Fuse Those Batteries

By Ken Larson, KJ6RZ

Alan Stine, KG6LVU, and I were having a discussion several weeks ago concerning the proper way to fuse the 12 VDC power cable between a battery and a mobile radio. We concluded that fuses should probably be placed as near to the battery as possible. The installation instructions for several brands of mobile radios confirmed that we were mostly correct. All of the manuals recommend that two fuses be placed on the battery side of the power cable connector, as close to the battery as possible, with a third fuse on the radio side of the power cable connector. The recommended approach is shown in the figure below. The two fuses next to the battery protect the battery from a short circuit. Hugh Bosma, KF6HHS, recommends that these fuses should be rated at twice the current that you expect to draw from the battery. A 50 watt mobile radio typically draws 10 to 15 amps of power, so 30 amp fuses are a good choice for protecting a car battery. 10 amp fuses will do a good job protecting a 5 to 7 ah battery if you are running a low power QRP radio. The fuse on the radio side of the power cable connector protects the radio. The size of this fuse will usually be specified by the radio manufacturer. Mobile radios typically use a 15 amp fuse.


              Fusing Batteries

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