In normal operation, a diode in series with VBAT becomes forward-biased, and the current flows through it. When the battery is installed in the reverse polarity, the diode becomes reverse-biased and no current flows.
The forward voltage drop across the diode shortens the usable battery life. For example, an alkaline battery capable of providing 1.5 V is limited to 1.5 V − 0.6 V = 0.9 V. The boost which follows the battery also suffers due to this drop. Because a Schottky diode has a lower forward voltage drop, it can be used instead of a regular diode to minimize these disadvantages.
In practice, this configuration cannot be used with a 1.5 V battery because the voltage drop is too high. This results in a very limited usable voltage range and very poor system efficiency.
Even with a 3 V battery, the efficiency loss is considerable when measured from the supply voltage. For a normal P-N junction diode, the loss is 0.6 V out of 3 V, or 20%. For a Schottky diode, the efficiency loss can be 0.3 V out of 3 V, or 10%.
The selected diode should have a forward current that can support the device in all conditions. It should also have a sufficient voltage rating because leakage current occurs when the battery is installed in reverse polarity. Using a Schottky diode is more expensive than a normal diode, but has a low forward voltage.