Can I connect two RAYOVAC® 9V lithium batteries in parallel in order to double the available capacity for my application? Is it necessary to use protection diodes and/or series resistors to balance the current drain between the batteries?

When connecting two 9-volt batteries in parallel, to prevent reverse current flow we recommend adding a separate protection diode (such as a 1N91) to each battery so that either may discharge into your application, but neither may charge the other. The fundamental disadvantage of using blocking diodes is the inherent voltage drop (0.7 - 1.0V), resulting in a lower available voltage to operate the device. To minimize voltage loss, some circuits use a Schottky diode, which reduce the voltage drop to typically less than 0.5V. We do not recommend using a resistor in series with each battery to help balance the current drain.