Yup.
If you got 5800 you're good to go lol just replace the batteries, charge the 'n900 battery' in the 5800

Instructions from Mugen Power Battery:
The first time, please charge the battery in the device for 12 hours using the mains power adapter. After fully charged, please use the device until the device tells you the battery is very low, then recharge. Repeat this process for 4-5 more times, where after you can recharge the battery whenever is suitable.
Important: NEVER drain the battery completely to 0%