Hello everyone.

I've been charging my N900 from solar for the past year or two using various methods. I am currently doing final testing of my proto USB solar charger Maximum Power Point direct regulator which charges to a maximum of 800mA from a 5w solar panel (ie %80 efficient into the battery). Before I tried this I direct charged my batteries in an adapted external charger. A home-built external charger can be easily modified to run off a bicycle dynamo (as opposed to off a solar charger) with the addition of a low-dropout full-bridge rectifier and some decent capacitance.

When designing any solar or alternative power solution there are two things to consider, the load and source. Work out exactly what your requirements are--the load--and design an appropriate charging solution to meet those needs in the required time-frame--the source.

Every single solar charger bar 1 or 2 on the market right now are glorified external battery chargers with a little bit of solar attached designed to look good. The best way to do solar properly you have to do it yourself (I realise this thread is about dynamo charging but many of the same principles apply).

First thing with a Nokia N900 (N9 too?) charging solution is this. You absolutely need to short the data lines out. Most off-the-shelf chargers do not have this. You need to open the charger up and either solder the data pins (bear in mind this will render the charger useless for i-poh) or make a special charging cable.

There are also several other factors which determine whether the Nokia N900 will charge properly. The main thing to remember is that the phone does check the charger to see if it can supply enough current--it seems to do this more than once actually. It's been mentioned that the phone "negotiates" as per the USB spec, which in effect it does do, but there is no bus or data communication between the phone and the charger to enable this process. The first time the charger is checked by the phone is to see if the power supply can provide at least a few hundred mA in the first second or three (yes that quick). If the phone senses the charger cannot supply this amount of current it clamps the charging current at 100mA (95mA exactly). Ie if the phone senses that your charger is not capable, your phone will be little more than "trickle-charged" the duration it is plugged in.
After that the phone seems to do another check a few seconds later at 500mA, if alls well it can accept a sliding range of current above 500mA which seems to be dependent on a couple of things, my guess is the main one is current the SoC (State of Charge) of the battery at any given point.

If you think this is battery charging hell, you should see what the i-people have to do to get their devices solar charged.