I have had good luck in buying cheap batteries, but I have had a few duds, too.

It's a great idea to get an external battery charger and put it in a place where a fire won't burn your house down.

If you can look at the battery before you buy it, look for a battery that superficially looks good. My favorite clone batteries just looked great from the moment I first saw them. Quality tends to show.

Watch new clone batteries carefully, and if there is something you don't like about them, throw them out. That's one of the good things about buying cheap ones, you can afford to discard them.

Pay close attention to reports here about cheap batteries and buy from known good sources to the extent possible. But you can get an idea of quality just from a catalog picture, or maybe even from the quality of the catalog itself.

One of the BAD things about the N900 is that opening the back to put in the battery is nowhere near as easy and pleasant as it was with the N800 or N810. So you are not likely to check them as closely, so having batteries you think are reliable is more important than it was with the other tablets.

I think that a lot of the clone batteries are from the same factories that produce the real batteries, so there is no big reason to fear them. And it's in the interest of clone battery manufacturers to produce a quality product for the same reason it is for a non-clone manufacturer to produce a quality product. Shoddy products are bad for business.

The capacity of a lithium-ion or lithium-polymer battery is determined by its physical size. There are no vendors out there with some magical process to create higher-density lithium batteries, so when you see a BL-5J battery announced as 1800mAh, you can be certain about
a) They are lying about the capacity.
And, due to a),
b) Why would you put trust in buying batteries from someone who are obviously trying to push fake stuff to people?

That there are smaller variations in advertised battery capacity is an entirently different thing. These batteries vary by some 5-10% or thereabouts, and, due to how conservative the advertising is, you may see the same battery advertised as 1050, 1100, 1200 mAh, for example. But if you see anything like a bump from the original BL-5J size of 1320 to -> 1800, then you KNOW they're lying. That's the kind of shops I don't buy from.

I did buy a replacement battery for my Palm with higher capacity, the original is 900mAh and the replacement around 1100mAh. But in this case the latter is physically larger and could only be (literally) squeezed in because it's lithium-polymer without a hardshell and it's possible to massage it into a shape that fits the limited room available.

Well, Mugen Power batteries have slightly bigger capacity while keeping same size and they indeed work as expected. So maybe 1700mAh is unlikely to be true, but 1500 is within expected range indeed.

What about those BLB-2's? +30-50% capacity was real.

Has anyone had bad experiences with the chargers that are currently offered?

I agree that for a phone like the N900 original accessories are the way to go, but regardless of money afaik Nokia simply doesn't offer standalone battery chargers for the BJ-5J, so I'm forced to buy from dubious sources. Any recommendations (with shipping to Germany preferably)

I only use the desktopcharger for charging my fake battery. it works good. no problems.

FWIW, I just picked up an "OEM" battery from amazon.com, via one of their vendors, ccmaccessories, for $26. Been using it for almost a week and all "seems" well. Getting about the same battery life, 10-18hrs with calendar, and Foreca weather widgets, (3) email clients all updating every hour, WiFi constantly on at 10mW, and about an hour or two "playing".

I had an interesting experience with my Palm Centro that is actually relevant here! I broke the screen and had to buy another one. Then I realized that since the screen was the thing broken, the rest of it seemed to work ok! And now I use the broken Centro to charge my batteries! It works great!

If you have a broken OR good N810, you might be able to do something similar. If you search, you can find a thread saying that you can shoehorn an N900 battery into the N810 and charge it! I think that's probably true, so I've sent for some cheap batteries (not from e-bay, which I am permanently boycotting) and will try it out. I would expect that an N810 with a broken screen but that actually turns on could be used just as well as a non-broken N810.

Also, I bought one of the cheap chargers for my N800, and it has been going strong for about two years now.

geneven,
you could hack any mobile phone or similar device to do the same.
But WHY when a universal charger can be mail-ordered for a few dollars?
Those gadgets have adjustable spring arms and polarity detection to make it easy. The same DX I linked above has them from $2!

N800 batteries have flat connectors - very different.

I purchased 4 cheap ones on ebay with chargers, etc.

The batteries work great, but the battery life indicator reports as full until right before the phone dies.

Ditto with the one I bought from Amazon for $19.99 (should have known better from the price of "$19.99") :s

EDIT: Not to mention it lasts about 75% as long as the real battery, guessing it is ~1000mAh