battery isolator vs relay
I've got a question about using a relay (cheap) vs a battery isolator/diodes (expensive!)
Before I go any further, I know about the "reason" why you should never use a relay, namely once the relay closes the drained battery will draw 47 billion amps from the charged battery, damaging both batteries and melting wires and causing large fires in the process.
But instead, what if the relay was attached to close only about 5-10s after the engine was started?
Then the power will be provided by the alternator instead of draining the primary battery, this should work, no?
Alternatively, can I just build a battery isolator using a couple of diodes? Hell, even those diodes are much more expensive than a relay. You'd need what, probably 100-120A diodes, or 2x60? At like $10 each you'll need what, 4 of them? Still not a very cheap venture.
[Edit: I'm on crack, you can get MBR6045's for like $3 each... $12 for diodes + wires + heatsink isn't that expensive. And yeah you can buy them for $40 or so, the place I was looking at was grossly overpriced.]
I tried searching for topics like this here and didn't find any.