12V battery switch and charger/maintainer as ultra-low-noise 12V supply

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Joined May 2, 2017
I need a very low-noise 12V supply for a low-power (0.25W) FM transmitter. I was considering using a deep-cycle lead-acid battery. The device would have a charging/maintaining mode and the output would be disconnected while the charger is active, and a "run" mode where the charger/maintainer would turn-off and the output would be connected directly across the battery. Does anyone sell such a device? I also thought about modifying a small UPS unit as this would have the battery and charger/maintainer circuitry. If anyone knows of a commercial device (and what one would call it) that performs this function, please let me know. I would consider just buying one if the price isn't too high vs making it myself. Thanks.