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Battery question 7.2v 2000 mAh — Parallax Forums

Battery question 7.2v 2000 mAh

Patrick DPatrick D Posts: 62
edited 2007-10-07 17:46 in Robotics
I am designing a bot around a BOE and BS2. I'm using 4 servos, and using several sensors, an LCD, etc. I wanted to use a battery that will last, so I picked up a Ni-Cd rechargeable battery pack at Radio Shack. It's rated at 7.2 V/2000 mAh - and was designed for RC cars.

If I plug this in my BOE, will I 'smoke' it?

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2007-09-29 04:32
    The BOE is designed for input voltages up to 9V. 7.2V is a good choice. Be sure to power the servos from Vin since you have 4 of them and they can draw quite a bit of current when starting. Servos will work up to about 7.2-7.5V. Beyond that and the brushes in the motors will wear faster than normal. Much more than that (like 9V) and you might damage the servo electronics.

    An alternative (which I use) is 5 NiMH AA cells in series which yields 6.00-6.25. I happen to have a charger that's not fussy about the number of cells you charge at a time. I use the 4 cell holder that comes with the BoeBot and an extra single AA holder in series and mounted on the back of the chassis.

    Post Edited (Mike Green) : 9/29/2007 4:37:24 AM GMT
  • D FaustD Faust Posts: 608
    edited 2007-09-29 11:24
    Be careful with the 7.2V they usually peak at a little over 8V off the charger. (At least the ones I've used) You will probably burn out you servos if you run them of 8V for long. Radioshack sells 6V batteries. Check out their site. You might have to wire two in parallel because they are 1000mAh I think, or you could just deal with decreased life. These batteries get up to about 7V when charged, I think.

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  • Patrick DPatrick D Posts: 62
    edited 2007-09-29 12:25
    Thanks for the info...
    I was also concerned about the current. One more question - if I were to connect two of those batteries in parallel - what about then? Would the voltage regulator on the BOE be able to handle the current? is there a better way to give your bot a longer runtime?
  • D FaustD Faust Posts: 608
    edited 2007-09-29 13:24
    My understanding is that the 5V regulator regulates only enough to supply the things running off it, not all that it can get.· Therefore, by connecting the two·batteries in parallel you are still going to draw the same amount of curret, but the life will double.· The servos won't be drawing power from the regulator so it should be fine.

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  • Patrick DPatrick D Posts: 62
    edited 2007-09-29 13:29
    great - thanks
  • k4shfk4shf Posts: 21
    edited 2007-10-07 17:13
    Mike Green said...
    The BOE is designed for input voltages up to 9V. 7.2V is a good choice. Be sure to power the servos from Vin since you have 4 of them and they can draw quite a bit of current when starting. Servos will work up to about 7.2-7.5V. Beyond that and the brushes in the motors will wear faster than normal. Much more than that (like 9V) and you might damage the servo electronics.

    An alternative (which I use) is 5 NiMH AA cells in series which yields 6.00-6.25. I happen to have a charger that's not fussy about the number of cells you charge at a time. I use the 4 cell holder that comes with the BoeBot and an extra single AA holder in series and mounted on the back of the chassis.
    Mike,

    How did you counter the weight being on the back?· I tried loading the blade with 20 grams additonal weight but seemed like there was less weight on the axles and traction is a problem....

    jumpin.gif·Tim T.
    ·
  • D FaustD Faust Posts: 608
    edited 2007-10-07 17:19
    I don't think that one AA cell could cause a weight problem.

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  • Mike GreenMike Green Posts: 23,101
    edited 2007-10-07 17:46
    The back of the chassis (for me) is the part above the pivot ball. As D Faust mentioned, a single AA cell and holder isn't very much weight. I have a PING sensor with the servo and bracket and that's on the other end of the BoeBot. Since I'm using a Propeller Protoboard rather than a BOE, the weight distribution is different anyway (with connectors, SD card, XBee, etc.)
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