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. An external charger can be easily modified to run off a bicycle dynamo with a low dropout full-bridge dynamo 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.

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. There are multiple factors at play here but the main thing to remember is that the phone does actually negotiate with the charger to see if it can supply enough current (it seems to do this more than once actually). The first time this is checked by the phone is to see if the power supply cannot at least a few hundred mA in the first few seconds. If the phone senses the charger cannot supply this 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" during the duration it is plugged in.
After that it 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.

Here is a helpful page
https://en.wikipedia.org/wiki/Univer...rging_adapters