If you read the Snidal manual, using the ES when the battery is low may result in a catastrophic ( his words!) failure of the starter motor. Because it seems that in that case, the drain on a low battery causes the ignition timing to inadvertently advance, doing the same thing to the motor, what a backfire can do to your foot. He also says that in later models, RE has incorporated a relay to prevent the advance from happening.
I would get the battery checked, as a minimum, before using it for ES.