Lots of possible approaches...

Why I would isolate: If I wanted to run a load that drained a battery significantly but wanted to be sure I still had sufficient power to crank the engine.

Why not? The engine should be running when winching.. If not its dead (the engine) and cranking it over is not a big issue.

The other would be that your primary battery is not a deep cycle and a long winching cycle would reduce its life.

Decent reason if you winch a lot. A better solution being two deep cycle batteries.

What I would go with: Two deep cycles batteries (starting capable) in parallel with no fancy isolation switch. An issue with letting one drain without the other is charging them. OT me it makes more sense to let them both run the load and dischage to the same voltage level. A good practice is to make sure they are both in good condition and similar chemistry.

As far as spikes and surges go: The winch motor can draw a lot of current. When you turn it off the load side of the motor ( connected to the windings) will see a large inductive voltage spike. The good thing is the relay contaces isolate the rest of the sytem from the motors windings as the relay opens. A good desing practice is some sype of filter across the relay contacts. Your relay should have this built in.

On the battery side you will also see an inductive spike. This is caused by the power cables since they are carrying current and will generate field.

Once the current is shut off this field collapses generating a voltage. The greater the current and the more inductance your cabling has the greater the level of a spike you get.

This says your winch power relay should have short cables to the battery to it to reduce the inductance.

The batteries will absorb some of this spike but their resistance goes up with the voltage. A good way to shunt this voltage is with some type of protection diode and or a capacitor that has a low ESR.

The alternator has protection diodes built in to it as do almost all vehicle electronics. Remember your starter motor is connected to the battery and will do the same thing. The size of the pulse again depends on the current draw and the inductance of the cables.

So far starting my vehicle has not caused any problems... There are a number of other tricks and tips to putting together a robust system, at some point its overkill.

A good way to find out how far you need to go is to put a acope across the battery terminals. Many people ignore these details and never have a problem. If it were my vehicle I would verify.


Kevin