Rechargeable batteries for the RCX

[Barry Bonzack]

I have been doing research lately on rechargeable batteries for the RCX. I have been using the discussions on VEX for guidelines on Mindstorms. I am helping run a 10 week camp, using about 12 RCXs, and replacing 6 batteries in each one every week does not seem economical. The Vex Inventor's Guide recomends Ni-Cd rechargeable batteries for their vex because they maintain their voltages reliably until they are almost completely empty (page 4*10). It highly discourages alkaline batteries, even rechargeable ones as well. Here the Vex FAQ also recommends NiHm

NiMH and Ni-Cad batteries have a low internal resistance making them a good choice to power the robot. NiMH does have a higher energy per volume than Ni-Cad and is an excellent choice.

However, I stumbled upon this today on LEGO.com

Is it safe to use rechargeable batteries with LEGO MindStorms products?
While it is safe to use any rechargeable batteries with LEGO MindStorms products, they will provide less useable time per recharge than non rechargeable batteries. This can lead to problems with the RCX, Scout, and/or IR Transmitter (Tower). If you do experience problems with any LEGO MindStorms product while using rechargeable batteries, we suggest that you try a set of fresh non rechargeable batteries to see if that solves the problem.

So here is the question, Has anyone used rechargeable batteries for the RCX for competition or non-competition purposes? What experiences or problems have been noticed with the performance of the RCX with these batteries?

[Al Skierkiewicz]

Barry,
Although this is a general statement, the rechargeable batteries most people think of for this application are NiCad. Since the terminal voltage is 1.2 volts (NiMH too) instead of the 1.5 of alkaline, the total voltage availabe for the RCX then is a full 1.2 volts less when using the NiCads. With only 7.2 volts available, the RCX gets unstable when using all three motors in high current designs. With good quality alkalines, you should have no problem running a one week camp. This year we had several teams run on the same set of batteries most of the FLL build season.

This from the Tech Support at Midstorms...
The RCX programmable Brick is designed to run at 9 volts. As the batteries drain the RCX programmable Brick should continue to function until the voltage drops to less than 6.7 volts. Please note that intermittent or complete failure of operation including IR communication may occur at any point less than 7 volts. At 6.7 volts the low battery indicator should display and fewer than 6.3 volts the RCX programmable Brick will enter the alarm mode beeping. The RCX programmable Brick does have a Firmware protection mode, which will shut down the highest draw devices as needed to protect the RAM stored firmware. Generally the first device to shut down is the IR Communication, followed by the output ports that motors and lights are connected to.

And...

While it is safe to use any rechargeable batteries with LEGO MindStorms products, they will provide less useable time per recharge than non rechargeable batteries. This can lead to problems with the RCX, Scout, and/or IR Transmitter (Tower). If you do experience problems with any LEGO MindStorms product while using rechargeable batteries, we suggest that you try a set of fresh non rechargeable batteries to see if that solves the problem.

Also, don't be fooled by the DC adapter plug on the RCX. It is not the input for a rechargeable battery pack charger it is merely an alternate DC input..

[FourPenguins]

I have tried using NiMh (I think. They may have been NiCd) rechargeable batteries in the RCX. As Al said, they have a lower terminal voltage, and performance suffers accordingly. Is it safe? Yes. Will you likely lose motor power and end up replacing the batteries more? Yes.
Another thing to consider is that rechargeable batteries are significantly more expensive than their alkaline counterparts. If you only plan to use these batteries for a few sessions, it might be more economical to buy alkaline batteries for each week. (Especially if you buy in bulk.)

[Billfred]

Unless you're putting some serious mileage on your RCXs, you might be able to get away with regular batteries. Where I'm working, our RCXs go the entire summer on one set (that's ten 70-minute activity periods, plus a bit more on final performance day) and we haven't had to change batteries once in two and a quarter summers. If you get some good batteries (my camera lasted through 23 hours on a train, BattleCry, Boston, and a lot of photography here in Maine--several hundred pictures--before getting the low-battery warning on Rayovac batteries), you might only need to change batteries once or twice.

You mileage, of course, may vary.

[Danny Diaz]

My greatest addition to this discussion:

http://www.philohome.com/motors/motorcomp.htm

I think this guy really nails home the difference between rechargeables versus non.

-Danny

[Evenstar_342]

My FLL team tried using rechargable batteries for their RCX in 05-06 since we've always had problems with batteries. Although we didn't do anything incredibly scientific, we found that the rechargables were used up a lot faster during the constant repetative runs on the table. We do a lot of those (fixing programs and drilling our table people), so we found that the best solution was to replace regular batteries every so many runs or before each table run at competition.

Another thing our team didn't like was how unrealiable Robolab's battery meter was. At the 2006 World Festival, one team was commended for writing a program that allowed them to measure their battery level (I guess showing it on the view screen). Has anybody else heard of this or know how such a program works?

[Billfred]

As an update to my earlier post, I apparently didn't knock on wood.

Late in the summer at camp, we had two robots emit the beep of death. Both of them were using Dynex batteries. I'm skipping that brand in the future.

[Al Skierkiewicz]

Evenstar,
I am surprised that you have had problems with batteries, especially the need to replace them every few runs during competition. Are you sure that your battery terminals were not deformed causing a high resistance connection to the batteries? Remember that the battery case is series wired and therefore there are twelve connection points to consider. The terminals are easily deformed with rough handling (mass vs acceleration) and dropping. If you turn the RCX over (without the battery cover in place) and batteries fall out, then the terminals are deformed. Damaged motors are another source of high current draw. Frequently a dropped motor will exhibit exceedingly high internal friction due to the rotor/frame spacing. A deformation here causes the rotor to rub on the frame. A simple test is to turn the motor shaft by hand. It should turn with a little resistance and continue turning when the shaft is released. If you cannot turn it by hand or it stops immediately when the shaft is released, then the motor is defective. As Danny's link above attests, these motors for the most part are not repairable. (I have had limited success realigning a bent motor mousing with the old motors.)
The Robolab battery meter is normally used with the robot idle (no motors running) so it is measuring no load battery voltage. It is still a good indication for the most part. A reading of less than 8 volts, no load, is a good indication that the batteries need replacing. Many FLL teams have not learned the direct relationship between large sticky tires and the high currents involved in turns. The large (3.5" diameter) sticky Lego tires on all four wheels with direct drive (wheels mounted directly on the motor shaft) is the highest current that can be drawn from the RCX. The addition of the friction imparted to the motor shaft by the direct tire mounting and the steering friction of turning sticky tires on the front of a robot combined with significant weight of robot design/attachments can significantly reduce battery life. As in FRC robots of old, high current draws during turns coupled with low battery can cause controller failure.
I have not heard of the need to change batteries during a competition other than to insure correct operation when the robot makes it to finals.

[Evenstar_342]

Mr. Skierkiewicz,

My team found your post very informative. Even though we've moved to the NXT, some of my fellow mentors investigated the battery terminals and found them to be just fine. Our rationale for replacing the batteries was the desire to keep as much constant as possible. Not being master programmers or electrical engineers, our students always had problems with friction and alignment especially. Probably some of the time that the robot was off was due to those issues instead of battery life. However, we felt that by having a decent idea of the freshness of the batteries would eliminate one variable, so we could continue problem solving more effectively. We were consistently underwhelmed with Robolab's meter, your explanation could account for several instances of robots dying at summer camps when the meter read almost full (I almost slept through that semester of physics though, so I could be totally wrong ). We're not complaining, a top two finish four out of five years is pretty good and, most importantly, we've inspired legions of students to discover science and technology.

[Al Skierkiewicz]

Even,
That is always good news. Our team does not have the resources for NXT yet but they look very impressive. Time will tell on battery issues with these platforms, since I have no available data on battery use yet. I think that FLL will have to migrate to a larger playing surface in the future for the NXT since the robots I saw were so much larger.
I am glad your team found the post informative. Let me know if I can help in the future.

[Danny Diaz]

Originally Posted by Al Skierkiewicz

I think that FLL will have to migrate to a larger playing surface in the future for the NXT since the robots I saw were so much larger.

I hope not. The table surface is already very large - it's wide enough that I have difficulty resetting the center "shared" mission, and it's long enough that I have a difficult time running up and down the table to help out kids playing on the game board because *they* can't reach their robots either. I would hope that FLL will not grow the game board size at all, and make the teams just design smarter. Already the "base" areas are huge - compared to previous years where RCX-only robots were competing - and the referees give tons of flexibility on that rule as it is. I'd like to see less delicate mission models and slightly more complex activities for robots to solve/complete - I loved the "Smart Medicine" mission with the Buckeyball from this past year, the "Protect the Pumping Station" from the mission in Ocean Odyssey, and the "Climb the Stairs" mission from No Limits - those were all simple in concept, but could be done in multiple ways and were as simple or complicated as teams wanted to be. The models were fairly easy to build (the stairs maybe were a pain), they were definitely solid, and they were fun to complete.

That's just my $0.02 as a head referee...

-Danny

[Andrew Schuetze]

Okay, so I just ran my first two summer camps with the Lego NXT and the Lego supplied rechargable battery pack and DC power supply. In the first camp which was a half-day version for six weeks. The teams only had to recharge the battery pack every couple of days as they only were in robotics for 1 to 2 hours a day. I don't have any good data for the full-day , two-week camp as I only taught the morning and witnessed the teams recharging over-night for good measure. Lego must have designed the NXT to operate on the voltage coming from these packs and I haven't a clue as to what type of system they are using, NiCad, LMH, Alkaline ,,,

[Al Skierkiewicz]

According to Lego this NXT only pack is rechargeable Lithium.