Backup Battery Circuits

Just thinking, how about a small wet cell like a lawn mower battery or a gel cell connected at the radio's power terminals. Connect a diode or selicon retifier in the hot line feeding the radio circuit.(size it for the max current of the radio load and battery charging load "15- 20 amp?") This will allow the radio to play and the aux. battery to charge during normal operations. When the main battery is disconnected the rectifier will prevent the aux. battery from dischrging through the other circuits There is a slight voltage drop across the retifier however your charging circuit should be putting out 14.6 volts and the aux. battery will still charge and the radio should work ok. Another way would be to connect a 12 volt relay's coil to the boat's inition or switched auxilary power circuit. Wire the above mentioned aux. battery radio combination power circuit through the noramlly open contacts of the relay. Whenever the main battery is disconnected or the ignition switch is turned off the relay will open and the radio / aux battery will be isolated from the rest of the boats circuits. Good luck and happy boating

Hello Sailer, If you want to lower the voltage drop over the rectifier (diode), then instead of a silicon diode use a schottky type. It has only about 0.3 volt drop compared to 0.7 volt for a silicon type diode. best regards Jens

Thanks, Gary and Jens. I appreciate the assistance. I'll give it a try!

Instead of tying in your current radio to an existing circuit you can power it directly to you main battery. Most radios have a wire from the harness that is used for clock and station memory. With this featurn you can tie this line to the main battery and then connect the main power source to the existing power hook up thus allowing the ignition to turn off the radio.

For Automotive battery isolation I used a relay powered by the D+ terminal on the Alternator. Some D+ terminals are 7v some 14v so measure and select if appropriate. The use of a relay eliminates the voltage drop associated with the diode and increases battery life as lead acid batteries need to be charged to the gassing stage.

Hey Gary. I may be over simplfying things a bit, but how about a DPDT switch and a small wet cell dedicated to the radio? The switch can be wired to switch the radio's power from main to backup, and the backup battery from the radio to the charging system at the same time. Simple and VERY reliable.

It will work, but someone has to remenber to throw the switch. The relay or diode will work idependently of operator error or negligence.

Well there's your solution... a DPDT relay that is wired to close on the radio side when you cut the main batteries assuming the main disconnect is manual. I assumed that this was a manual switch. That's why I suggested the other manual switch. They could bath be thrown at the same time. Not hard to remember, especially if you mount them both in close proximity.

1. Do the boats have Cogarette-Lighter Socket like Cars?
2. Clocks Radio etc remain on-power even if Ignition Swith is off in cars. Why not in boats?
3. Boats battery is large enough to support for days if not weeks Radio etc.
4. How long the power-outages [disconnected from mains power] are?