Big sound on your car battery
Big bass pulls a lot of power. Here's how to keep the music going and still start your car.
Where the power goes
See what happens to your battery in each setup.
Tap to try each one
How long can I play?
Engine off. Pick your setup.
That’s short. A second battery will help.
Playing music on your car battery isn’t advisable. It’s a starting battery, built for a big burst of power (CCA), not to be run down like a deep-cycle battery. Can you? Yes. Should you? No. The time above is only how long before it may not start, not a safe playing time. A weak battery drops to about 10 V when you start the car, so the starter works harder and longer, and the car may not start. Fit a second battery for music.
A second battery is for play time, not a capacitor. Capacitors only smooth out short bass dips. They don’t give you hours of music. If you fit a bigger second battery or a battery pack for a big amp, your car’s standard alternator may not recharge it fast enough while you drive. You may need a higher-output alternator, a proper dual-battery (DC-DC) charger, or both. Ask us to size that with the battery.
How we worked it out
- What your amp pulls: amp size × how loud ÷ 9.6. Music doesn't use the amp's full power all the time: background uses about 5%, party about 12.5% and full bass about 30%. The 9.6 is 12 V × 0.8, because an amp loses about 20% as heat.
- What you can safely use: battery size × the part you can use. Car battery only: about 20%. That's how far it can go before it may not start, not a safe amount to play. Second AGM battery: about 50%. Second lithium battery: about 80%.
- Play time: what you can use ÷ what your amp pulls. With the engine running, the alternator tops the battery up.
About capacitors
A lot of people are told a capacitor will fix a music system that drains the battery. That isn't quite right.
A capacitor only smooths out short bass dips, the quick drop in voltage when the amp hits hard. It doesn't add play time, and it doesn't protect your car battery the way a second battery does.
If you want longer play and a car that still starts, fit a second battery and charge it properly. A capacitor is optional for big installs that already have enough battery. It isn't the main fix.
Guideline only, see our Terms.
How not to do it
And what to do instead.
What setup do I need?
Go by your amp size.
Fine with the engine running. With the engine off, fit a second battery.
Handles the bass spikes better. Use thicker cables.
AGM or lithium, with a DC-DC charger so your car battery stays full. How it works
Is your battery struggling?
Watch for these signs.
Pop in for a free battery test. What the result means
Not sure what to fit?
Send us your amp size and car, and we'll suggest the right battery.
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General guidance only. Have big systems fitted by a qualified installer.