original battery:

UP325385

SONY FUKUSHIMA
UP325385 A4H
7DLD20D
1200 mAh

replacement battery:

2200mAh 3.7V UP425585 A4H

================================

- the original battery in HDD100/120 is a Sony Fukushima battery, model UP325385 A4H. Those 6 digits are actually dimensions of the battery: 3.2mm x 53mm x 85mm.
There's also 1DLC21D writing on the battery, but I haven't figured out so far what that stands for.

- find an iPod 1G/2G battery Lithium-Polymer ("Li-Po") 3.7V, 2200mAh or 1600mAh. I got mine from eBay for as little as US$ 11, from Hong Kong.. (DealExtreme.com sells them! USD 9, worldwide free shipping! Look for "2200mAh Replacement Battery for iPod 2G" or SKU 576)
It is marketed as UP385385, as is even written on the battery label. But the printed data on the battery bode says:
Jingyou425585
, meaning that my replacement battery is slightly thicker and wider: 4.2mm x 55mm x 85mm.
The other line of text is E2L05-1 and I have no idea what that is.

- two main problems here are different connectors and number of wires on these batteries.
The replacement battery has 2 wires and a slightly smaller 2pin connector than the original 3pin connector in philips player.

Original connector's pins are:
"EB+" RED wire,
"TH" WHITE wire,
"EB-" BLACK wire.

Replacement battery's wires are
"P+" RED wire,
"P-" BLACK wire.

Shortconnecting onboard connectors pins that correspond to original BLACK and WHITE wire and connecting them to the replacement battery's BLACK wire enables using the replacement battery. Quite simple.
Red wire goes straight to corresponding red wires pin.
As simple as that.

In other words: RED to RED, and BLACK to (WHITE + BLACK)

battery lasted for 18 hours and 48 minutes

http://www.flickr.com/photos/77977833@N00/sets/72157594346118923/