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I'm working on a school group project which involves setting up a remote controlled car with a BeagleBone, webcam, WiFi module and a powered usb hub.

To handle the mechanical side, we bought a cheap car from Walmart and hacked it. It came with a battery compartment that holds 5 AA batteries that the motors run off of (at about 2 amps each. I think one measured 2.5). With Alkaline batteries, that gives us 7.5 volts at full charge. With rechargeable batteries (what we hope to use in the end), that is 6 volts. That's fine for the motors, but the issue I'm running into is the BeagleBone and hub. I'm using 7805s to provide power to each the BeagleBone (which requires one amp) and the hub (which should be about a half amp). The 7805s require at least 7v to function. So, I bought a 4AA battery holder to step up that voltage. Now, if I were to hook up that voltage to the motors, it would be way to much.

Final questions:

  • Could I safely run a wire from the middle of the battery pack to the motors?
  • Would that end up killing one set of batteries before the other?
    • If so, I've read that the less dead set will try to charge the other set. Would that matter if I'm using rechargeable batteries?
  • Is there a way to step down about 4-5 amps without too much cost or wasted energy (via heat)?
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In place of the LM7805, using a Low Drop Out (LDO) regulator such as MIC29300-5.0WT would allow operation at as little as 5.37 volts, providing 3 amperes at that voltage, enough for your Beagleboards but not the motors.

The more expensive MIC29500-5.0WT at $5.46 apiece on Digikey would provide the 5 amps you might want instead.

  • Tapping between batteries is best avoided with unbalanced loads.
  • And yes, the batteries loaded higher would deplete earlier, with unpredictable outcomes.
  • In stepping down 6 to 7.5 volts to 5 volts at 5 amps using an LDO, up to 12.5 watts of heat would necessarily be dissipated at the regulator: Power dissipated as heat = voltage difference * average current = 2.5 * 5

However, realistically the batteries won't be at 7.5 volts under load, and not for long anyway. Also, the continuous current drawn would not be 5 amperes through the regulator. Your design would ideally provide the motors with unregulated power, so your "step down about 4-5 amps" deserves a revisit. Besides, voltage is stepped down, not current, at least in this context.

Switching (buck) regulators would waste less heat, roughly around 15% that of the LDO, but they usually need more voltage headroom to operate. YMMV, searching DigiKey might yield what you need.

Why is your hub needing to be powered from the same power as your car motors? Your hub should be at your base station, the car would just carry a WiFi dongle of some sort, ideally?

Suggestions:

  1. Edit: [15-Oct-2012] Provide regulated power only to the logic parts, drive the motors directly off the battery, and add a hefty decoupling capacitor before the regulator to work around motor inductive spikes. If your batteries can handle 4 motors of 2 amp each, they can handle the initial surge the capacitor will need.
  2. Provide a diagram of some sort, of what is being attempted, and some clarity on how the powered hubs got aboard the moving car. This will enable more helpful answers.
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  • \$\begingroup\$ On re-reading your post: If you are using the powered hub to power and access your webcam, that webcam is perhaps a bad choice. A lower power WiFi webcam powered directly off the battery or the regulated line would work better. \$\endgroup\$ – Anindo Ghosh Oct 15 '12 at 5:55
  • \$\begingroup\$ The hub is on the car to provide power to both the WiFi adapter and webcam. It also serves to multiply the single USB port on the BeagleBone. Edit: Seperate battery packs would work with an LDO, but I wasn't sure about what to look for. Added weight might be an issue as well. The motors are already pretty weak. Also, we were hoping to put a charging circuit on the car at some point. Wouldn't seperate battery packs make that more complicated? (We're using NiMH to keep things simple. From what I found, they're fine to charge at very low amps without major circuitry.) \$\endgroup\$ – Chris Cartwright Oct 15 '12 at 6:16
  • \$\begingroup\$ If you are planning to charge batteries on board, then a single battery pack is the way to go. The USB hub will add payload weight to your car, and as you say the motors are already weak. A WiFi integrated camera modified if necessary to run off the 5V rail from your regulator would be preferable. \$\endgroup\$ – Anindo Ghosh Oct 15 '12 at 9:58
  • \$\begingroup\$ A WiFi integrated webcam wouldn't work because we wouldn't be able to interface with the BeagleBone remotely. Going with a single battery pack, what would be the recommended cap size? \$\endgroup\$ – Chris Cartwright Oct 15 '12 at 21:13
  • \$\begingroup\$ Again, please provide a diagram of what you are planning, for a better understanding and response. Recommended battery would be a LiFePO4 6.6v rechargable, the type used in RC cars and aircraft - it would give you a good combination of power density, recharge life, and safety, with a low enough voltage headroom to the target 5 volts, that LDO heat dissipation would be low. \$\endgroup\$ – Anindo Ghosh Oct 16 '12 at 3:11

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