I’m getting ready to help out with a summer workshop using XRPs. I’ve been building and playing with some XRPs to prepare, and standard alkaline batteries die incredibly fast.
The SparkFun video suggests NiMH, but I have a couple of reservations:
The Warning Light: Given the nominal voltage of a 4-cell NiMH pack, it seems likely we’ll be staring at a glaring red low LED for almost the entire usable life of the batteries.
Hardware Specs: The official hardware guide explicitly states the “minimum to run the system is 5V.”
I really want to avoid 3D printing a bunch of new bases just to accommodate a different size battery holder. I started looking at these instead: XTAR AA 1.5V 3300mWh Lithium Batteries
These look pretty interesting, especially since they have a warning mode where they drop the voltage right before shutting down (which should perfectly trigger the XRP’s low-battery warning).
The downside is that their protection circuit will trip if the load exceeds 2.5 amps. That seems dangerously close to the realm of what you would see from a stalled XRP if a student drives it into a wall. Especially if they are adding extra motors and servos.
fwiw, we ran an XRP camp last year where students played the Orbit Odyssey game. We got a set of rechargeable batteries for the camp with enough so that there are two sets per robot. One set being used while the other is charging. In the past we bought EBL 2800mAh Nimh rechargeables. In a second round of purchases we bought Cardiens 2800 mWh Li-Ion batteries. Both met our needs and as far as we know never caused significant issues around low power situations.
Thanks for the info. It sounds like I shouldn’t get stuck on the ‘5V is the minimum to run the system’ specification. I’ll order a set of the NiMHs. I really appreciate you taking the time answer.
If you have any other tips on the Orbit Odyssey game, let me know. We are working with middle school FTC students.
Regarding Orbit Odyssey, perhaps a quick summary of what we did to see if any of it is interesting to you.
We are an FRC robotics team and are working to get more of a student development pipeline set up. Last year we started two FTC teams for middle school students. As a way to help them get up to speed on how a FIRST robotics program operates we ran the 5 day XRP robotics camp, 9-4 every day.
We had the FRC students be “counselors” and help the campers as well as give different presentations to the campers.
We printed mechanisms that students could attach to their XRP base to be able to play the game. This was good and bad. It was good because I am not sure how well the students would have made something from scratch. Bad because many of them did not try to improve the mechanism which meant they were out of things to do by the afternoon of the forth day.
We split all of the campers into teams of two
There was a “competition” on the last day with all of the teams playing a round robin sort of tournament. That was a lot of fun to watch and having that sort of competition at the end helped create a motivation for many of the team members. If you decide to do this - there is a scoring web page that someone put together to help manage the matches.
To expose the campers to CAD we required them to model a badge in TinkerCAD that was 3d printed and put on their robots. One red and one blue depending on which alliance they were on.
We debated if we should put the XRP base together before camp started. The concern was that some of the pins are easy to bend. In the end we had the campers assemble the XRP and no pins were bent!
fwiw, we are going to run an XRP camp this year and do the new game, Iron Acres. It is going to be a little more challenging for the campers as we are only providing a kit of parts they can use to build a mechanism to put on their robot.
We did try to get some outside time right after lunch to burn off a little extra energy.
Ah, we should amend that! The board will actually remain powered down to about 3.5V before browning out, though you obviously want some margin above that. Don’t think about that 5V as a minimum requirement, more as a minimum recommendation. We may also decrease it to 4.8V to make it clear that NiMH is okay.
On that note, NiMH actually sustains a higher voltage while under load and at lower state of charge than alkaline. Here’s a discharge curve for comparison:
What are you going to give them for parts? I’ve been exploring this more lately. I looked at the XBS system, but it’s still listed as in ‘alpha,’ and I would need to reprint the bases. I’ve also thought about using LEGO Technic.
I saw the Iron Acres game and I really like it; I think that will be a better game overall, with more variety for build challenges. We’ll be sticking to Orbit Odyssey for now, but I’m going to gauge interest in a second round with that game.
I think that update would be helpful. Do you know offhand when the low-voltage light illuminates? It would be nice to know so you can gauge how close you are to over-discharging your batteries.
The other thing we looked at is the lego technics system. We pursued that a little bit last year and if you go that way, let me know. there are some parts you can buy that make connecting the legos to servos better than what is on the printables site. ( Search models | Printables.com ). The problem we had with the technics last year is that people got distracted by the legos and started building all sorts of non robot things.
Batzone lithium batteries from Amazon. Had no issues last year and are using them again this year. Also got two sets aper bot although our half day camp seemed to do ok with just one usually.
That’s a different XBS set than I originally saw on Printables. It looks like this set is designed for specific FIRST online training classes. That might be useful for giving the kids a good starting point.
Thanks, that’s good to know. It also sounds like a 2 amp limit might be ok. Did any of the kids build a Mecanum drive, or other XRP with four motors and multiple servos?
Around 4.3V. In my experience with NiMH batteries, the LED starts flashing under load when around 50% SoC, and stays on almost all the time when <10% SoC.
For reference, the stall current of the motors is around 750mA at 6V. The stall current decreases as they heat up, and with battery voltage droop, so if you were to actually stall 4 motors in the real world, I’d expect the total current draw to be around 2A. While it’d be close, I think you’d be fine in reality (it’s rare to stall all the drive motors at once).
