Unofficial FLL FAQ06-c - Batteries
UFAQ Table of Contents

Q8.1) Can I use rechargeable batteries rather than alkaline inside the RCX.
A) Yes with a few warnings. Use batteries that are rated for 1.5 volts a battery or less. Anything more can/will damage the RCX. Most rechargeables have lower voltage than alkaline batteries. The performance of the RCX will therefore be different. Another issue with rechargeables (especially Ni-Cad) is that they provide constant voltage and then over a short period die quickly. Alkaline batteries die slower giving you some early warning.

Q8.2) I have an RCX with an external power jack. Can I use it to recharge the batteries inside?
A) NO.

Q8.3) We have heard of some teams buying cases of batteries. How many batteries will we need?
A) Depending on the robot, a set of batteries will last from 1 to 20 hours of use. A heavy treaded robot will gobble batteries at an alarming rate. A light wheeled robot will last much longer. Do not be alarmed at 'buying a case' of batteries. A case is normally 144 batteries or just 24 changes. A large team with several robots can easily use 24 changes in a season. See the following coach's manual for more details: Handbook. You should also read the information on rechargable batteries elsewhere in the UFAQ.

Q8.4) Are the kids required to change the batteries or should/can the coaches change the batteries?
A) It's not very hard to change them, so let the kids do it. In fact, by having them change the batteries, they will consider twice before burying the RCX/NXT deep into the robot.

Q8.5) What is the proper way to change the batteries in an RCX?
A) It is possible to change the batteries without losing the firmware if you use the following procedure.
  1. Remember that they all face the same direction.
  2. Use a magic marker to scribble over the old baterries while they are still in the RCX. This allows you to keep track of them should they get mixed up during the change!
  3. Remove only one battery at a time.
  4. Replace with a new battery as soon as possible.
  5. Wait 5 seconds before changing the next battery. This allows the RCX to recharge its internal parts.
Having two team members work together to change the batteries may help keep the new and "used" batteries separate, as well as encouraging teamwork.

Q8.5.5) What is the proper way to change the batteries in an NXT?
A) Unlike the RCX, the NXT will not loose its programming with the removal of batteries. Changing them is as simple as opening the door, pulling out the old batteries, and inserting the new. As in all electronics, make sure you observe the proper polarity.

Q8.6) How can you test your batteries?
A) In RIS, you can check the battery level by going to the settings area. In Robolab, open inventor 4, click on the project button at the top of the screen in the Robolab program, and then click on the interrogate RCX button on the drop down menu. The RCX battery level is displayed on the left side of the screen. You need to have your RCX on and in front of the serial tower for this to work. In NXTG, you can access the power level from the "NXT Window".

Q8.7) Can cold affect the performance of a robot?
A) Anything and everything can affect an FLL robot! Certainly the cold will greatly reduce the performance of a battery. Consider keeping your robot and batteries warm on that long car drive to a competition in cold climates.

Q8.8) Is an RCX robot sensitive to voltage?
A) The power of the Robot is dependent on the voltage of the batteries. This is not always the case with electric motors, frequently it is the power, not the voltage, that matters. But with the RCX higher voltages will produce more power at the motors. At 9V (alkaline batteries) the motors have 40~60% more power than at 7V (NiCd or NiMH rechargeables). See Phlohome's great page on motor power www.philohome.com/motors/motorcomp.htm.

Q8.8.5) Is an NXT robot sensitive to voltage?
A) Yes it is, but less apparent if you are using the synchronized "Motor Move" icons.

Q8.9) What are some of the pros and cons of various battery types in an RCX?
A) The following answer was supplied by James Sluka 
Nominal Voltage (6 cells): 
Alkaline: 9V 
Rechargeable: 7.2V
(RCX motors are more powerful at 9V than at 7.2V)

Voltage profile:
Alkaline:  Starts at high voltage (>1.5V/cell) but quickly drops to 1.5V,
           longish time period of more or less constant power output,
           slow final death.   
           The initial high voltage period can cause problems, 
           wheels may loose traction etc.
           The slow death of the battery means robot reproducibility 
           will not be very good.

Rechargeable:  Starts at a lower voltage (1.2V/cell),
               very minimal initial high voltage period,
               rather abrupt power loss at end of charge.

Personally, I prefer the rechargeables power profile, but the Alkalines 
do give the robot more power.

Cost:
Alkaline: If you shop carefully you can find good quality alkalines for 
          50 cents each. Batteries are much cheaper in packs of 12 or more, 
          for example $11.45/36 batteries or 32 cents/each.
          Figuring a case (144) per year per robot, a single team 
          spends $46 for the year. Or, figure a new set of batteries each 
          meeting, 2 meetings/week for ~4 months comes out to 48 batteries 
          per robot/season, about $15. The key is to find the biggest package
          of batteries you can. Don't buy 4-packs, the batteries could 
          be $1.25 each!

Rechargeable:  Many electronics stores sell a four cell charger + four 
               batteries for ~$25. Additional batteries are $3 to $4 each. 
               A team needs at least 12 batteries for the first robot (6 in 
               robot while other 6 recharge), and an additional 6 batteries for 
               each additional robot. For a single robot figure charger plus 12
               batteries, total cost $50 to $60. Each additional robot (6 batteries)
               is about $20. This is quite a bit more than the alkalines but of 
               course you get to use the same batteries next year. And, in keeping 
               with the spirit of FLL, much more environmentally friendly.
-James Sluka


Q8.9) Why does my NXT recharable battery only shows about 8.4 and not 9.0 volts with a full charge?
A) Recharable batteries do not have the same voltage as alkaline batteries. Alkalines are 1.5 volts a cell for a total of 9.0 volts in an NXT (or RCX). NIMH cells are about 1.2 volts per cell. If the battery had six cells inside you would expect to see about 7.2 volts. But the NXT recharable battery has 7 cells for a total of ABOUT 8.4 volts. As such, 8.4 volts is about what you should expect to see.
The fact that you have 'lost' 0.7-0.8 volts compared to alkalines should not be a concern. What you are really are looking for is a battery voltage that does not change significantly over time and usage. The rather beefy NXT recharable battery should be just the ticket.

UFAQ Table of Contents
Home

Disclaimer: This FAQ is not an official FIRST document. It is an accumulation of knowledge derived from six thousand messages posted to the FLL forum over three seasons. It has been reviewed by numerous people, but may still contain errors. Use at your own risk.
Readers are encouraged to submit errors, suggested wording changes, new topics, or comments to Skye Sweeney at skye@fll-freak.com
Copyright 2003-2006 Skye Sweeney; Last Updated on 10/28/2006