I bought a BL-5J battery, so now I have 2 (and I have marked them so I know which is which ).
I have difficulties charging/using this battery.
What happens:

• connect to charger, it sometimes starts charging for 1-5 minutes, then goes green
• it then sometimes boots succesfully, the battery meter shows full charge
• it then dies unceremoniously after a few minutes - always before I managed to do the calibration dance.

Note "sometimes" - other times, it goes into some sort of boot loop or the yellow light doesn't come on at all.
I tried with 2 different chargers.
I tried a full power cycle, the behaviour wasn't significantly different after that.

If I put in my first battery the device boots normally.
The status bar battery meter shows 0%, but bnf shows 99% (I guess this could be fixed with another calibration - anyhow, that's not the topic of this thread).

I know what the most likely answer is here, but just wanted to make sure if there's something I haven't tried yet?

After extensive testing (taking into account that the N900 is confused by the unknown battery) I decided that this battery is f***ed. Sometimes holds a charge, sometimes doesn't, and worst of all: it has some sort of loose contact so whenever I twist the phone slightly, it switches off. 100% reproducible, does not happen with my other battery.

This battery was sold to me as "brand new and unused" (and without a picture), ergo the seller is a liar (see image - clearly taken from a trash phone). I did get my money back though.

"BL-5J"

That's where you ****ed up m8, should have gotten a PolarCell.

https://wiki.maemo.org/N900_Accessories#Batteries
https://www.polarcell.de/handyakkus/...kia-bl-5j.html

Any battery that goes from empty to full in a few minutes is f***ed. I could see that from your first bullet point. No "extensive testing" necessary.

Unfortunately, it has come to the point when buying a battery for the N900 is a lottery. Most of them will be DOA by now.

It is still quite easy to find working batteries for N900. They may still be found even from your nearest mobile accessory shop. I found some (pretty crappy quality, but working at least) from Motonet (Finnish people know this) in my hometown. Just be sure you don't buy anything which is advertised to be "original Nokia n900 battery". All original stuff is dead already, but there's some good quality accessory/third party batteries still available.

The problem with this particular battery was that the seller clearly lied about it being unused & in original package.
But yeah, a "new" one that spent 10 years on a shelf somewhere wouldn't necessarily be much better...

You know I did scour the forums and various online selling platforms before making a choice.
Can you tell me where to get this fabulous PolarCell for N900?

https://www.polarcell.de/en

Or, https://www.ebay.co.uk/sch/i.html?_f...rcell&_sacat=0

Thanks, glo-worm & pichlo! Not sure how I missed that.
The ebay offer seems to be from the same company as pichlo's link. Must order now.