Grounding for a battery
You can put ground wherever you want; the grounded points in your circuit will then all be at 0V (by definition/convention), and, theoretically, they will all look as the same node (ie, no impedance whatsoever between them).
Look at it this way: Batteries, and other voltage sources, only set a voltage difference between their terminals, which means that ungrounded circuits have an indeterminate voltage at every point, with only voltage differences being defined. So hooking some point to ground is simply a matter of fixing an origin for the scale.