I have a dive flashlight that charges via a proprietary USB connector and exposed pads on the exterior. My dive computer also has similar pads that don't charge but allow for data download and programming. I have seen similar things on other marine electronics that are potted in epoxy and just have some wire loops sticking out to hook onto for a data interface.
My question is what kind of circuit do you need to prevent a short, battery drain, or other bad things I can't imagine from happening?
Best Answer
The pads used for charging the battery should be isolated from the battery during a dive. If there is no current flowing through the pads and the salt water nearby, no damage is caused by electrolysis. It is not necessary to isolate the + 5V pad and the GND pad together, isolating the + 5 V pad only will do. The isolation can be done with a diode between the pad and the + connection of the battery if the leakage current of the diode is small enough. Another way is the use of a reed relais, the contact between pad and battery and the coil between GND and + 5 V pads. If the external charger is connected, the relay closes the contact between battery and pad.
The pads for data transmission may be isolated with other relay contacts. Using analog switches for isolation is possible too if the leakage current is small. A data input may be isolated with an opto coupler, there is no current flowing through the LED if nothing is connected to the pads.