Under voltage radio link

We are now able to use FM radio band from 88mhz to 108mhz FM to transmit a very low RF signal of about 10mw people are now useing this for MP3 players in cars so you can listen to the MP3 player on the car radio, idear is to use this RF link to transmit a tone from the submarine back to the shore when the main drive battery is going low unit could be made to monitor lipo batterys problem with monitoring lipos is that you have to monitor each cell as no matter how good the pack is there is always one cell weaker than the rest, and if you take the weakest cell down to 2.85 volts and then go below this the cell will start to lose capacity every time you got below 2.85v but the circuit could also be used to monitor lead acid battery or what ever you have in your boat. The RF side of the unit the transmitter would be the size of a half pence piece and it would be fully tuneable within the FM radio band and would be all SMD components on PCB would be interested to hear your comments on this. I have done alot of messing about with lipo cells in model planes one of my pattern ships is pulling 3.4kw which is a lot of power.
John Robinson

Sounds a very good idea. The alternatives are, I suppose, throttle cut, a flashing LED or (the method I use) a sounder in the sub. Both have their problems - the main one being connections between the internal electronics and the outside world.

Would an FM transmitter be able to function from within the hull or would it need external aerials on the hull?

HI David
I suppose you could do some thing like make the main drive motors throttle up and down a few times to let you no battery was down. Like you said David you could have LEDS flash off and on port and starbard nav lights on conning tower. The arial would be a thin piece of wire stuck in free flood area in side boat take for instance the FM transmitter was on 88mhz a full wave antenner would be 133.16inc in lenght but if the antenner was in free flood area believe it or not the antenner would be about 8.2 inc in lenght which is not hard to get inside the sub if you where to use 1.8mhz to transmit to the sub the arial would be about 501 feet in lenght in air but if it was in the water would be about 30 feet long it gets quite interesting
john