After school got out, I had some time to conduct some FRC specific battery testing. In anticipation of the build season, our team purchased a carton of batteries (two per) from two different brands (four total) for preseason testing. We selected the MK ES17-12 and Interstate 1116 for testing. The batteries are made with 6in 4 AWG leads and terminals, torqued to 7 ft-lb. Although two is not a large enough sample size, we will keep the data in mind when purchasing our competition batteries for the season. This setup is part of a larger project (my IB EE) but this is a nice way to make sure everything is in order before I do this for real. Without further ado, here are the results.
Test 1: Capacity
Batteries are charged fully (Initial voltages below) with the Dual Pro RS3
Then I ran a discharge test (10A to 10.5v) on the CBA IV
In both tests, we see that MK has a slight edge over the interstate batteries. Whether or not this is enough to justify the extra $8 per battery, is up to teams. The only thing I regret is not bringing a battery beak (I did this on vacation). I will try to get the internal resistance values at kickoff. In the future, I would love to test how temperature affects battery capacity and voltage drop (this has been a topic of conversation among members of our team). Additionally, there is a “Power Profile” test on the CBA which can calculate the peak power output of a battery. It would also be interesting to add more brands to the test (perhaps Duracell (no online reseller?)) or compare some of our older batteries.
On an unrelated note, it is worth noting the subpar levels of discharge across the board. Getting only ~11ah out of a 18Ah battery is pretty poor (I understand that this is influenced by cutoff voltage and test current). This just goes to show how outdated SLA battery technology is (50 years old!). The results of this experiment are very intriguing; I wonder what the magic number of cycles is for peak battery performance? Too bad the CBA charge controller is out of stock and $100. Coming from the RC/hobbyist POV, I am used to seeing much deeper levels of discharge from lithium batteries. They are more energy dense, and besides combustion vehicles, by far the most popular. They are slightly more dangerous, but again, this is largely dependent on battery chemistry. This can be remedied with proper protection (hardshell case, BMS) and placement within the robot (how often are SLA batteries now getting punctured?). Charging and storage are the only other concerns; lithium batteries require slightly more involved monitoring. The batteries are more expensive, but teams would not have to purchase/replace as many (superior battery health compared to SLA). Alas, we will probably never see lithium batteries in FRC, however cool it would be.
To everyone who who helped edit this post, thank you so much! Especially @Shrub_Balsa! Thank you for the amazing graphs!
Neat tests, not only for comparative capacities, but also just showing the variations between identical batteries. It would be interesting to see a larger sample size, maybe with some Duracells mixed in too, but obviously that’s not the cheapest thing to test.
The other thing to consider when comparing SLAs to Lithium is that typically current COTS SLAs can handle a far higher peak current than what comparable-capacity Lithium batteries are rated for. While we do have a 120A breaker on the robot, depending on the situation, current draw can spike upwards of 400A or more for several seconds without popping it (speaking from experience). I suppose you could have a vendor produce a custom cell for FRC or use multiple batteries (like what’s common in RC applications), but this adds complexity, cost, and additional safety concerns and it’s unlikely FIRST would pursue either, at least in the foreseeable future…
And even when it does become too much for the battery, I think HQ has all but said they’re content for that to be the failure mode. A robot hitting brownout protection is relatively safe and has a reasonable chance of not even killing the robot for the whole match (depending on whether a radio reboot is triggered).
That said, I appreciate the analysis and validation on what we’ve felt for years. Would love to see this tested on more brands like the Duracells and Power Sonics that a lot of people seem to like.
Bingo. The only available lithium technologies that can take the high current cycling for more than 50-100 cycles are LiPo (big fire), LTO (terrible energy density), and certain specialty Li-Ion (big fire). I had some supposedly high cycle LiFePO4 batteries tested and it lasted maybe 80 cycles at 10C (180A on a 18Ah battery).
Also, BMS that can handle 400A transients tend to be pricey and very large; we would want the BMS to be placed on the robot, separate from the battery pack somehow.
To the OP, these are awesome tests. Thank you for doing them!
Have you looked at the battery spec sheet? Getting ~11ah out of it at a 50a load is great and higher than the listed specs.
If you plan on doing this some more I’d use a standard relay so you don’t have to worry about frying it or needing a heat sink and fan to keep it alive.
BTW, my team is working on a cheap build-it-yourself 50 Amp battery capacity tester… I’m trying to get the CAD kiddos to design a box, in addition to the electrical kids working on their side. Its in a thread already, if someone wants to jump into building.
This is a good test, but I’m not sure I am 100% onboard with your acceptance criteria. Had you picked 10.5V instead of 9.5V as the cutoff, the Interstate batteries would have lasted a little longer. The MK batteries you tested do seem to show a slightly flatter curve, but I think It would take a lot more battery tests to show a significant trend between manufacturers.
That said, these are the only tests I’ve seen at currents as high as ~50A. They’re useful to show that the batteries tank pretty quickly below 10V at that current. Thanks for sharing!
The capacity test was conducted at a 10A load. Going off the provided capacity values, mine fall somewhere between the 1C and 5hr rate (keep in mind this test was terminated at 10.5V).
Thanks for the recommendation. For my intended use (not this lol), the 80A would be more than adequate (only under load for about 5 seconds). The prices are pretty comparable (+prime next day) and the low voltage control of the solid state works better for my switching method.
This weekend at SVR I was able to test a new duracell battery courtesy of team 7413 (tysm!).
The setup was not exactly the same (long ish 6awg leads (multimeter broke so no good resistance values)) but I did use the same test parameters (10A to 10.5v).
Keep in mind, I was only able to get one test in so make of it what you will. This is probably not enough for any sound conclusions, but interesting nonetheless.
