I have had a Nokia BH-214 Bluetooth headset for about six months, it is really great since I can use my own preferred headphones for whatever activity as it has a 3.5mm audio jack and it is fully compatible with the N900 for pause, skip forward and back, answer call, and volume; it also has a mic so your phone can stay safely inside a bag. While I suspect mine may be an eBay counterfeit, even per official spec the otherwise amazing BH-214 shares one major issue.
That issue is the 5-6 hour battery life or less which is too short for really long bike rides where I need my music, audio from GPS, and phone calls, I power the N900 from a hub dynamo/generator and a converter to 5v USB power.
The stock BH-214 battery is 2mmx30mmx20mm 110ma/h Li-Polymer without its own regulator.
I found a 4mmx35mmx25mm 400ma/h Li-Polymer battery that had a regulator soldered on, since the BH-214 has a regulator on the board I removed the tiny regulator circuit board that was covered with transparent yellow tape, of course marking the + and - poles on the battery first.
Installing meant carefully grinding away any obstructions with a Dremel tool, and using a butane soldering iron with the tip removed as a heat gun to very carefully soften the bottom of the casing as I pushed the circuit board into its mounting clips.
Once everything cooled I soldered down the power leads.
The mod leaves the circuit board sitting too high so I cut a square of plastic so it fit inside without getting caught in the snap downs of the case, I like the look of the BH-214 front buttons better than blank white so I filed off the edge that held the button in the bezel, I glued the modified four way button down with hot glue.
Everything works as before, just the buttons feel a bit less clickey, but I have yet to run the battery down after all day listening, not sure why Nokia didn't choose to use a larger battery in the design.
This mod and a tough pair of headphones, search eBay for flat cable earphones, and I am good for all day cycling.
The clip is fragile and I have cracked it, melting the cracked area with a soldering iron fixed that but now I hang the BH-214 from a safety pin on my collar or t-shirt.
This headset battery mod is perfect to match with the Mungen battery lid and the double Scud super 3000ma/h N900 battery, or just when your headset wont fully charge anymore.

I should also mention that I use the Nokia CA-100 USB to 2mm charger to provide the 5.7v power the BH-214 needs when powering off of my Bicycle power system or I am near a computer, it is smaller than carrying both a microUSB and 2mm mains/wall charger. Using a regular 1200mA Nokia 2mm phone charger caused the headset battery to really heat up and not take a full charge but the 300mA charger included with the BH-214 never caused problems.
I have not tested a Nokia phone charge with the new battery mod but the CA-100 works great.

Update
I have run the modded BH-214 for two days straight between charging twice now, music or podcasts running most of the time between 8am and 8pm. That makes for about 40 hours with 16 of that running audio. Massive improvement in convenience and usability especially since the charge time does not seem to be much longer than before the mod.

Can you post pics? Also interested in your bike set up and how you mount the N900 while ridding.

I dont know that I will take apart the BH-214 again as I dont want to break it, but I will try to get around to photographing the finished work as well as my bike holder.

Nice mod.
I use my BH-214 daily at work and I have been thinking of ways to extend battery life.

I have used a chinese copy of Nokia DC-11 battery extender to charge the BH-214. This is great because the BH-214 can be charged while being used

I understand, and no need. I am more intrigued by the mounting method (and/or setup) you have for the N900 on your bike.

Wow, I must have a fake BH-214 then, a bit late to complain though, I can NOT play audio and charge at the same time.

Hello, biketool ! Would You pleas tell me how to disassemble this headset. I have got fake one from chinese shop and it sims to be broken. So i would like to repair it but can't figure out how to disassemble. Thank You.

If you have a guitar pick or fingernails you can take the BH-214 apart, I think I might have a fake and that is what I did, the bezel around the button is just gently lifted/pried until the catches pop loose. THe circuit board is held in by I believe four plastic catches that require a bit of pressure and bending to let release. The battery is held in with double side tape.
I take no responsibility for cracked cases, I have taken apart many small gadgets and know where to look and how these little snap connections feel and how to open them.