Solution: N900 Charging from dumb power sources *UPDATED*
 

I have been wondering for 2 days since I received my N900 why charging seems hit and miss from non-PC based USB ports, for example car cigarette lighter USB adapters and USB Battery packs. Here's the answer and solution.

The Micro USB spec says that power draw should be under 100mA until the phone and connected power source device (ie laptop, computer, etc) can negotiate a higher current. This negotiation happens over the data pins (the center ones in the big USB plug). If the power source is a "dumb" device which is not capable of negotiation (for example the dedicated wall socket charger), the spec says that the data pins (D+, D-) should be shorted - indicating to the phone that it can draw as much power as it needs (up to 1A?)

My problem is that several USB Power sources that I have (a few car cigarette lighter to USB adaptors, and an APC Portable battery), have the center pins open. The phone therefore tries to negotiate with the dumb power source and obviously fails, and hence proceeds to ignore it.

There are a couple of solutions. For cigarette lighter to USB adaptors, you can probably just pull them apart and solder the 2 center pins of the USB socket together on the PCB. Another solution would be to cut your USB to micro USB cable in the middle, short the D+ and D- (usually white and blue wires from memory - check with a multimeter first!) and neatly join/heatshrink the cut. This would render the cable useless for anything except charging from the dumb power source.

In my case I neither wanted to sacrifice a USB cable, nor did I want to short the pins on the USB socket of my portable battery, as I wanted to retain its "passthrough mode", which transparently passes data through from a PC. Shorting the D+ and D- pins would destroy the passthrough mode.

Instead I salvaged a male to female USB part from an old USB hard disk enclosure (the cable that used to provide extra power to the USB hard disk). I cut the power cable off it, and heatshrinked the wires so they won't short. I then carefully soldered the center two conductors together in the USB socket.

Voila - a "Force Dedicated Charging" USB adaptor, for dumb USB power sources.

Attached are photos. (incidentally, taken with the n900 camera in macro mode - stunning. I had to downscale them for the forum, but the originals are here if anyone is interested)

I wouldn't recommend you plug this adaptor into a PC - although it probably would do no harm as the ports are meant to be current limited, but shorting the data pins together will obviously terminate any communication on the USB bus.

UPDATE:
A lot of people have pointed out that the FM transmitter output power is reduced when plugging any sort of charger in (rendering the FM transmitter close to useless). Nokia have implemented this power reduction due to the fact that the charge cable will act as a transmit antenna, which brings the possibility that the transmit power will be well over legal limits.
If you would like to over-ride this power limitation at your own risk, with the possibility that you'll be transmitting at radio power levels over the legal limit, then grab the modified fmtxd binary from here:
http://jacekowski.org/Maemo/FMTXD
or here (zipped, post 8):
http://talk.maemo.org/showthread.php?t=60567

Make sure FM transmitter is off, then copy the new fmtxd to your n900 (root of the mass storage), then run the following in terminal:
You should now be able to charge as well as simultaneously transmit (possibly illegally) on FM.

UPDATE 2:
Added photo of my current solution (last photo in below series). These weren't hard to find on ebay, (usb male to female adaptor) and the same solder bridge trick was applied. Much neater than original hack.

Re: Solution: N900 Charging from dumb power sources
 

Is it possible to buy a cable that has the pins shorted? How would they be idenitified?

Re: Solution: N900 Charging from dumb power sources
 

Definitely won't be possible to buy a cable with the pins shorted.

Let me be clear that this cable/adapter is USELESS for transferring data, or for charging from a PC USB port.

The sole purpose of this hack is to force the phone to think that it's plugged into a dedicated charger (which I call a dumb power source).

You only need to think about doing something like this if you have a USB socket which the phone will not charge from (and you want it to).

For everybody else, I recommend you buy dedicated wall chargers and dedicated car chargers - these will already have the D+ and D- pins internally shorted, and never need to transfer data.

Re: Solution: N900 Charging from dumb power sources
 

Cool. I just opened up my USB charger lighter adapter, and bridged the two inner data terminals as you suggest, and now my N900 will charge from it. Unfortunately, the FM transmitter becomes very weak when connected.

This aspect of the N900 is very lame indeed - it works fine when not charging, but why on earth would I want to flatten the device's battery when I'm listening to it through the car stereo?

Re: Solution: N900 Charging from dumb power sources
 

I haven't had a chance to experiment with the FM transmitter yet, but I would assume this is a hardware limitation, possibly to do with grounding or something that occurs which reduces the efficiency of the radio transmitter. It's early days: it'll be interesting to see what hacks people come up with to solve this problem.
UPDATE: told ya! see solution in opening post.

Re: Solution: N900 Charging from dumb power sources
 

When I tried using the FM transmitter while plugged into a laptop, the n900 flat out told me that the FM Transmitter couldn't be active while USB is connected.

Re: Solution: N900 Charging from dumb power sources
 

The FM transmitter's most obvious use case would be as an in-car music system, so it's indeed very strange that it won't work right when also connected to a car charger.

Re: Solution: N900 Charging from dumb power sources
 

Sorry for going off topic here.

I've never seen that happen, and I've experimented a fair bit. I can always hear the FM TX on my handheld FM receiver when it is activated. I get about 3 metres range with no USB cable attached and a good fully-quieting signal (no hiss), depending on device and antenna orientation.

When I plug the wall charger, or my newly functional car charger in to the N900, the signal level drops drastically, such that when the N900 and FM radio are right next to each other, the radio still hisses and pops, although I can hear the transmitted signal.

Re: Solution: N900 Charging from dumb power sources
 

Yes, these cables exist but they are usually built as 2-into-1, so you can pool the available amperage from two USB ports to power a single device.

Re: Solution: N900 Charging from dumb power sources
 

You just saved me 30 bucks!

I have one of those 12V Cigarette adapters that has 2 USB ports on it. I could never get to work with my N900.

So, I popped it open, heated up my handy dandy soldering iron, jumped the two center pins on each USB port on the circuit board, re-assembled it, and Vioala! The N900 charges!

Thanks again! :D

Re: Solution: N900 Charging from dumb power sources
 

No, it's bug 6792, fixed internally and slated for inclusion in some future firmware update.

Re: Solution: N900 Charging from dumb power sources
 

Are you sure this is the same thing? The FM transmitter does work, with the USB charger attached, it just works very, very badly. I get no error message at all.

Re: Solution: N900 Charging from dumb power sources
 

Pictures for the curious would be most appreciated! (If possible)

L

Re: Solution: N900 Charging from dumb power sources
 

Wow, you made my day with this post. I went ahead and soldered the two data leads of a flex USB adapter I have and it works like a champ. I'm not good with a soldering iron. It's an ugly joint but it works. I have confirmed that this now works with an iPhone USB wall adapter and the Belkin USB/3 AC outlet adapter. The flex adapter was easy to modify because it is held together by two screws and is not molded together. I got this awhile back at www.cyberguys.com but they don't seem to have this anymore.

Here are some pictures:
http://www.flickr.com/photos/brandersnaxe/4210350827/

http://www.flickr.com/photos/brandersnaxe/4211116724/

http://www.flickr.com/photos/brandersnaxe/4210352687/

http://www.flickr.com/photos/brandersnaxe/4211120280/

http://www.flickr.com/photos/brandersnaxe/4211122794/

Re: Solution: N900 Charging from dumb power sources
 

wow, I hope I don't have an issues with my power block. I just ordered this:Zagg Sparq I was planning to use it to charge multiple device and also hoped it would charge the N900 on the go. So I guess I'll just cross my fingers and hope for the best =(

Re: Solution: N900 Charging from dumb power sources
 

I have a Zagg Sparq and it does not charge my N900 with a normal USB cable. I have ordered the "official" nokia adapter (which changes old-school nokia charger output into mini-usb, presumably with the data pins shorted), as well as a couple of extra usb cables which I intend to hack along the lines illustrated in this post.

Re: Solution: N900 Charging from dumb power sources
 

can a software patch fix it instead of a hardware modification?

Re: Solution: N900 Charging from dumb power sources
 

I need to buy a Nokia adapter somewhere (besides eBay since I don't want to bother with eBay unless it's the last option). NokiaUSA doesn't seem to sell them.

Re: Solution: N900 Charging from dumb power sources
 

Short middle pins.

Re: Solution: N900 Charging from dumb power sources
 

For anybody who doesn't want to hack anything together, I have found another solution. Get a Motorola SKN6252 mini-usb to micro-usb adapter. Then use a regular A to mini-b USB cable from the dumb charger. I have verified this to work. I got the adapter from Amazon http://www.amazon.com/Micro-Adapter-.../dp/B000IBLD3E. I am using this 'shorty' cable from Cyberguys.com: http://www.cyberguys.com/product-det...roductid=10030.

Here is an action shot:
http://www.flickr.com/photos/brandersnaxe/4227462726/

Re: Solution: N900 Charging from dumb power sources
 

Are you sure that it isn't just luck of the draw? Meaning, maybe the amperage is within tolerance to work. I had one dumb usb charger work and another that never did.

Seems like grounding those pins might be the only full solution.

Re: Solution: N900 Charging from dumb power sources
 

I do not think this Motorola adapter is luck of the draw. Even the instruction diagram indicates its use for using a car charger with mini-usb for use with a micro-usb phone. Here is the usage sheet:
http://www.flickr.com/photos/brandersnaxe/4227505038/

Re: Solution: N900 Charging from dumb power sources
 

Wait so you have to get both? Since the Amazon one only goes from Micro to Mini USB.

Re: Solution: N900 Charging from dumb power sources
 

You need the motorola adapter and a decent mini-usb cable. I got the adapter for 99 cents on Amazon. I just tried it with an old PSP USB cable I have. Had to plug and unplug a few times, but it is charging now.

Re: Solution: N900 Charging from dumb power sources
 

Interesting... though, i'm wondering if the motorola phones it's replacing would have the same issue with other "dumb" micro usb chargers.

Does that make sense?

Re: Solution: N900 Charging from dumb power sources
 

what would happen if i just ripped out the two pins?

Re: Solution: N900 Charging from dumb power sources
 

Okay here is a gotcha with this adapter. Make sure the adapter is plugged into the N900 FIRST.

THEN plug in the DC mini-usb cable. I couldn't get charging to start otherwise..

Re: Solution: N900 Charging from dumb power sources
 

Last time i tried soldering something I burnt a hold clean through a graphics card. I think its probably best if I give this a miss.

Re: Solution: N900 Charging from dumb power sources
 

One more gotcha. Apply power only when the N900 is on the home screen, otherwise charging may not start. Don't ask me why. It seems to start every time when on the home screen.

Re: Solution: N900 Charging from dumb power sources
 

nothing compared to unmodified ports. the pins must be shorted, theyre insulated by default and ripping those out would still keep the pins insulated.

Re: Solution: N900 Charging from dumb power sources
 

I have SKN6252 and it does not make my dumb mini USB chargers work with n900. I have about 8+ of those and only 1 works with N900 using SKN6252.

Re: Solution: N900 Charging from dumb power sources
 

Thanks for this. This is exactly what I suspected. Shorting the pins is the only for-sure answer.

Re: Solution: N900 Charging from dumb power sources
 

Just shorted the center pins on my cheapie cig lighter usb adaptor and it now works like a charm. Charging using the supplied n900 usb cable. Many thanks for this.

Mike C

Re: Solution: N900 Charging from dumb power sources
 

is there a way to short the centre pins without a solder iron?

Re: Solution: N900 Charging from dumb power sources
 

Not safely. You have got +5v and ground on either side. Shorting these may not be good.

Re: Solution: N900 Charging from dumb power sources
 

I've found this cable that seems to be just what we need to charge our n900. I purchased it based on the "bad" reviews which in our case is exactly what we need. So charge only, no sync.

http://www.bestbuy.com/site/Rocketfi...1MRA&cp=1&lp=1

I have a zagsparqq that only worked occasionally with the provided cable, with this cable it worked right away. You can find it on ebay for much cheaper. Hope this helps.

Re: Solution: N900 Charging from dumb power sources
 

I have a ac adapter with a usb port on it. I was just wondering if it is safe to use with the supplied usb cable as a second charger.

It says output dc 5v max: 1000ma min: 150 ma

Would this be safe to use with my n900?

Hmm I plugged it in, it workd, but do you think this could damage my n900 in any way?

Re: Solution: N900 Charging from dumb power sources
 

It depends on the quality of the charger. It's difficult to tell without checking it out with instruments. But if you can get your hands on a multimeter and measure its voltage it could give an indication. If it provides between 5V and 5.2V or so, and not something wildly different, then it may be OK. No guarantees though. Many China-made chargers are total rubbish, while others are completely fine. If it was provided as part of a kit with some decent, well-known product, then it's probably OK as well.

Re: Solution: N900 Charging from dumb power sources
 

Nah actually it was a cheap chinese kit I ordered off ebay for my sansa mp3 player. I just may skip using it with my n900, not taking any chances on this one lol. Will try to purchase another usb cable instead.

Re: Solution: N900 Charging from dumb power sources
 

It does NOT charge the N900 -- on 20-40 min hold with Zagg to see how they intend to resolve.