jherran's answer helps you with disabling Spotlight indexing, but does not address the part of the question about toggling automatically when AC power is connected or disconnected.
You could use
sudo mdutil -i off / and
sudo mdutil -i on / in conjunction with a listener for when AC power is disconnected. Using something like ControlPlane will satisfy your requirement for automatic toggling. It supports "Current power source" as an event to listen for. It is free and open source.
You might be against using a third party solution. To roll your own, try writing a script that loops checking
pmset -g ps | grep -c 'AC Power' with a sleep interval in between. If AC is connected, it outputs 1 (0 otherwise).