Looks like you can buy the cradle, the battery, and/or the charger module individually. If you want to charge a battery outside of the 901, you would just need the cradle, and use your own battery and charger. If you want a spare battery for longer play-time, you would buy the cradle and an additional battery and use your charger. You just have to make sure that one is charged while the other one is charging.
I suppose you could buy the battery, charger, and cradle, and be able to charge them both at the same time - but how often would that be necessary? I would imagine that these batteries require "smart" charging, and the current charging unit is probably not equipped to monitor 2 batteries at the same time and adjust their charging cycles accordingly - we're talking a small, specialist audio company, not HP, Canon, Nikon, etc......
I'd love a pmp the size of a credit card, with a 9" screen, 6 month battery life, able to drive 86 dB speakers, 1 Tbyte of storage, DSP eq of actual in-ear response with specific iems, thought-control, and at a retail price of $99.99
sorry for the rant. It's nice to wish and want - let's try to be a bit more realistic..... now, where's my credit card for that cradle and battery....