battery disconnect switch in car

My uncle recently purchased a "Battery Doctor/Mini Master disconnect switch with on-off knob for top or side post batteries" from Autozone, and wants to install it under his 1972 Chevelle dash. Is this possible? I know these battery disconnects are meant to be installed close to the battery, but he has had a constant battery drain on the vehicle that no one has been able to fix, so he is trying to find a temporary solution that doesn't require opening up the hood every time he turns off the car. I would appreciate any information I could get. Thanks

Anything is possible but doing this is a very bad idea. Very heavy gauge would have to be run into the cabin of the car, a high amperage fuse or fusible link would have to be installed at the battery, and electrical performance would suffer from the extra length of cable and extra connection points. Meanwhile, there's more places for a high-current short to occur and possibly start a fire. The inconvenience of opening the hood to operate a disconnect switch is far less than the inconvenience of a finely roasted '72 Chevelle. The best thing to do is find the battery drain, the electrical system on that car is quite basic so finding the parasitic drain shouldn't be too difficult.

Thanks for the reply. That is basically what I told him to do. He is going to be replacing all the electrical connections by next spring, so hopefully that will fix the problems he is having. But just to clarify, the battery disconnect would be connected through the negative side of the battery. The installation scenario you described sounded like it would be connected through the positive. Right? As far as I can tell the Chevelle has two main ground wires. The chassis ground (connected to the fender) and engine ground (connected to the bracket above the alternator). If I can't convince him to focus on the battery drain itself, the connections would be both ground wires on one post of the battery disconnect and the other post would be connected with a cable leading right to the negative of the battery. Correct?

See newbie problem, engine kill switch where I discuss such issues. I have never understood why people have used hot +12V isolation remote from the battery. As to competitive sports that have not banned +12V isolation (in the cabin or remote vehicle corners)... what can I say? Isolating the GND has none of the risks of hot cabling. The battery GND (-ve) terminal connects to the isolation switch and all grounds are taken after the switch.   

So how would I make the connections? One post on the switch would be connected to the negative post on the battery. What size cable should I use? The cable will pass through the firewall and be around 4 to 5ft. The second post on the switch needs to have both chassis and engine ground. So would I connect the second post to ground inside the vehicle and, using the factory engine ground point, connect the engine ground to chassis ground in the engine bay? If that is correct, where is the best place to connect those wires? Would the firewall be good? Thanks

The way I'd do it is chassis to engine in the engine bay, and battery to switch to chassis in the cabin. Otherwise battery to switch & back out to the chassis. Two cables from the output of the switch is undesirable. You can have multiple cables from the chassis to the engine, or gearbox etc. (I usually run 2 or more in case one fails.) IMO firewalls are not good - they're too thin. The cable has to be heavy enough to not cause excessive voltage drop nor cable heat during cranking. Normal cable current with engine running is far less than cranking, in fact it should only be the battery recharge current. If the existing cable from battery to starter ground (engine) has been sized to a minimum, then the new cable will have to be n times its copper cross section where n is the the length of the new cable compared to the old - ie, n times longer. However the existing starter ground cable may be heavier than is needed. The minimum gauge can also be calculated if you know the length and the starter motor current and the max voltage drop it can have (taking into account the +12V cable drop, plus connections).

This is a really bad idea, find the drain first, probably a door switch for dome light, glovebox light or trunk light switch. Any isolator switch must be at the battery AND at the negative side for safety reasons.

Hmmm... now that I read the full OP... I agree that it is a bad idea to fit an isolation switch to overcome a battery drain. Find the drain. Maybe it's a failing alternator diode or other component or wiring failure which will eventually burn the car to the ground. And I assume disconnecting the battery is known to work - ie, it's not a failing battery. Isolation switches are only fitted for vehicles are rarely used, or competition and maybe oft worked on vehicles - the latter 2 requiring disconnection of the vehicle-battery ground.    As for safety, it is not unsafe to isolate GND wrt shorts etc (and you may have picked up how stupidly moronic I consider hot-side isolation in cabins etc to be!) EXCLUDING the damage or stalling that could occur due to the longer cable - ie, longer the cable, the greater the risk of failures. But AFAIAK, the latter risk has not resulted in a 3rd ground being fitted to OEM vehicles - it is still common to only have 2 cables to join battery GND & chassis/body & engine/alternator without the 3rd "redundant" cable.

oldspark wrote: The way I'd do it is chassis to engine in the engine bay, and battery to switch to chassis in the cabin. Otherwise battery to switch & back out to the chassis. Two cables from the output of the switch is undesirable. You can have multiple cables from the chassis to the engine, or gearbox etc. (I usually run 2 or more in case one fails.) IMO firewalls are not good - they're too thin. The cable has to be heavy enough to not cause excessive voltage drop nor cable heat during cranking. Normal cable current with engine running is far less than cranking, in fact it should only be the battery recharge current. If the existing cable from battery to starter ground (engine) has been sized to a minimum, then the new cable will have to be n times its copper cross section where n is the the length of the new cable compared to the old - ie, n times longer. However the existing starter ground cable may be heavier than is needed. The minimum gauge can also be calculated if you know the length and the starter motor current and the max voltage drop it can have (taking into account the +12V cable drop, plus connections).

Thanks for the input. If he is dead set on doing this, I will recommend that he upgrade his cables to 2 or 0 gauge Stinger wire. And because of that, I think only one wire will go through the firewall. Leaving chassis ground connection in the cab, and engine ground connection(s) in the engine bay. So the best kind of ground would be a direct connection to the actual chassis?

Thanks for your responses. I agree with both of you. The drain should be the priority. He took it to his local mechanic, and the only thing they could supposedly find putting a drain on the battery was a gps smart tracker. Obviously, it has to be on all the time, but the manufacturer claims a continuous amp draw of only 50mA. That shouldn't be enough to drain overnight. Right?