Adding a 2ed battery the easy way..

I work a electrical distributing warehouse where we sell Isolators for boats and the Sure Power Isolator’s really seem the best way to go.

How much for the islolator and the switch?

Your 1st pic is exactly how my setup is. Works perfect. But the 2nd - isnt it redundant to have both isolator and perko? Dont they both do pretty much the same thing?

The first picture allows you to run auxiliary equipment (stereo) off the second battery and have it charge off the engine alternator without the operator having to operate any switches.

The second picture allows the same thing, plus it allows for starting the engine off the auxiliary battery. Eventually you will get to the point where your main battery is dead (perhaps it is old, or perhaps you left the running lights on all night) but the auxiliary still has enough to get you started. Just turn the switch to BOTH, start the engine, then remember to turn the switch back.

The installation as shown, and the "Sure Power Isolators" mentioned, are the rectifier (diode) style and have a significant drawback: the diode drop will significantly reduce the amount of output you will get from the alternator (note that the top wire in the diagram is the connection to the engine alternator).

Unless your alternator has "remote sense" (not available on any stock alternator, sometimes available on after market alternators) the diode drops will result in the voltage the battery sees being 0.2 volts up to 0.7 volts (depending on the style of diodes used in the isolator)lower than what the alternator is putting out. Since the alternator is regulating based on what it sees it will cut the current back much sooner which will result in the battery charging taking much longer than it would otherwise.

Note that you will still be able to get full current out of the alternator for running the stereo, lights and so on, but if you have run the battery down and need to get it charged back up it will take a lot longer.

I prefer the use of the "battery combiners" which eliminate this drawback. They really aren't any harder to install and eliminate this problem.

Well for the most part this set up is easy and low cost and it is one of the right ways to do it, I am not saying that Rod is wrong, But there is a few different ways of doing this and this is one of them, I am setting a second battery up like this and I am using the Optima blue top battery for my second battery, If I drain my battery then something is wrong here, In other words I would be spending more time on the beach than on the boat boarding and skiing. With a blue top battery and 15000 watts I know I should get a few hours of sound and not having to worry about draining my battery. With this set up and my cost on everything including the battery about 225.00.