Good batteries (which are usually rather expensive), including hologram-tagged Nokia batteries, come with a special protection circuit inside. It triggers if the charging voltage goes above 4.3 volts for some reason (say, a failure in the phone's charging control circuit). In addition it protects the battery from a couple of other issues too. Without this extra protection a random failure in the phone charging circuit will make the battery explode violently. In addition there's the packing material, already mentioned. If it gets punctured the battery can easily catch fire.

Stay away from cheap batteries from unknown sources. Some of these can be of OK quality, and meant for radio controlled planes and the like. Those batteries don't have a built-in protection circuit because they are meant to be assembled into aggregates with a separate protection circuit added by the RC amateurs. They are easy to get by, and cheap enough, but don't put one in your phone. Problem is, some unreliable re-sellers may try to make a quick buck by re-packaging these kind of batteries into something that fits a phone.

Oh, and the super-high capacity is a dead give-away of an unreliable seller, because you just can't squeeze more than a certain amount of capacity into the given volume of the N900 battery compartment. Maybe a little bit more than the original 1300 mAh (IIRC), but never 1700.

What if I was ready to pay "alot" for a better battery? As spare batteries are not too expensive I guess they are pretty cheap to produce.. with todays nano tech etc would it not be possible to make a really really good small battery but for a higher price?

Well looking at so many people wanting an extended power for the BL-5J, why not nokia just make one which is over 1500MAH BL-5J and use the same BL-5J in next maemo with the same size that fits 5800 N900 and the new one what ever is coming that way all we are safe of buying that OEM BL-5J which is over 1500MAH.

The people with the latest technology wont talk to you unless you want to order millions.

For "standard" capacities, smaller energy densities than BL-5J, and in standard sizes (which BL-5J is not), you can find people who do minimum order of 1000s So just find someone who will do custom size at 10,000-100,000 units, maybe they'll come up with a 1000mAh battery. Then you make the label say "1700mAh", bonus points for shipping a few hundred to various bloggers and reviewers, pick ones who are too clueless to tell the difference, and you're all set.. Oh wait.. This make-money scam is already taken

heh, I see your points. Lets hope we se a Mugen battery or someone abit more serious then. I would rather not make it thicker though but I suppose it is a must to cram in more maH...

And how many exploding batteries have been reported on this site, ever? If you include the ITT site, the answer is

zero.

Congratulations on helping to support an urban legend.

I don't have any Nokia batteries in my N800 or N810, just cheap clones, since I have found them to be more reliable.

It's not an urban legend just because nothing has been reported on ITT! Sheesh.

It's not difficult to find references to real incidents elsewhere. And if you just bother to look, you can go to brighthand (which is a forum similar to this, dedicated to mobile devices) and find, in just a single thread, lots of references, reports and pictures about batteries swelling to double size, leaking, and other issues. I was following such threads closely in the past.

There are definitely issues with cheap batteries (and also non-cheap batteries, but not as much), even if you haven't experienced any yourself, or seen any reports on this single forum.

A report of batteries swelling to double size, leaking and other issues, is
not a report of an exploding battery.

However, here is a report of a possible exploding battery.

The fact that an exploding battery has never been reported here surely tends to indicate that it isn't very likely.

Note that the above report seems to be of a legitimate battery, not a clone. This tends to back up my claim that many of the reports about battery problems are not of clones at all.

I also claim that many clone batteries have been produced by the same factories that produce the real batteries. To the extent that this is true, I don't see why clone batteries produced by a factory would be less reliable than 'legitimate' batteries produced by the same factory.

I have seen many reports on this site of apparent battery-related problems with legitimate Nokia batteries. I am surprised that critics of clone batteries don't jump on such reports -- it almost makes me wonder if these critics aren't as objective as they think they are. I have seen many reports thru the years here on this site that seemed obviously to me to indicate that someone had a dead or faulty battery. These were ALWAYS of legitimate Nokia batteries as far as I know. Does anyone have different information?

Ignoring the chance it may explode, set on fire, crash the world economy, et al. I'd like to see someone with one of these 'extended batteries' run a few tests to see if they really are extended.

okay, then I'll relativate my post.

happy swelling to double size, leaking and other issues...

damn, this sounds not as good as happy exploding...