For my project, I will have 3-9 198650's in a 3sXp relationship as a power source in conjunction with a wall wart 12V DC source. I would like to be able to install a switch, such as this one?, http://i00.i.aliimg.com/photo/v0/253934072/3_Way_rocker_switch.jpg, to enable me to switch back and forth between the two power sources. My question is, how do I connect up the negative leads of the battery and the DC wall source?
Good 18650's are expensive, and I don't want to degrade them if I can help it, hence my concern. With this in mind, would it be a good idea to wire the negative leads of the wall wart and batteries together?
As best I can determine from that picture you appear to have a simple SPST switch (just two terminals)? To switch from two alternate voltage sources to a single load device it would be best to use a DPDT switch if you want total + and - isolation.
The contacts for a DPDT switch is shown in the lower right of the picture. You would wire your load device to the two common terminals, your battery to the two NO contacts and your wall wart output to the NC contacts. Keep all the negative wires to one contact set and all the positives on the other contact set. That would allow complete isolation of both + and - of both voltage sources from each other. Most times there is no harm in just hard-wiring all the grounds together, (batt, wall wart, and load) and just use a 3 terminal SPDT to switch the common terminal (wired to the load device) and wire the wall wart positive to NC and battery power to NO. So total isolation or only positive terminal isolation, your choice, just select the proper switch contacts you need DPDT or SPDT and your off to the races.
