47 CFR 80.917 - Reserve power supply.
(a) Any small passenger vessel the keel of which was laid after March 1, 1957, must have a reserve power supply located on the same deck as the main wheel house or at least one deck above the vessel's main deck, unless the main power supply is so situated, if -
(1) The vessel is of more than 100 gross tons; or
(2) Beginning March 25, 2009:
(i) The vessel carries more than 150 passengers or has overnight accommodations for more than 49 persons; or
(ii) The vessel operates on the high seas or more than three miles from shore on Great Lakes voyages.
(b) The reserve power supply must be independent of the ship's propulsion and of any other electrical system, and be sufficient to simulataneously energize the radiotelephone transmitter at its required output power, and the receiver. The reserve power supply must be available for use at all times.
(c) When the reserve power supply consists of batteries, they must be installed as high above the bilge as practicable, secured against shifting with motion of the vessel, and accessible with not less than 26 cm (10 in.) head room.
(d) The reserve power supply must be located as near the required transmitter and receiver as practicable.
(e) All reserve power supply circuits must be protected from overloads.
(f) Means must be provided for charging any storage batteries used as a reserve power supply for the required radiotelephone installation. There must be a device which will give continuous indication of the rate and polarity of the charging current during charging.
(g) The cooling system of each internal combustion engine used as a part of the reserve power supply must be adequately treated to prevent freezing or overheating consistent with the season and route to be travelled by the particular vessel involved.
(h) Beginning January 2, 2013, any small passenger vessel that does not carry a reserve power supply must carry at least one VHF handheld radiotelephone.