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Don't just assume a bigger sub will kill your battery without checking the actual power draw specs - that's where people go wrong. A 12-inch subwoofer's impact depends on its RMS rating and how hard you're pushing it, not the size alone. A quality powered sub rated at, say, 200-300 watts RMS will drain noticeably more than a smaller one, but modern car alternators handle that fine if your electrical system is in decent shape. If you're already running a lot of other gear or have an older battery, you might need an upgrade there, but the sub itself shouldn't be a dealbreaker.
The real battery killer is continuous high-volume use, not the subwoofer size itself - a 12-inch with modest RMS ratings will draw far less power than a smaller one cranked to max all the time. What people miss: check whether your alternator can handle the sustained draw, because even if your battery survives the initial surge, a weak charging system will drain it faster than the sub itself uses power.
I've swapped out car audio gear enough times to know people panic about battery drain way too early in the process. The thing is, a 12-inch sub won't automatically murder your battery just because it's bigger. What actually matters is the RMS power rating - that's your sustained power draw, not the peak numbers manufacturers love to advertise. A quality 12-inch rated at maybe 500-600 watts RMS will draw less juice than some cheap 10-inch that's pushing 1000 watts peak all day long.
The previous answers are pointing at the right issue but missing some nuance. Yes, continuous cranked-up listening will drain faster than occasional use, but that applies to any subwoofer. Where people really get burned is pairing a power-hungry sub with an undersized alternator or a car battery that's already working hard for other upgrades. If your car's electrical system is solid - decent alternator, healthy battery - a properly rated 12-incher won't be a problem for normal driving. Even highway use where your alternator's running at full capacity should keep things in balance. It's the combination of factors that kills you: cheap high-wattage gear, a tired battery, weak alternator, plus hammering the bass nonstop.
Check the RMS specs on whatever 12-inch you're eyeing, compare it to what your car can actually charge (alternator amperage), and be realistic about how you'll actually use it. Most people aren't running full volume for hours straight anyway.
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