I need to set up an emergency cut-off DPDT switch for a circuit that at worst case would pull 45A+ total from a pair of lipos (one getting pulled a bit more than another) at 6 - 12v. Searching on google led to inadequate finds. I also heard about "Battery cutoff switches" which seem to be popular in the marine transport and automotive business, but these seem like overkill and are also massive (the ones I found) - I need something discreet. I was wondering if my best option would be to set up a smaller switch connected thru a transistor to be able to handle the high load, but I am weary of the extra power dissipation this would add to the circuit (do transistors dissipate significant power when "on"?). Thoughts would be appreciated.
At 45+ Amps you’re in relay/contactor territory. Unless you get a super-low-on-resistance MOSFET (0.001 ohms), and even then I’d worry about it blowing up while turning off and on. You could put a bank of MOSFET’s in parallel, say 10x <0.01-ohm MOSFET’s each carrying 4.5A (we hope). Now that’s more reasonable. Just make sure your MOSFET gate driver is strong enough to turn them ALL on and off at exactly the same time.
The Aussie Shield: breakout all 28 pins to quick-connect terminals
E-stop... i would do some type of risk analysis to determine the safety category(level) of your system. Dc with high current are not so fun. However, you have "low" voltage levels so its not worse than any battery circuit in a vehicle. (I would worry more about balancing when charging.) Use a fuse near lipo-cells and proper dimension of your cables.
If you need to cut off dc i think you could use some ssd relay. They will "steal" some voltage though. Also, switching dc is expensive so i would do as mentioned by RuggedCircuits and build something with a mosfet, igbt, smartfet or whatever they call it. And be sure to keep I^2*Ron as low as possible (powerloss in your switch). I would forget about using a relay especially if you have some inductive loads.
In general, to build a safe system the key is double everything and [u]monitoring the state of your switches[/u] so you will [u]never be able to start a defect system[/u].