Daniel Wee at clubsnap forum did the first teardown of the Olympus OM-D EM-5 Third Party Battery that can be found at ebay.
Those third party batteries seem to work well with camera but there are some caveats:
- OEM charger for the Olympus OM-D EM-5 battery BLN-1 does not charge this third party battery requiring a charger provided with third party battery
- The charger for the third part battery has a design where the main connector swivels out saving some space. However this may hamper long term reliability
- Seller of the Olympus OM-D EM-5 Third Party Battery claims it is made with Sanyo battery cells and a capacity of 1450 mAh. However they are actually made by Zhongshan Tianmao Battery and has a capacity of 1020mAh so they hold about 84% of the charge of a real Olympus BLN-1 1220mAh battery
So it is up to you to see if the Olympus OM-D EM-5 Third Party Battery although with some caveats is the right fit for you.