Powering a Cell Phone from the Battery Port

Discussion in 'The Projects Forum' started by trader007, Mar 11, 2011.

  1. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
    19
    hello all. been awhile.

    The charger on my phone is broken, so i decided to use this phone as a tethering device for my carcomputer.

    its a palm treo pro, and i would rather power it from the battery bay anyway since that would shut the phone off when i disconnect the power.

    i cant get the phone to turn on by just supplying 3.7v to the battery terminals. there are 4 ternminas, and the two outside ones are definitely the main voltage, but the two terminals inside do something that tells the phone its OK to use the battery.

    anyone know how these work? to get 3.7v i just used 4 diodes off a 5v usb cord and 3.6ohms of resistance.

    thanks!
     
  2. thakid87

    Active Member

    May 23, 2009
    122
    0
    Have you tried reversing the polarity? It is very possible that it is polarity sensitive.

    Try jumping the two middle terminals.
     
  3. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
    98
    It probably wants a regulated supply, how did you determine four diodes would drop 5V to 3.7? Are they Schottky diodes?
     
  4. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
    19
    shorting the two middle terminals only works on the usb pinouts. thats how the phone knows to draw full power from the usb rails. otherwise it needs communication from the windows computer on those pins so the computers usb port wont get too many amps pulled through it.

    but as far as the battery goes, the middle pins are for temperature and battery status as far as i can tell. one pin should have resistance from the neg- pin on the battery, telling the phone that the battery is not overheated. i tried matching up those ohms, but it still didnt work.

    and its actually 1 diode that drops the voltage to 3.7v from 5v start point. i dont know why i was getting the readings i was before, i dont think my cheap voltmeter is very reliable.

    so anyway, i just put the power wires to the phones two outside terminals, and i stuck the battery in. it works. the phone is charging however it worries me that the battery might be hurt by this over time.

    maybe it wont matter though. its 3.7v so it wont overvolt, and i only drive for a couple hours at most so the battery shouldnt be hurt by the available current either.

    still, i would like to get the phone running without a battery at all. but if nothing else, ill just put a relay inline from the battery to the phone, that runs off my computer's 12v line. that way when my rigged 'charger' shuts off, the battery also gets disconnected and the phone shuts off. this would stop unnecessary draining of the battery everytime i park the car.
     
  5. trader007

    Thread Starter Active Member

    Feb 27, 2010
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    edit- ive decided on a simple yet slightly crude way of doing this. basically, leave the battery in the phone, and hook my own "dumb charger" to it.

    i have a 3.7v button protection pcb, and it cuts off voltage at 4.2v. it doesnt limit current though (2a circuit trip), so ill add 2x 10ohm resistors rated at 10w. one resistor will be hooked to a switch, so i can change this "charger" from 500ma to 1000ma easily. if i find i dont need 1000ma, ill let it run at 500 and go easy on the battery. my power comes from a homemade USB hub, that has a really nice meanwell regulated 5v supply running it. so my charging power will be clean.

    basically, charging the battery this way will lead to a shorter battery life, however cell phones charge these batteries to over 4v anyway so i might not even notice the difference. also, ill try to find a low fvoltage diode to get around a 4.5v output, that should help strain on the battery and still be enough to run everything, but if i have to i can run it on straight 5v as thats what the phone normally charges at anyway.
     
    Last edited: Mar 13, 2011
  6. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
    19
    success! at least so far.

    its ghetto, but actually built pretty well. the reason i brought all four battery contacts outside the phone was for future configuring. right now, the battery doesnt disconnect when power is lost, so the phone will stay on until it does when i park the car. this is actually a good thing i think, because it will help keep the battery from being overcharged.

    [​IMG]
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    [​IMG]

    so its just two 10ohm resistors (10w ratings), a protection PCB and a molex connector for easy removal. ideally i would have left the battery inside the phone, and just run two small wires to the voltage pins... that way i could disconnect the phone and still use it. but then again if i ever figure out how to eliminate the battery alltogether then im all set up for that.

    ill post back with results on how well it actually stays charged while under heavy use. that seems to be the hard part!
     
  7. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
    98
    It could also lead to the battery catching fire.

    Can't fathom why you simply didn't want to cough up the few dollars it would have cost to replace it with a proper charger.
     
  8. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
    19
    the cost would be a whole new phone. the charger port on the phone is broken, and i cant fix it.

    and i could use a external li-ion cell charger, but they only run around 200ma and i need to power the phone PLUS charge the battery. thats around 1000ma.

    this is working though. the battery pack has a cut-off circuit in it, and i also added my own pre-made protection pcb. i think the chance of fire is low.

    one thing ive found is the resistors simply arent needed. not when you plug into a wallwart or a usb hub anyway, because those are already current limited. however, i would rather limit the current on the device itself for safety reasons, so i think i will add a switch to the resistor-bypass wire so i can turn on the current limiting when i hook this up straight to my pc's power supply.
     
  9. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
    19
    just wanted to update on this. its been working pretty well!

    if i leave it on too long, the circuit just shuts off charging. the battery never gets above room temp, so it seems safe.

    and it will not start charging again until i unplug the phone and plug it back in, so i dont see this ever running away on me or doing anything weird.

    so if anyone else needs to get a phone working without a charge port, this does work!
     
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