Dimming lights? Voltage drop? Let's size it right.

Car Audio Battery & Big Wire Calculator

Enter your alternator output and amplifier power. You'll see how many amp-hours of AGM battery to put in the back, how many 1/0 OFC power runs to bring from the alternator to that rear bank, and every step of the math.

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01Alternator output at idle

amps

What the alternator makes at hot idle (stopped at a light, parked). Usually 50–70% of its max rating — check the spec sheet.

02Alternator max output (full RPM)

amps

The rated output at high RPM — the number on the alternator's label or listing.

03Total amplifier power

W RMS

Add up every amp's RMS rating at the lowest impedance you'll run it.

04Amplifier efficiency

Use the manufacturer's figure if listed. Mixed amps? Use a blend weighted by wattage.

05How hard you play it

Music averages well below an amp's full rating. This sets the average draw used for play-time math; peaks always use full power.

Fine-tune the assumptions

Vehicle's own load (lights, fans, ECU)

Play time at idle to plan for

min

System voltage under load

AGM usable depth of discharge

Max bass-peak draw per Ah of AGM

A/Ah

Front-to-back run length

Max voltage drop on the runs

Continuous rating per 1/0 OFC run

Charging-system ground rules

Do the Big 3 first. Upgrade alternator +, engine ground and chassis ground to 1/0 OFC.
Fuse both ends of every battery-to-battery run, within 18 in of each battery.
Ground it as well as you power it. Match each run with a solid chassis ground at the rear bank, or run matching 1/0 grounds.
Match the bank. Same brand, size and age for every AGM in the rear bank.
Batteries only buy time. If the alternator can't keep up while driving, upgrade the alternator.

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Your results Rear AGM bank + front-to-back 1/0 runs

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