We had some frustrating matches at TRI due to brown out. That lead me to load testing our battery fleet… Two our our 2022 batteries were down to 10% capacity!
Since I had 40 tests worth of data, I’m betting the Delphi crew would like to see it! Its all done at 12 Amp rate. The vertical trace is milli-Amp hours. Each trace is one battery, with higher numbers being later in time. I didn’t have a great way to distinguish between pre and post season tests on this graph. Other than the failed batteries there’s not much change over time; just a gradual decline. Of course, the older batteries aren’t seeing matches, so that may not be very surprising.
Does anyone else have similar data they’d be willing to share?
Average internal resistance (including cables and Anderson) 0.0178 Ohms over all the test I have that data for. 0.016 was the lowest measurement and 0.023 was the highest.
We got the 2022 batteries from AndyMark. Interstate brand.
We have had a similar issue with Interstate batteries. We got a bad batch of batteries and that lost us a few matches at Buckeye. Some of the batteries read up to 14.5Ah, while others are as low as 1-2Ah.
Our 2020 batteries were all MK brand and most did very well throughout the season. Additionally, before Worlds, we ordered a big batch of batteries from MK, and those batteries not only didn’t fail us once at worlds, but have anecdotally lasted much, much longer than Interstate batteries.
I don’t have the Ah readings for the Interstates available on hand currently, but I’ll see if I can get them to you. And for future reference: try out MK batteries for the next season.
We borrowed Interstates from other teams for Championship this year. The oldest one was labeled that it was put into service in fall of 2021.
After one run on our CBA, they were marked as pit-only batteries.
I don’t know the full provenance of the batteries–it’s possible the other teams weren’t as cautious about overdrawing the batteries in practice, or that they ran way more practice than we do to rack up the cycles. But the point remains, they were less than a year old and not fit for the field.
Far as I’m concerned, MK is the yardstick for FRC batteries right now. I understand from talking with other mentors that the Duracell batteries are pricy, but perform even better and are available locally. Any other brand is going to need to prove itself.
The instantaneous answer for checking battery readiness is the Battery Beak. Plugs right into your battery and checks state of charge and voltage under a couple different amp draws.
The better test when you’ve got 90-120 minutes to burn per battery is the CBA, which draws the battery down to a set voltage, measures battery capacity, and plots voltage over the course of the test to suss out any bad cells inside. We got ours between Electric City and Championship, and it’s how we were able to make informed decisions about which ones to run. We write the amp-hour reading on the side of the battery in Sharpie afterward so the data can’t be lost.
Some teams will forego the CBA tests and simply demote all of last year’s batteries to practice duty, since most teams only get 1-2 seasons of competition-level lifespan out of a battery. Might not be dollar-efficient, but it does save a heap of time in testing.
(EDIT and apologies!) The BatteryBeak does a great job measuring the current state of CHARGE of the battery. It does not show any differences between the 10% ENERGY capacity battery and 100% ENERGY capacity batteries… IE, the 160 mAhr terrible battery and the 16,000 mAhr good batteries test the same for Voltage and state of charge when they come off the charger.
Three possibilities: the CBA from AndyMark at @Billfred mentions.
I just got this in and I’ll post what results I get.
This is what I’m using right now. It allows you to test at 12 Amps, which speeds things up quite a bit! I can get a couple of tests in a FRC meeting: 1:06 is normal. Its extremely under-packaged and not very friendly to use… https://smile.amazon.com/gp/product/B08MQ1G57D
A cool thing that -someone- in FRC should dream up is a 1/4 Ohm resistor with some kind of semiconductor switch in series with it. You would turn on the load resistor (pulling 50 Amps) and start a timer. It would turn off at 10.5 Volts and display the time to get there. This would give you a load test that is sorta close to the match load, doesn’t over-amp the Andersons, and works pretty quickly (about 20 minutes). One challenge is how do you get rid of 60 Watts with something that isn’t too expensive! Even better would be a big Mosfet with a high power series resistor (maybe 0.2 Ohms or so) and some smarts to make it constant current. That will make the tests more repeatable, but also adds some cost.
One of these things plus a timer and a 50 Amp relay might make a good start part.
Thinking on it harder, the heaters above are PTC heating elements, so their resistance will vary a LOT depending on air flow. There are 200 and 300 Watt resistors on FleaBay… Hmmm.
We buy direct from MK. They have always given us good pricing, this year it was $38.50 each plus a per order “logistics” charge ($35ish). In a normal year we can pick them up from a distribution center near us (MA) and avoid shipping; this year with supply chain issues they could only get them into California and ended up doing a freight shipment to us at no extra charge (I think this was only because of the unusual circumstance & wouldn’t expect that to happen normally). We do try to buy 6-8 new batteries at a time and often share an order with another team so it’s a decent size order; this year it was a pallet of 12 batteries.
Ok, I did the first test on the BD250. It looks like it can only measure up to 9999 mAhrs, so it could be used to weed out bad and terrible batteries (under 50% capacity), but can’t really compare good batteries (18,000 mAhrs). For bonus fun factor, it had completely shut itself off after the test, so no clue what it measured!
On the good side, its pretty easy to use and is well packaged. Plug it in, set current, cutoff Voltage, and press and hold to make it start. I’d prefer something that saved its settings… The other one I have -does- save the cutoff voltage but not the current.
Unless I get some useful feedback from SkyRC this sucker is going back.