Hi! Does anyone know if we can use a charger with lower output to charge the Nokia N900?

Charger option spec:
OUTPUT: 5V - 850mA

Nokia N900 charger:
OUTPUT: 5V - 1200mA

Any idea if using this lower output charger long term will cause any battery degradation? or any other problems?

there probably will be no problems. the problems start occuring when current drops to about 500mA. and then the problem is that device can use more power than it gets from charger so battery keeps draining.

so only thing that will happend probably is that charging only takes a bit longer.

All Lithium-Ion battery based devices have a special charging chip in them. This chip will charge as best it is capable, and will prevent an inadequate charger from doing any damage.. Up to the limits of that chip. Beyond that limit(say you tried putting 12V into it), the chip would probably fry, but the battery would be unharmed.

In this case, the N900 will be fine. More than likely, your charger there is rated for 850mA continuous at some high temperature, and will put out more when its cold out. On top of that, while the charging chip limits the -maximum- current it can draw to(iirc) 1000mA(or 1200, I forget), it may draw less than that, depending on whats needed to charge the battery properly. So, more than likely, it will work fine.

If the charger is cheap, or improperly spec'd, one of two things will happen:
1. When charging from near-dead, after a while your charger will overheat and shutdown for a while.
2. If the charger's a cheap knockoff, theres a chance that, in the above scenario, it will simply overheat and fry something in the charger, resulting in a broken charger. But, there is a very small chance of this happening.

Either way, nothing should happen to your N900, so go ahead and get it.

Question is if the Charger works. See the D+/D- problem and stuff like that. But with this adapterthingie it should work.

It seems like charging chip limits total consumption to 3.5W.

Phone consumes ~420mW while cpu is idling, screen is on and wifi off, as battery charger chip reports. Charging in the same settings with battery at 25% gives (-3010mW), so total is about 3.42W. At 5V corresponding current will be 700mA if DC-DC coverters are used. Then there are losses in conversion (10-20% I guess), so current draw from charger should be about 800-900 mA.

If charger is high quality, it should be able to handle some overdraw. If it's not, then it's output voltage may be with ripples and that may confuse charger chip and led to improper charging cycle. That's pure speculation on my part since i haven't read any hardware's datasheets.

USB charger spec requires powersupplies to respond to overload by reducing voltage until power draw is at acceptable level.. So if the charger is properly designed it'll just balance itself nicely.

Thanks!!!

These are all very good feedback. The alternative charger I have is actually an original Motorola bluetooth headset charger. I don't have any other devices with micro USB except this (and the N900) and was wondering if I could keep 1 charger in the office and use the N900 charger at home to avoid travelling with it so often.

I'll give it a try (knowing that theoretically, it won't fry my N900 and/or battery) and let you guys know the result. Thanks again.

Looks like the lower output (850mA) charger could not charge the phone

Guess I'll just have to use the USB instead.

The output from the charger should be fine, what may be an issue is that the n900 expects the data pins on a dumb charger to be shorted (connected together), its normally fairly easy to fix this if you have a soldering iron and solder handy. I have a couple of car cigarette lighter socket to USB port chargers which both work after I performed the operation to fix this issue, they are rated at 500mA and 800mA.