It is the current (mA) rating, not voltage that matters when comes to USB chargers. Voltage is always 5V no more, no less. USB1/2 - doesn't matter
For proper operation of D3 you need charger with current rated at 800 or more mA.
When charging the phone off less powerful charger, you may experience issues like charging taking a lot longer or even not charging at all. 550mA is the bare minimum you need (this is how much PC USB ports supply). Phone will charge slower if charged from a computer, but will eventually charge up.
Any idea if there's a maximum current you should use? The playbook charger is (I believe) 1500 mA, which is twice what the stock charger is, and I don't want to risk just plugging it in. Does anyone have any insight?