batteries banks.

[audiofanaticz]

The only reason to use an isolater is when you are playing your system with the car off, you would then use the isolater on your main car battery that starts the engine so it does not get drained.

Isolaters are old news, and any other reason to use one is not beneficial in any way, especially if your electrical system is still going to be tied together off the same charging point IE the alternator which would bypass said isolater.

Isolaters are bad, playing your stereo with the car off is bad too. All thats going to happen from that is your going to turn the vehicle on and put your alternator in overload mode trying to charge those dead batteries, and thats how your alternators crap out.

If the battery is half dead or deader you should charge the battery on a charger before even thinking about starting the vehicle.

 

As for the small battery where is this going to be located?  In the back with the big batteries? In the front as a starting battery?

If its in the front away from the big batteries I wouldn't worry much since current will pull from the path with least resistance and that would be from the rear battery by the amp which will take the grunt of the abuse for your electrical needs, and whatever it doesn't get from there it will start coming from the front battery and alternator. if you have a clamp meter you can test this yourself, the current going through the wire between your amp and back battery will be much larger than the current going through the wire that connects your alt and front battery to the rear battery.

As said above the voltage thing is also something to look at, typically as long as they are all AGM batteries they should have similar resting voltages (resting meaning the vehicle off for 24+hours). If one of those batteries has a lower resting voltage the higher resting voltage battery will discharge to the lower resting voltage battery and it becomes a drain on the system. The only time the smaller batter having a lower resting voltage would matter would be when the vehicle is off for extended periods of time and the float charge from your alternator(s) from the last time the vehicle was ran starts to go back down to the batteries normal resting voltage.
 

Now Im not sure what your goals are, but it seems to me like you want to blow money on senseless cheap battery crap because its cheaper from what Im gathering with your recent battery posts.

So what I would do is stop right there, buy a good battery (XS Power, Northstar, Odyssey, BatCap, hell even Kinetik) since these batteries are more less designed around our use and abuse we give them.
If you dont have enough cash to buy multiple nice batteries right now just buy 1 or 2 and save up some more like everyone else does becuase buying a bunch of batteries that are not geared to how we use them is only going to end in additional wasted money when you buy the real batteries that you should have got from the start.

[ShadeTreeMechanic]

The batteries should be the same size ideally. When a small battery is in parallel with a big battery, everything is fine until the reserve capacity is used up in the small battery. The big battery has to try to charge the small battery because their voltage is the same from being in parallel. At this point it would be better not to have the small battery at all. The reason people can get away with having a smaller starting battery in parallel is because when the engine is running, the alternator charges the small battery. This takes away from the alternators capacity to charge the big battery, but the big battery is free to run the amps because it also gets a charge from the alternator.