There are two numbers that decide whether a battery bank works, and almost everyone only calculates one of them. Amp-hours tell you how long. C-rate tells you how hard. Get the second one wrong and your bank has plenty of capacity and still collapses on the first note.
This is the actual maths, worked through with real numbers. It takes about five minutes and it will save you from buying the wrong bank twice.
Step 1: Get your real RMS wattage
Not the number on the box. Not peak, not “max power,” not what the sticker says. RMS, the continuous rating. Add up every amplifier in the car.
If you are not sure what your amp actually does, look for a CEA-2006 rating or a bench test. Plenty of amplifiers rated “3000W” make 1,800W real. Plenty of good ones make more than they claim. Guessing here poisons every calculation that follows.
2 × sub amps @ 4,000W RMS = 8,000W
1 × mid/high amp @ 1,200W RMS = 1,200W
Total = 9,200W RMS
Step 2: Know your charging voltage
Measure it. Engine running, system idling, meter on the battery. This single number decides your chemistry and changes every wattage figure that follows.
- 14.4V — stock or near-stock
- 14.8V — mild boost
- 15.5V — high voltage setup
- 16.0V — full competition
Below 15.2V, LTO is off the table entirely — it cannot reach rated output and undercharging damages it. Above 15.5V, sodium ion is at its ceiling. Our chemistry guide covers that decision in full.
Step 3: Convert watts to amps
This is where the number gets uncomfortable and people start to understand why their wiring matters.
9,200W ÷ 15.5V = 594 amps
Five hundred and ninety-four amps, continuously, through your cable, your lugs and your grounds. That is the number that makes a 0.01Ω bad connection cost you six volts. It is also the number your battery has to actually deliver.
Step 4: Size by wattage
Take your total RMS and divide by the pack’s rated output at your charging voltage. Use the True Amplifier Rating if the manufacturer publishes one — that is the figure adjusted for amplifier efficiency, and it is what your system actually sees.
9,200W ÷ 7,000W (KILO HP40 @ 15.5V) = 1.31 → 2 packs
Always round up. 1.31 packs does not exist, and running a bank at 100% of its rating continuously is not a plan.
Step 5: Size by discharge rate — the step people skip
A battery’s C-rate is how many times its capacity it can deliver per hour. A 40Ah pack at 17C delivers 40 × 17 = 680 amps continuously. That is a hard ceiling regardless of how many watt-hours are sitting in it.
594A ÷ (40 × 17) = 594 ÷ 680 = 0.87 → 1 pack
Here the wattage calculation was the binding constraint. It is not always. Compare the two chemistries at the same load:
| KILO HP40 (sodium) | AE15 (LTO) | |
|---|---|---|
| Capacity | 40Ah | 15Ah |
| C-rate | 17C constant | 50C constant |
| Current ceiling | 680A | 750A |
| Packs for 594A | 1 | 1 |
| Packs for 9,200W | 2 | 2 |
| Bank capacity | 80Ah | 30Ah |
Both banks deliver the current. The sodium bank holds nearly three times the energy for roughly the same money — which is why it plays longer between recoveries. The LTO bank holds voltage harder on a burp because of the higher C-rate. Different tools, same load.
The rule
Calculate both ways. Whichever number is bigger is how many packs you need. A bank sized only on watt-hours will sag; a bank sized only on current may run out of music.
Step 6: Sanity-check the runtime
Take your bank to 50% depth of discharge, which is where you want to stay for longevity.
(80Ah × 0.5) ÷ 594A × 60 = 4.0 minutes of continuous full output
Four minutes sounds short. It is not, because nobody plays at full RMS continuously — music has enormous dynamic range and your alternator is refilling the whole time. For burp use, four minutes of full output is somewhere around 48 five-second hits before you are at half charge.
Step 7: Now size the alternator
The alternator is not what holds your voltage up during a note. The bank does that. The alternator’s job is refilling between hits, and how big it needs to be depends on how hard you are working the bank.
| Total RMS | Alternator |
|---|---|
| Under 2,000W | Stock is usually fine |
| 2,000 – 12,000W | 200 – 250A high output |
| 12,000 – 30,000W | 300A+ high output |
| 30,000W+ | Dual 300A+, for recovery only |
At the top end, stop thinking of the alternator as a power source. You are running off the bank and topping it up between runs.
Common mistakes
- Using peak wattage. Doubles your bank size and your bill for no reason.
- Sizing only on amp-hours. The most expensive mistake in this list — you get sag you cannot explain.
- Forgetting to round up. Running a bank at its exact rating leaves no headroom for a hot day or a hard song.
- Calculating at the wrong voltage. The same pack makes very different power at 14.4V and 16.0V. Use the number you actually charge at.
- Mixing chemistries to make the numbers work. Never. One chemistry per bank.
- Stacking packs instead of fixing wiring. If your amp sees two volts less than your battery, another battery will not help.
If the answer is more than five packs
Stop and talk to someone. At that point there is usually a better way to build the bank — a bigger single unit, a different chemistry, or a wiring problem masquerading as a capacity problem. We would rather help you get it right than sell you thirteen batteries.
Or let the calculator do it
All of the above is built into our Battery Advisor. Enter your amps and your charging voltage and it runs both calculations, sizes sodium and LTO side by side, shows you the cost of each, estimates your runtime and burp count, and gives you an honest verdict — including when the cheaper chemistry is the right answer.