save and useful lower value to set it to?

Thanks!

[DonRotolo]

10.5 will help avoid damage to the lead-acid battery. You can test down to 9.6 volts, but in FRC I do not think you will get any additional data as to battery condition from doing that.

I advise using the data comparatively: That is, don't sweat the "actual" Ah rating, but use the results to compare one battery to another. After you test a few you will get a feel for what is "normal" and what isn't.

[iambujo]

Thanks Don. I ran through the 7 batteries I brought home, and attached is the combined plot. "Battery 8" has the spike because I paused the test for a bit. Overall all the curves look clean, 2 of the batteries we bought in 2011 discharged a good amount quicker. They may not have been charged as well.

The brand new 2012 KOP battery's plot is the cleanest but it has a voltage dip from about 10 to 11 Ah, anyone know what that might mean?

The good news is none of them show bad cells from what I can tell.

[IndySam]

I have never seen a dip like that, might be a software error.

I also never use the first reading of the season. I like to run them through a few cycles and then test.

The two batteries that died early is fairly typical of week batteries. I would only use batteries like this for practice if they consistently test bad.

[Al Skierkiewicz]

The curves look pretty normal for the varying ages of the batteries. I can't think why the manual would suggest that you don't go below the 10 volt cutoff. I will have to look into that. On our CBAII I set the cutoff for 8 volts so that we can compare to curves printed for the battery and to get a little more resolution on graph. The step seen on the new battery may have been a disturbance in the power line or it might be an indication of a bad/dirty terminal. One of the first things we did was change the CBA connector to a 50 amp Power Products connector to match the robot battery connector. There are a variety of factors that change as the battery ages. All of them contribute to a lower amp hour capability. Our batteries have a stated life expectancy of 400 charge/discharge cycles under normal use. I class our use as heavy and would discount that figure to less than 350.

[iambujo]

Al - I think you are right. I found much to my surprise that it was easy to get the graph variations by moving around the battery or the CBA III during the load test. We did put the Anderson battery connector on the CBA, and all of the harnesses are good and tight, so I was surprised how touchy the setup was. It makes me wonder, if the load test setup was that touchy, wouldn't the battery under stress in the robot have the same issue?

[Al Skierkiewicz]

No, likely the connector was dirty or had a spring issue. If you were clamping directly to the terminals on the battery, who knows what was on them. Now that you have a more reliable setup, it would be nice to perform a second test on one of the shorter life batteries to see if it duplicates your earlier tests. Normally, a higher current would have broken through some of the surface dirt.
While we are on the subject, when we put terminals on the battery, I put a star washer between the terminal and the battery terminal. This serves two purposes. One it bites right through any surface crud on either terminal and second it prevents any twisting that might loosen the hardware. Once tight, I believe it to be pretty gas tight and we don't worry about it. We have some larger heatshrink that covers both terminals and the hardware pretty nicely.