Electrical – Conserving battery life in a stepper motor circuit

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).

Your understanding of slip rings seems correct.

There are many companies that make slip rings. So answering specifically to those is 'off-topic' for electronics.stackexchange. You might have to read through several pages of web-search results to find them.

Typically, slip ring costs increase rapidly the more separate contacts you need. So 4 power signals for one stepper motor, and two for the inner motor might be much more expensive than only two for power. It may also cheaper and easier to get a multi-circuit slip ring with two 'power' connections (for power and ground) and several 'data' connections.

Edit: One use for slip rings is connecting the buttons and airbag on a steering wheel to the rest of a car. So automotive slip rings are a mass-produced; they would be a low cost, relatively robust part. So you might find something to experiment with from a car 'junk yard'. It might be low-cost to get something there, and experiment with it, before trying to find 'exactly' what you need.

You might consider putting the control electronics 'on each motor', so that you only have to get power and ground through the slip rings. Then the electronics 'on the motor' would use that power to drive the motors power connections.

So the first/outer motor would be connected directly to its controller.
Power and ground would be fed through a 2 power-circuit slip ring.
The controller on the second/middle motor would take power and ground, and convert that into a signal to drive the motor.
Power and ground would be fed through a second 2 power-circuit slip ring. The controller on the third/inner motor would take power and ground, and convert that into a signal to drive the motor.

I assume the electronics on each motor would need to understand its position. That might be done using a gyroscope sensor on the motor controller, or use a low-cost radio.