dual battery questions

[buster 10+ year member]

I have two batteries with an isolator. I started off with the system running off the extra battery, but the voltage dropped on bass notes. I now have it running off the main starting battery that is closest to the alt and there's no voltage drop, but this defeats the purpose of the second battery because it isn't helping now. Why can't the alt keep up when the system is ran off the extra battery that is after the isolator. Do the 2 batteries have to be the same or can they be different, brand, size, etc. ?

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[n2audio 5,000+ posts]

If you don't have a seriously upgraded alternator all an extra battery will get you is more engine-off listening time. With the car running its just another load your alt. can't keep up with.

 

[buster 10+ year member]

The extra battery and isolator are stock, so I don't know if they upgraded the alt to compensate or not. The way it is set up, it will not give me more listening time because the batteries are isolated from each other when the engine is off.

 

[badboyzx2 10+ year member]

stock? what do you drive?

 

[buster 10+ year member]

It's a '98 k1500 chevy truck. I special ordered it from the factory and an "auxiliary battery" was one of the options. All the diesels come with dual batteries, but I don't think they are isolated. No one can tell me much about it because I guess it isn't that common for a gasoline engine to have stock dual batteries. I finally found some schematics on it and think it uses a relay to isolate the extra battery when the key is turned off. When the engine is running it charges it back up. I started with my system running off of this battery because if I drained the extra battery it would be isolated and the main battery would still start my truck. The problem is while running I have much much more voltage drop when running off the extra battery. Running off the main battery close to the alt, it doesn't have that much drop at all. From the alt the power wire goes to the main battery then splits off and goes to the relay and fuse box and finally to the extra battery, I assume this is the cause of the voltage drop. So now, nothing is running off the extra battery and it is serving no purpose and is probably just an extra load on my alt and because it is isolated from the main battery, it won't help start the vehicle if the main battery is drained, I think that's how it's set up. Can I just remove the battery and disconnect it or do I need to take out the relay and all that? Can I just leave the regular extra battery hooked up and get an optima for the main, could I mix different batteries the way it is wired? I've had many questions about this setup for years, but no one seems to know much about it so I turn to you guys.

 

[ftgu 10+ year member]

The main battery probably charges faster, being that it closer to the ALT. I would replace the aux. battery with a good DEEP-CYCLE,high cranking amp, marine battery with a slow discharge rate.