File:Economy relay coil drive for battery-powered devices (simplest - series RC).png

English: Powering a relay coil continuously at the rated voltage of the relay is too inefficient. At the rated voltage, the smallest relays consume 100 mW; a relay with 6A, 8A or 10A contacts will probably consume 400 mW to 1W, etc. However, this energy is only necessary to turn the relay on. Once it is on, it will stay on until the voltage across the coil decreases to a very low release voltage value. It is usually safe to assume that the operate (turn-on) voltage is 3/4 of the rated voltage (e.g. 9V for a 12V relay), and that the release voltage is less than 1/4 of the rated voltage. Always check the datasheet for specific values and terms of application.

The simplest battery-saving arrangement is to wire a series resistor RS into the relay coil path, and shunt this resistor with a capacitor CX. At turn-on, CX is a short and the coil sees almost all power supply voltage (less the voltage drop on the switching device) and turns the relay on. Then CX charges up, and the voltage across the coil drops to the desired safe level - but the contacts are still on.
The shown example circuit is a time delay relay with the TL431 driving a single Takamisawa RY-12W-K relay (Rcoil of 960 Ohm, Vturnon of 8.5V, Vrelease of 0.6V) in a 4S lithium battery environment (Vbatt of 14...17V). Without RS, the coil would draw 12..15 mA. In bench tests, the circuit operates reliably with RS of up to 8.3 kOhm, corresponding to a mere 1.3...1.6 mA steady-state current. CX may be 220 uF or more (although anything above 470 uF is an overkill). With higher RS or lower CX values the circuit fizzles. It clicks but won't stay on (high RS) or won't turn on at all (low CX).
The circuit must provide discharge paths for CT, CX and the inductive surge of the coil, hence the three diodes. Any low-power diode like the 1N400x will suffice.

Note that any component in the coil current path (RS, CX, reverse diode) adversely affect the energy available for switching the contacts. In case of a low-power signal relay, as shown here, it is unimportant. However, if the relay contacts carry substantial currents, then even the diode alone can disrupt operation of the contacts. It's complicated... (if large currents and/or power are involved)