A4988 and battery power
A4988 data sheet here  

(1) The data sheet says that the /ENABLE input does not destroy "state"information. 
However, as Anindo notes, while there is no external torque, the motor will apply "pull in torque" when depowered. It MAY be possible to use the current limiting setting to reduce holding current to a minimum and to increase it when actually stepping, but this is not a certain solution, and it  adds complexity which may make the overall solution unattractive. 

On page 10 it says:

Enable Input  /ENABLE

This    input   turns   on  or  off all of  the  FET    outputs.
When    set to  a   logic   high,   the outputs are disabled.
When    set to  a   logic   low,    the internal    control enables the outputs 
as  required.
The   translator  inputs  STEP,   DIR,    and MSx,    as  well    as  the internal    sequencing  logic,  all remain  active, independent of  the /enable INPUT STATE.  



(2) You may also be able to dynamically alter the effective sense resistors that control maximum current. These MAY latch, on a step by step basis, which would make this method impractical.

Data sheet says:


At   each    step,   the current for each    full-bridge is  set by  the value   of  its external    current-sense   resistor    (RS1 and    RS2),   a   reference   voltage (VREF), and the output voltage  of  its DAC (which  in  turn    is  controlled  by  the output  of  the translator).