I bought this battery two months ago at Proporta's headquarters in English Close, Brighton. Nice people but not a nice location (no direct bus there)! Their previous office in Hove, Brighton was a more convenient location wise but small.
They even have a little Japanese guy there who was helpful in getting me a discount (10%).
The battery in question is a made in Japan Mugen Power Lithium-Ion battery rated with charge capacity of 2000mAh. It was made specifically for Hewlett Packard's iPAQ h4150.
Yesterday's 13 hour flight was an excellent time to test the battery. And boy did it pass. Helped by a CPU hack utility (XCPU-Scalar) I managed to coax out an incredible 11 hours of total usage out of the battery. Wonderful.
It was mostly for listening to music (GSPlayer) although I did some light e-book reading (iSilo), some note taking and a short quick game of Michael Schumacher's Kart Racing 2002.
The audio files used were OggVorbis files encoded at 64kbps. The quality isn't bad. In fact in my ears it sounds the same quality as mp3 files encoded at 128kbps (using my Sony MDR-EX71SL).
Back to the battery. The battery itself is bigger than the original iPAQ's 1000mAh battery in terms of physical size. As a result, Mugen (or Proporta) has provided a new door. It creates an extra hump on the back. Manufacturing quality is so so. The black packaging around the battery has started to show signs of wear and tear since I frequently remove it. I have to add that it does not affect the performance.
The battery costs around £35. It is cheaper than HP's optional large battery (RRP around £70) and has a larger capacity (2000mAh Vs 1800mAh).
Update: The 2000mAh battery costs £39 ex. VAT. Proporta do ship worldwide standard shipping for £2.12. You will only need to pay VAT if you are a resident in the European Union. They also sell a version for the h2210 for £34.