My 1989 Johson 60HP has been running good - no alarms UNTIL this weekend. This weekend I over engineered my boat by installing a 4-position battery switch (batt1, batt2, batt 1+2, and off). I also installed a new marine staring battery for the motor which raised the voltage to 13v which is highest I've seen for a while (I've been using old battery). I triple checked my wiring over two days so I feel pretty good about my work. While the switch was in Batt1 position I started the motor and it was running great. WIthout thinking, I switched the switch to batt1+2 position. (Batt2 is an old battery barely hitting 12V.) and within several seconds a solid alarm sounded and my ultrasound fish finder went nuts; still spins but light is out. All other electronics are good, the motor starts, and I haven't found any blown fuses or breakers. The solid alarm sounds as soon as I turn key to first position even after 30min of engine shut off (cooled). Although my oil reservoir is about 1/3 full and although the motor wasn't running long, I don't think its a oil or heat issue. After looking through many on line schematics I saw that there is a diode in line with the fuse. Could I have overloaded the diode and or something else? What regulates voltage? What the purpose of a rectifier in a DC circuit?