As someone might know, the N950's battery can be upgraded to BV-4D 1400mAh. Let me just quickly introduce it.

From the specification sheet, the N950 uses BV-4D 1320 mAh battery. This is also the same as the early prototype of N9-00. (Click to see the picture.)

However, as the orginal N9-00 Deli was canceled as a consumer device, it became N950 as what we see now. Nokia downgraded the battery when ship the device to BP-4L battery, at 1200 mAh.
This can be confirmed in this page.

There exist some early BV-4D 1320mAh battery on ebay, listing as battery for N9-00. (Although don't know if it is true or not.) This raise some confusion between N9-00 (N950) and N9-01 (N9). N9 uses BV-5JW 1450mAh but N950, indeed, uses BV-4D orginally.
Even Steve Litchfield in AAS was confused about this - see the comment of this link!

Finally, the improved BV-4D 1400mAh is used in 808 Pureview. And I did find a trusted source for the genuine BV-4D battery, without buying a 808.

I'm excited and currently waiting for the delivery. Ideally, the battery in current N950 will be improved from 1200mAh to 1400mAh, which is a big improvement.

I will give the result once I tested the new battery.

In my N950 was a 1200mAh BL-4D battery installed, i changed to a 1320mAh BV-4D battery.
I test it.
I have no idea where I get a battery with 1400mAh...

After searching a bit in ebay I found a GOLD 2680MAH BL-4D! which I am sure it is not original from nokia (will make your n950 explode )

@MAX9:
Is there a big difference in battery life between the 1320mAh BV-4D and the original 1200mAh battery?

Off-topic: the n950 doesn't come with a wall charger, is it safe to charge it with the n900 wall charger?
Charging via usb will take much longer compared to using a wall charger.

You can use the N900 wall charger. I have done so many times without any problem. I have also used other USB wall chargers. I believe that any device that implements the USB charging standard will only draw the current that it needs to charge, so you should generally be able to use any USB wall charger.

I am waiting for a 1400mAh BV-4D...
A explode N950 is very uncool

Installed the battery only 2 hour ago...

I used a AC-16E Charger from the N9 with the CA-179 USB cable from N950

I only found trust-able BV-4D 1400mAh battery from Taobao (the Chinese eBay).

http://item.taobao.com/item.htm?spm=..._u=6v7snk35cfe

Using Google translate if you don't undertand Chinese.

I know there are a lot things are fake in China (maybe inc. the explosive super battery above ), but according to the picture, comment and discription, the battery is genuine.
The Chinese on the back saying that this battery is only for China, Hong Kong and Taiwan market for 808 pureview, hence I'm not expect it appear on ebay UK or US...

There is a problem posting batteries, as DHL and UPS are very strict with battery. I'm trying with ems.

Once the posting method is confirmed I will purchase and share the details.

Do you think it could do any damage using a 3.7V battery in a phone that takes 3.8V battery?

no.
I measure the batteries:
N950:

• BL-4D 1200mAh 3.7V = 4.01V
• BV-4D 1320mAh 3.8V = 3.85V
• BV-4D 1400mAh 3.8V = coming soon, in delivery

N900:

• BL-5J 1320mAh 3.7V = 3.87V

N9:

• BV-5JW 1450mAh 3.8V = 3.83V

SCUD made a bigger (but same physical size) battery for the N900.

It could be found on Alibaba.com

The ordered 1400mAh battery...