Charging: USB and AC differences

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  1. robnoble

    robnoble Member

    If I use my HTC Desire for Google Navigation in the car, with the phone plugged into a USB power adaptor for the cigarette lighter socket with the cable that was supplied with the phone, the battery still runs down.

    A dialogue pops up when the battery is getting low saying words to the effect that the phone is using more power than is coming in via the charging cable.

    I notice that the phone distinguishes between being charged on AC and being charged by USB. In Settings->About Phone->Battery it says, "Charging (USB)" or "Charging (AC)". I believe that charging from a PC USB socket is limited to <500ma by the USB spec. I guess the phone knows that it can draw more current when connected to the AC charger.

    The supplied AC charger must have some way to tell the phone that it is connected. Perhaps it communicates with the phone via the USB data lines, or something simpler like the data lines being shorted together?

    The USB power adaptor I have in my car can source 1A. If I can work out how to convince the phone to switch to "Charging (AC)" mode then perhaps it can do Google Navigation and charge the battery at the same time...?

    Does anyone know for sure how the AC adaptor identifies itself to the phone?


  2. jabtas

    jabtas Well-Known Member

    That's the $64000 question right there
  3. robnoble

    robnoble Member

  4. robnoble

    robnoble Member

    Gradually answering my own question here...

    The Wikipedia page for MicroUSB refers to a Battery Charging Specification:

    Universal Serial Bus - Wikipedia, the free encyclopedia

    (last para after the "Power" heading)

    and says that a Dedicated Charging Port shorts the D+ and D- pins with at most a 200 ohm resistance.
  5. OFI

    OFI Well-Known Member

    Going by pinouts on other USB devices this seems likely.

    Handbook of hardware pinouts, cables schemes and connectors layouts @ have a look through some of the USB pinouts on there i'm sure I read one on there a while back about charging cables.
  6. robnoble

    robnoble Member

    Throwing caution to the wind, I have just prized open my car cigarette lighter USB power adaptor (currently on special offer at Maplin - Micro DC to USB Adaptor : Caravan & Camping : Maplin), and shorted the two middle pins that were unconnected with a blob of solder.

    Now when I connect my phone to it, it says "Charging (AC)".

    I'll need to test it on a long trip to see if a) the battery is kept topped up whilst using Google Navigation or other power-hungry apps, and/or b) the car USB power adaptor fails/overheats/catches fire.

    I'm hopeful it will be ok. I was impressed to find what looks like a switched-mode PSU inside the adaptor, rather than a linear regulator. Hopefully this will current-limit if the phone tries to draw more than the 1A is is designed for.

    mikeh and mrlongbeard like this.
  7. nx1977

    nx1977 Well-Known Member

    Will be interested to see how this works.

    One interesting point I've noticed. Been using the Wife's Samsung Spica's charger which is rated at 700mA. Now from 5%, hers (1500mA battery) charged in around 90 minutes.

    My Desire was put on at 18% earlier, and is still at 89% after 2.5 hours!

    So what gives?
  8. sandozer

    sandozer Well-Known Member

    Be careful there Rob, if the regulator cant handle the extra current you may finish up with 12v going straight to the phone, it may work now but be seriously overloaded. As you have shorted the data lines the phone will draw around 800ma (I have measured this ) Be safe and get a yourself a 1amp capacity USB adapter.
  9. mrlongbeard

    mrlongbeard Well-Known Member

    The one he's linked to is rated at 1A
  10. sandozer

    sandozer Well-Known Member

    If its a true 1 Amp unit the Data out and Data In should already have been linked.
  11. SurfXombie

    SurfXombie Well-Known Member

    I have the Brodit car mount with integrated lead that plugs in the cigarette socket.
    I was surprised at how fast it charged even while navigating.
  12. robnoble

    robnoble Member

    It is supposed to be 1amp capable (see Micro DC to USB Adaptor : Caravan & Camping : Maplin) but since I don't want to toast my shiny new Desire it's worth checking, thanks.

    I'll first need to work out a way of getting to the connections so I can insert an ammeter and attach a voltmeter.

  13. sandozer

    sandozer Well-Known Member

    I see it is supposedly 1Amp capable, wonder why the data connections are not tied?
  14. robnoble

    robnoble Member

    Ok, armed with an assortment of crocodile clips, two multimeters, one butchered USB extension cable, a selection of resistors and a Scalextrix speed controller (no, really), I've measured the voltage output of the Maplin car USB power adapter at various currents. I've also recreated the smell of Scalextrix racing and reminisced about my childhood days.

    Basically it's starts at around 5.25 volts (no load) and decreases to around 3.6 volts at around 2 amps (double the rated capacity!). Somewhere around 2.5 amps it starts pulsing the power on and off.

    Possibly if I'd left it on for longer then it would have started cutting off the power at a lower current level, or gone super-critical and triggered a thermonuclear event.

    Anyway, the important thing is that the voltage didn't go higher.

    Furthermore, I can now tell you that with D+/D- shorted, my phone draws 750ma from the charger input (whether connected to the car adaptor or the original AC adapter), and with D+/D- disconnected it draws ~450ma. This is when using the lead supplied with the phone, but also connected through the doctored USB extension cable, croc clips, etc.

    However, if instead I use another cable which has two pins shorted in the MicroUSB plug itself (I assume the D+/D- pins, but didn't check), then although the phone says, "Charging (AC)", it only draws 500ma.

    So the bottom line is that the choice of USB cable does seem to affect the charging current, as does the state of the D+/D- pins in the charger and/or cable.

    chknhwk, ky90025 and dj_si like this.
  15. RobDickson

    RobDickson Well-Known Member

    Thanks for your time. Somewhere I've got a USB-USB adaptor that might have the pins shorted on. I'll check and try it next time I have a car.
  16. sandozer

    sandozer Well-Known Member

    Rob, those figures stack up with my results too. I used a Lab powersupply rated at 3Amps set at 5V output.

    I found the two charging rates on the phone are 400ma USB only and a shade under 800ma with the data lines tied together.
  17. wisewood

    wisewood Active Member

  18. nx1977

    nx1977 Well-Known Member

  19. pms

    pms New Member

    What is different in this charger than any other 1A cigarette charger? I have one 1A cigarette charger and still when running the GPS the battery is discharging instead of charging.... Is this charger somehow different from any other 1A charger?
  20. marshalex

    marshalex Well-Known Member

    that belkin one linked doesn't charge the handset at the full 1amp recommended, found that out at my peril today when driving back from manchester using the sat nav. also checked using the battery status in the phone and in the car it only shows charging usb when connected. must mean that these don't output whats required

    btw whats the easiest way of tying the 2 data lines together? tried with a piece of foil on a spare micro usb cable but this didn't seem to work
  21. jabtas

    jabtas Well-Known Member

    I think the bottle neck is being caused not by the type of charger, but by the supllied USB lead from HTC themselves

    I originally posted this on the Google Navigation thread, but it also fits in here

    Just carried a little experiment, with interesting results

    Last week I bought a car holder and charger from eBay (item no 170467443547)
    The holder is a little plasticy but fully adjustable, but what do you want for less than 7 quid delivered

    The interesting thing is, that the supplied charger say output 600 - 750ma

    I just did a 35min journey, Google Navigation + GPS + Auto screen brightness
    According to the battery graph produced by Battery Snap, the charge state rose from 64% to 69% in that 35mins

    It would appear that the Desire is able to pull more charge than 400ma, as long as the charger is able to supply it
    Maybe the bottle neck is caused by the supplied USB lead from HTC
  22. brunop

    brunop Member

    Hi Rob

    I've got the same USB car charger from Maplin's, is it the two centre points I've highlighted in red that you need to solder together? (Image attached)

    And can you confirm that when using this and the HTC supllied lead it charges the battery whilst using navigation?



    Attached Files:

  23. marshalex

    marshalex Well-Known Member

    so it would appear that i'll have to prise open my new belkin adaptor to make this change then. might drop their cs an email and see what comes back from that
  24. marshalex

    marshalex Well-Known Member

    just spoke to belkin about this and its been escalated with regards to that adaptor posted. hopefully this will get sorted in the next few days
  25. nx1977

    nx1977 Well-Known Member

    Belkin are very good usually.

    Did you buy from a legit store, or ebay?

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