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USB battery cable / bracket thingy

don mcintyredon mcintyre
edited November 2012 in Hardware developers Vote Up-1Vote Down
John - Is the circuitry voltage protected? I'm assuming yes, since I believe the standard usb voltage is larger then 3 volts.

If so, how about a cable or clip on device that would connect a standard 9volt battery direct to the mini USB slot?. Cable would work, but a holder that a 9volt battery is inserted into, with a mini male plug would work also.

Regards - Don

Best Answer

  • 9 Comments sorted by Votes Date Added
  • Paul FreemanPaul Freeman
    Answer ✓ Vote Up0Vote Down
    That's a much better solution Bill, I'd suggest 3x batteries in series into the USB port would be fine, that's closer to the voltage it operates from. However from what I can tell it uses a linear circuit inside to reduce the voltage, so there's waste there (linear circuits turn the extra voltage into heat). Attaching 2x D cells to the inside battery connectors would be a simple task and would be the most efficient use of available power.

    Don's comment about a stepdown circuit is correct too. There's pre-built, cheap, efficient switch-mode modules that have pre-set input/output voltages that are suitable. But 9v batteries are the least space efficient of all the usual ones, and also the least price efficient. The rectangle shape is convenient though. When you figure it out, a stepdown circuit would be say 90% efficient, then the TWINE internal regulator about 60%. So you waste about 50% of the available power.

    I've just ordered 2x AAA size Li-Ion batteries, which will sit in the usual battery places. Some basic rewiring of the connectors will run them in parallel, doubling the available current to 1200mA. A tiny surface mount charger IC with only 2 external capacitors and a resistor will charge the batteries any time the USB plug is inserted. The circuit will just sit inside the case, but I'm concerned about possible heat issues while charging. I'm expecting about 2-3 hours to fully charge it. Running time should be similar to standard Alkaline if not longer, however the low-battery report will be very very close to the end of its life. Li-Ion's run at 3.7v, and when charging go up to 4.2v. Firmware updates may allow reporting of the battery voltage, which can provide a better low-battery alert setup. Most of critical components are rated up to 3.6v, so I'm possibly risking the components doing this.

    We'll see how it all goes in a while, Christmas will slow down the experimenting. I'd be interested to hear if anyone else has tried similar.

Answers

  • Paul FreemanPaul Freeman
    Vote Up0Vote Down
    Don, I wouldn't suggest putting 3x the rated voltage into any electronic device, this is way over the spec. I doubt it would handle it. Besides, you get more efficient power from 2xAAA's than you would from a 9v battery, and AAA's are cheaper.

    I'm looking into a rechargeable internal battery myself, I think that's the better way to go.
  • don mcintyredon mcintyre
    Vote Up0Vote Down
    Paul -

    Good comment. Probably the staff could answer. I was looking at the simplicity of the 9v connector setup and making a case for it, maybe with some sort of step down circuitry built in. For a long term (one month+) in place alarm.

    I've noticed that battery life is really effected when experimenting, moving it around, etc., vs just letting it sit with one trigger.

    Regards - Don
  • Bill SpencerBill Spencer
    Vote Up0Vote Down
    Why not use the USB cable to connect a pair of D-cells? They'll last longer and Radio Shack has battery cases/holders for them, as well as C & AA. Even though the normal USB voltage is about 5, I'll bet feeding 3v to it will work fine with the Twine. (Rhyme not intended)
  • Paul FreemanPaul Freeman
    Vote Up0Vote Down
    Bill, I've tested my TWINE at 3v (equivelant to 2 cells) at the USB port, and unfortunately it was unreliable. I'm assuming this is due to it going through the regulator circuit, which has generic losses. If you want to attach external batteries through the USB port, 3 cells work fine, which produces 4.5v. However you will not get low battery reports when do this, as it does not report the battery voltage when connected to external power.

    The best solution is to run another connector of some sort, and have 2 external cells connected to the battery terminals. You get all the benefits of the extra power reserves of AA, C or D cells, plus battery voltage reporting, and no issues with going over or under voltage. It's also by far the most efficient method. AA's are highest power per $ value, and they're approx 3x the capacity of AAA's

    However I'm still going down the rechargeable path. Initial tests have shown it should work perfectly. With the exception of the low battery warning point.

    Anyone wishing to follow my experiments on a rechargeable battery for TWINE, can follow it here...
    http://community.supermechanical.com/index.php?p=/discussion/comment/780#Comment_780
  • Chris WillsonChris Willson
    Vote Up0Vote Down
    But there is nothing wrong with plugging my Twine into a garden variety USB cell phone charger, is that correct??
  • ChristinaChristina
    Vote Up0Vote Down
    We use garden variety USB cell phone chargers, works great!
  • Joel FinkleJoel Finkle
    Vote Up0Vote Down
    I'd be skeptical about the value of using a 9V battery: there's no more mass to that than the two AAAs, so even if the step-down circuitry were 100% efficient, you shouldn't expect more battery life.  I like the D-cell idea better.

    Currently, I'm using a cheap micro-USB adapter, and considering the batteries to be essentially battery backup for when the house power is off (my WiFi router and cable modem are on a UPS in a nearby room).
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