Which of these devices can be used to supply auxiliary power to alarm systems?

The option that is accurate for supplying auxiliary power to alarm systems is the battery charger. A battery charger serves to provide charging power to batteries that are part of the alarm system, ensuring that they remain charged and ready to supply power even when the main power source is unavailable. This is especially crucial for maintaining the functionality of alarm systems during power outages or failures.

The other options, while they play important roles within an alarm system, do not specifically supply auxiliary power. A keypad is mainly used for user input and control of the alarm system, allowing users to engage or disengage alarms and enter security codes. A circuit board houses the components necessary for the system to operate and process signals but does not inherently provide power. The control panel is central to the operation of the alarm system, managing signals, notifications, and system settings, but it relies on a power source rather than being a source of auxiliary power itself.