N900 charging from ‘dumb’ charger

Updated 27/5/2011 – updated to reflect comment that really using 200ohm resistor is at the upper limits of spec and more likely to cause a problem than no resistor at all.

This weekend I took Kat up to London for a couple of days. Knowing that my n900 is a bit of a battery vampire I took with me a generic USB charger an my nokia usb cable.

At the end of the first day we returned to the hotel and I plugged in my n900 as it was on it’s very last gasps of power. Only to discover that it did not register any charge. I previously noted an issue with this charger when I’d completely run out of charge and found that whilst it did register charge nothing became responsive until I plugged it into a computer.

Very odd.

The next morning I went out early to grab a new usb charger from the maplin that was down the road from our hotel. I bought one with 2 USB sockets on the basis that it would be helpful to charge multiple devices. But when I tried it out I found that I still got no charge. At this point it would have been good to be able to do some research on line. However… no charge!

So during the day, after going to the natural history museum, we took a trip to the Nokia store on Regent Street, and I bought a new official Nokia charger. I did ask if they knew any reason why my generic usb chargers weren’t working, but the sales guy knew nothing.

After some research I found that the issue of charging is down to the spec for charging from micro usb. It turns out that the N900 is adhering to the spec, that says it can determine a ‘dumb’ charger if the the d+ d- data lines are connected via a 200 Ohm resistor.

I found this on the talk.maemo.org forum written by shadowjkYou can find the Battery Charging spec at usb.org, in a zip. batt_charging_1_1_FINAL.pdf

Page 7 shows the connection, “A Dedicated Charging Port is required to short the D+ and D- lines with a resistance of RDCHG_DAT”, page 29 specifies max 200 ohms across D+ and D-. Although the wording on page 7 suggests it should be exactly 200 ohms… Also on Page 29, a minimum of 2 MEGA ohms of resistance between D+ and ground or vbus. That is, not connected together at all.

Several people on the forums found that simply soldering d+ and d- together was sufficient, but I decided I’d go with the resistor option, since I had a 200 ohm resistor laying around anyway. – However as noted below this is actually the max allowed resistance and could potentially cause problems. People that know more about electronics than I suggest that simply shorting the wires is the intended specification and the best thing to do. Since it’s also the easiest thing to do, I wouldn’t bother with any resistor if I were doing this again.

I had a cheap 4 port usb hub that I wasn’t using, and I figured I could rip a socket and the plug out of that. So I broke out the soldering iron and desoldering pump , desoldered one of the sockets, and the plug. Then used a bit of strip board and soldered the pieces back on, but with a 200 ohm resistor joining the two middle strips. And voila! easy as that I could plug this into any generic usb charger, and plug my normal cable and get charge.

usb cable with d+ d- joined with resistor

Here it’s on a strip board which I later cut down to size and fit the assembly back inside the original case


I could probably have made the whole thing smaller, but it’s not too bad. And now I can use any generic usb charger.

This weekend really made me appreciate how much I want battery tech to improve to the point it will last weeks of heavy usage no just hours.


14 responses to “N900 charging from ‘dumb’ charger”

  1. What I don’t understand is why does dumb usb chargers don’t come prepared for this if it’s clearly part of the standard specification.

    • it is annoying. I would guess that most were made before the spec was final. hopefully over time new chargers will be fine. i can only hope they will make it clear on packaging that they prperly support this standard.

  2. Another issue is not all phones follow the spec. Did you know there is a YD/T-1591 spec from China that talks about how a dumb charger is supposed to look.
    And then there is APPLE phones and MP3 players that do not follow any published spec at all. If you short D+ and D- together with 200ohm resistor, and connect an Apple product to it, the phone/mp3 player will not draw any current.

    The apple product needs ~2.7V on D- and ~2.0V on D+.

    RIM Blackberry needs to see a high voltage >2.0V on both D+ and D-

    It seems that the perfect charger is NOT a dumb charger, but a SMART charger. One that can detect the “phone-type” and then either short D+ and D-, or output required voltage levels.

  3. just FYI, if someone decides to buy a cheapo motorola fast CAR charger P518 and alikes:
    1. charged very unreliably, sometimes when you plug it in, does not charge, sometimes (more rarely) charges
    2. open the charger’s microUSB plug carefully
    3. cut the resistor (which is 200kOhm between pins ID and GND, see USB spec on wikipedia)
    4. close microUSB plug

    voila! charges N900 100% every time now!
    (note the middle pins are already shorted on this charger)

  4. Please not the specs say “MAXIMUM 200 Ohms” which means you SHOULD just short D+/-. There’s definitely nothing in the spec mentioning any *minimum* resistence! So using a 200R to do this shorting is a really bad idea.

    • As stated in the post, linked from the original source I found, the spec is a little unclear, whilst the definition for the value RDCHG_DAT is MAX 200Ohm, there is no min specified, and page7 states “A Dedicated charging port is requried to short the D+ and D- lines with a resistance of RDCHG_DAT.I used a 200OHm resistor and it worked fine, I know others have merely shorted the connection and it’s also worked fine. I’m not sure I’d classify using the max value as a really bad idea, at worst it is needlessly high. Personally I felt that less harm was likely to come from using a resistor than from not using one.

  5. the value RDCHG_DAT is MAX 200Ohm“A Dedicated charging port is requried to short the D+ and D- lines with a resistance of RDCHG_DAT.”

    What’s unclear with this sequence of conditions?
    I clearly says you have to make sure your short is <=200Ohm, otherwise it might not work. For an EE that's a clear indication the *best* value is ZERO Ohm.

    When your fuse in your home power distribution says "16A" – then do you refuse to connect devices that use <16A?

    Also you forgot the spec is meant from receptacle contact to receptacle contact. The 200Ohm value is meant to make sure your contacts and wires have less than 200R, but definitely when using a resistor of 200R/+-0% then your *effective* value is somewhat higher than 200, due to those parasitary resistances. And that's what it's all about.

    There are no harm what so ever in simply shorting D+- as it's meant to be by specs. There *might* be real harm from using 200R, as some devices might operate outside specs when connected to 200+x where x are your parasitary resistive components in total impedance.

    I think I know my way around designing HW and esp USB charging, and I discourage people to use any resistor for no good reason, to short D+ and D-

    BTW shadowjk learned about that stuff from me – at least a huge part of it.

    Openmoko HW & synergy

  6. Hi, whit short between D- and D+ which is the current for charge the battery? 500mA the max of USB or 1000mA as normal AC Wall Adapter?
    Thank you

    • D+- short indicates “fastcharger” which has no clearly specified upper limit, so N900 will use as much power as it can handle, which is ~850mA iirc. It’s generally safe to require fastchargers to supply 1000mA at least – and definitely all charger PSUs have to be shortcircuit/overload resistant, so nothing to worry really about all that.

  7. Know nothing at all about electronics but I have shortened the two pins in the car charger and it does actually seem to work. I simply used a drop of solder to cover both pins, no resistor in between (which I wouldn’t know how to apply anyway).
    Thank you for this insightful discussion.

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