Customers often ask whether they need to add an extra battery when installing a large car audio system with multiple amps, speakers, and subwoofers. The answer depends on how the system will be used — and in many cases, an alternator upgrade is the more important consideration.
Understanding the Problem
Your vehicle leaves the factory with a battery whose primary job is to start the engine. The alternator keeps the battery charged and supplies power to the entire electrical system while the engine is running.
Large audio systems use high-output or multiple amplifiers that draw significant current from that electrical system. There are two scenarios where this becomes a problem:
- You want to run the stereo for extended periods with the engine off.
- Your system's current draw exceeds the alternator's output capacity even when the engine is running.
The solution is different for each scenario. Calculate your system's current draw first — then you'll know which one applies.
Scenario 1: Music With the Engine Off
If your only concern is listening with the engine off, you can estimate run time using this formula:
10 × 75 ÷ 500 = 1.5 hours
If you need more run time than this formula produces, add another battery. Each additional battery of the same rating adds proportionally to the total run time.
Scenario 2: System Outruns the Alternator
If the audio system's current draw exceeds the alternator's output while the engine is running, the dashboard and headlights may dim when loud bass notes hit. The power shortage can also cause amplifiers to overheat and shut down.
Calculate your system's current draw using Ohm's Law: I (amps) = P (watts) / E (volts).
I = 1,000 ÷ 14.4 = 69.4 amps
Most stock alternators output between 60 and 120 amps total — for the entire vehicle. A 1,000W system alone can consume most or all of that.
Alternator Upgrades
Although upgrading the alternator costs more than adding a battery, the electrical system will perform significantly better in the long run. Some very high-power installations use multiple alternators to achieve adequate amperage. The MTX Jackhammer Dodge Ram build shown in this article uses two batteries and three alternators to support a 4,000W RMS system.
When upgrading the alternator, also evaluate the cable between the alternator and battery. Car manufacturers size this cable to limit current as protection for the alternator. Upgrading the ground wire is also necessary when installing a higher-output alternator. If you keep the factory alternator, leave the factory wiring in place.
