Do I need a battery isolator?

[shpatb 10+ year member]

I don't play music with the car off, ever. I have heard that each battery tries to charge the other one and they drain each other. I just want to get a second battery to be safe. Don't notice any dimming but I do have HIDs.

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[smoka 10+ year member]

You are right. They best way to not have to use an isolator is to get two identical sized batts with the near same datecode on them. This is your best chance that the resting voltages will be the same. But if you want to mix an old and new batt, then better use an isolator...