batteries or caps?

i have to agree with haemphyst..... caps are mostly hype... not worth the price of admission most times..... do the big three and u will be in better position to bump hard and do it for a long time... and your equipment will thank u for it.........

********smacks himself***********

don't mind me... i'm just hung over........

If your alternator is big enough to handle both batteries no isolator is needed.  The batteries won't kill each other because once they equalize there will be no difference in potential so no electrons will flow between them.

The problem most people have is they arn't willing to spend the money for a bigger alternator, so they throw a 2nd battery on the OEM sized alternator, causing it to strain. 

I'm confused.

Doesn't "equalizing" mean that the battery with the lower ESR drain off the other one - and - uhh - kill it?

Are you kdding? Two years?  If this was going to kill the alt it would have done so already.  Hooking two (or three!) charged batteries up to an OEM alt doesn't strain it any more than one does - this depends on amperage demanded of the batteries - not just having two hooked up.

I want you to know dude - I've been hammering away at this point - much to my popular deteriment - on this board for years.  Your sig confirms it, HO alts are totally worthless - what matters is current at 12.5v, everything else is a red herring. 

No, it means that if battery A is 13.654 volts and battery B is 13.652 volts (and they are identical batteries) then battery A will charge battery B to around 13.654 volts.  At that point, both power supplies are equal, and since there is no difference in potential no current will flow.  It's the same concept of installing a cap (a power storage device).  Once the two power storage devices stablize then no current flows.

This is an exception, and not a rule.  All cars are different, and most alternators are different.  If you put three charged batteries in the vehicle I would agree that at first there would be minimal load added.  But, as you draw current away from the batteries at a rate faster then the alternator can charge you'll stress the alternator.  Heaven forbid you leave the lights on for 30 mintues with the car off (or listen to the radio with the car off).

Now, your alternator has 3x the load to charge back up, which will make it work 3x harder.  It's simple math if you have a 100 amp alternator and a 150A load eventually you're going to have problems.

Huh.  Okay - I'm not trying to argue, just looking for a more complete explanation (and you do have that "i" and that third star) - but how does the ESR thing play into this?  Is this too a red herring?

I guess this is the biggest issue I have with your explanation - you take careful pains to paranthetically state "(and they are identical batteries)" - is that you don't explain why they 'equalize' with identical batteries, but you avoid what happens when diffferent batteries are used or how this impacts your logic here.  I feel this is a wholly incomplete explanation that, as stated, is full of holes.