Actually they are compatable...

BUT it seems some are not compatable with iOS 4... sounds strange. i know

here's something to read http://www.ifixit.com/Answers/View/28809...

you can change the circuit board of the battery pretty easily though

edit : To Change the board---

you need a battery that is good but has the wrong number, and a bad battery with the right numbers or just the right battery board

- peel off the yellow tape from the bottom of the battery- carefull not to tear the power ribbon

- with the sticky side of the battery up... and the board end facing away from you.... the board will kinda unfold (away from you) from the battery

- you will see 2 metal strips connected to the board... and you will notice they are folded a couple times near the board... unfold the strips as much as you can,

-break the strips off as close to board as you can. but don't totally rip the metal off the board.

-solder the good board to the strips of the good board.

note - i havn't tested this yet but as far as i can tell it should work.

Noting oldturkey's comment it's probably better to just get another battery, there not that expensive anyways

i try it 3g battery put it in 3gs it not recharge and shut down every 4 minutes..so my answer is they are not commpartible

Hmm, good question. The teardown that iFixit performed last year didn't indicate any differences other than capacity, but I'm not sure. I can't see why it wouldn't work, other than Apple moving connectors a bit. It looks like the exact same battery form factor.

my 3G has a 3Gs battery now for over a year and has great battery life! My whole back cover (except for dock assembly) is from a 3Gs!

yea definitely not compatible. i just ordered a battery for my 3gs and it appears to be a 3g battery. i shuts down every 4 mins and the restore gives error (29) which is hardware issue. so obviously its NOT compatible.

A 3g battery on iphone 3gs is not compatible because create error 29 but a 3gs battery on iphone 3g is the best thing.

No problem in many years,i fix iphone and i've fixed about 400 and replaced more than 60 battery and never problem with this method.

The big problem is to find a good battery (truly original).

The best type of battery in iphone are the white model with apn:

616-0391 for iphone 3g

i have just obtained a 3gs and have a 3g to build i ordered a battery for my 3g my mrs also had a 3g so i got one for that too now the 3gs come along so i thought instead of putting my 3g battery in the 3g i put it in the 3gs then put it on charge powered up good then i unplugged it , dead!!! i then noticed the other battery i ordered looked different. 10 minutes resarch and this is my conclusion....... the 3gs is totally different in many ways so always put the intended parts in the phone as all the connectors may look the same but are just slightly different and easily broken. apple must love it.

Update

if you look at both batterys they are blatently different as one has lots of tiny component on it and the other not so many and i found out the 3gs battery has got a circuit that stops it from overheating and overcharging.

i just put a 3GS battery on a 3G phone and it does not work! When i charged the phone it would not go over 23% percent. left all night charging and when i woke up it was at 4%. got to work and now is dead.

I own an I phone 3g which had a battery APN:616-0346 and i replaced it with a 3gs battery APN:616-0435. The phone works perfectly fine and charges properly.

It runs iOS 4.2.1.

They are not really compatable. For instance do not use a iPhone battery number :APN 616-0435 into your 3G phone .It is for the Iphone 3GS only. Models for the 3G have lower numbers like 616-0364.

They have chips which control them. The 3G is not the same as the 3GS so why expect them to work well with mutual batteries (sic). Beware of relabled batteries also.

If you try to fix a 3G with a 3Gs batttery it will cause all kinds of problems...innability to sync with iTunes ect. On OFF low power, no power.

Be careful out there.

Sibo