Car Audio Battery and Alternator Sizing: Lithium vs AGM
Size a car audio electrical system from the amplifier’s verified RMS output, its operating voltage and the way you use it. Then compare demand with the alternator’s available output after vehicle loads. The battery must cover shortfalls and provide enough usable energy for the session.
Quick sizing sequence: amplifier current → available alternator current → battery current and runtime → compatible charging, cable and fusing.
A larger battery can extend reserve. A larger alternator can replace energy faster while the engine is running. Both depend on a suitable current path and charging voltage.
Calculate Amplifier Current from RMS Power
Use the RMS rating at the intended speaker impedance and supply voltage. Add every amplifier in the system, including mids and highs. Peak or MAX figures are unsuitable for this calculation.
Estimated DC current (A) = amplifier output (W) ÷ [voltage (V) × efficiency]
The table uses 80% efficiency as a Class D planning assumption. Use documented or measured efficiency where available; it changes with amplifier design and operating conditions. These figures describe estimated demand while delivering rated output, not average current during every song.
| Amplifier RMS output | At 14.4V, 80% | At 15.8V, 80% |
|---|---|---|
| 1,000W | 87A | 79A |
| 3,000W | 260A | 237A |
| 5,000W | 434A | 396A |
| 10,000W | 868A | 791A |
| 20,000W | 1,736A | 1,582A |
Higher voltage reduces the calculated current for a fixed output and efficiency, but increasing voltage is only acceptable when the batteries, amplifiers and all connected vehicle equipment support it. Measure at the amplifier during load; the alternator setpoint may be higher than the voltage that reaches it.
Separate Music Demand from Continuous Test Output
Ordinary music, extended rebassed demos and test tones draw different average current. An SPL burst can demand high current for seconds while using relatively little energy. A long demo can remove much more energy, even if its peak current is lower.
Do not assign every system the same music duty factor. Record average DC current over representative playback at the intended volume, alongside short peaks and loaded voltage. Before measurements are available, calculate several clearly labelled scenarios.
Find the Alternator Current Available to the Audio System
Available audio current = alternator output at the operating condition − vehicle electrical load
Cooling fans, headlights, pumps, engine electronics, steering and other accessories use part of the alternator output. A 320A label does not mean the amplifier receives 320A at hot idle. Ask for an output curve and confirm engine speed, pulley ratio, temperature, charging voltage and vehicle fitment.
For a published example of this distinction, Delco Remy’s 40SI specifications list 190A idle output for both its 300A and 320A versions. These are heavy-duty 12V alternators; the figures illustrate rated versus idle output and are not a passenger-car fitment recommendation or evidence of performance at 15.8V.
Use the high-output alternator sizing and fitment guide for the mechanical, regulator and installation checks.
Worked Example: 5,000W RMS and a 320A Alternator
Assume a 5,000W Class D amplifier operating at 15.8V with 80% efficiency. For this example only, assume the 320A-rated alternator can provide 240A when hot at cruising engine speed and 180A when hot at idle, while the vehicle uses 60A. Replace those assumed outputs with data for the actual alternator at the intended voltage and RPM.
Full-output amplifier demand: 5,000 ÷ (15.8 × 0.80) = 395.6A
Audio supply at cruise: 240 − 60 = 180A
Audio supply at idle: 180 − 60 = 120A
| Example condition | Amplifier current | Battery shortfall at cruise | Battery shortfall at idle |
|---|---|---|---|
| Continuous 5,000W output | 395.6A | 215.6A | 275.6A |
| Assumed 2,500W average music output | 197.8A | 17.8A | 77.8A |
The second row assumes average music output is 50% of the rated output, with the same efficiency. It is an illustration, not a universal music figure. Under that assumed idle condition, ten minutes removes approximately 77.8 × (10 ÷ 60) = 13.0Ah from the battery.
13Ah is the estimated charge used during that example, not the recommended bank size. The bank also needs sufficient current capability, usable capacity, voltage margin and remaining reserve. Changing voltage, temperature, losses and cell condition affect the result. The alternator needs surplus capacity during quieter periods to recharge the bank.
Check Battery Current Capability and Usable Energy Separately
Amp-hours describe charge capacity. Watt-hours describe energy and also account for voltage. Neither rating alone tells you whether a bank can safely deliver a high-current event.
Current from a specified C-rate (A) = capacity (Ah) × C-rate
Approximate stored energy (Wh) = nominal bank voltage × Ah
Use the actual cell capacity, parallel count and approved continuous or burst rating. Burst duration matters. The finished bank may be limited by its interconnects, terminals, fuses, cable, temperature or loaded voltage before the cell-level calculation is reached.
Toshiba’s SCiB high-power cell specifications list the 2.9Ah cell at 2.4V nominal and state the conditions for its calculated pulse-power figure. A cell’s short-duration power figure does not establish a finished bank’s continuous amplifier rating.
For engine-off listening, alternator contribution is zero. Estimate usable watt-hours and divide by the average total electrical load. Use the battery playtime guide for that calculation and the 3,000W to 30,000W+ battery size guide for conditional bank-selection starting points.
AGM vs LiFePO4 vs SCiB LTO
Compare the exact battery and charging requirements, rather than treating all lithium batteries as interchangeable. AGM is a lead-acid design; LiFePO4 and LTO are different lithium chemistries.
| Battery type | Where it can fit | What to verify |
|---|---|---|
| AGM | Daily systems where an appropriately rated lead-acid battery meets demand within the available space and weight | Charging profile, usable capacity, discharge performance and whether repeated audio cycling suits the model |
| LiFePO4 | Systems needing usable energy with lower weight than a comparable lead-acid installation | Exact continuous and burst limits, charging voltage, temperature limits and intended application |
| SCiB LTO | High-current daily, demo and SPL builds using an appropriately configured bank | Cell model, series/parallel configuration, charge compatibility, available reserve and the complete current path |
A healthy existing battery and charging system may be sufficient for a modest daily setup. Confirm that with loaded measurements. High-current SCiB cells are useful where delivery and charge acceptance matter, but more current capability does not automatically provide longer playtime. The LTO, AGM and LiFePO4 comparison covers the chemistry choices in more detail.
Match Charging Voltage to Every Connected Component
For applicable Evolution Lithium 6S SCiB LTO banks, the preferred charging range is approximately 15.6–15.9V, subject to the instructions supplied with the bank. Six 2.4V nominal cells give 14.4V nominal. The 16.8V absolute bank ceiling is not a daily charge target.
Before raising voltage, check the limits of the starter battery, vehicle electronics, head unit, DSP, amplifiers, regulator and accessories. Do not directly parallel different chemistries without a charging and isolation arrangement approved for the actual combination. An active balancer equalises cell voltage; it does not limit alternator current or make an incompatible charging voltage safe.
Install lithium banks securely in the rear cabin or boot, never under the bonnet. See the front battery and rear lithium wiring guide for the system-planning considerations.
Allow for Cable, Grounds and Fuse Limits
Select conductors from expected current, total route length, installation conditions and acceptable voltage drop. Protect cables through metal, use suitable crimped lugs and confirm the ground return path. A Big 3 upgrade can reduce resistance; it does not increase the alternator’s generating capacity.
Choose fuses within the safe ratings of the protected cable and hardware. A cable joining two batteries can receive fault current from either end, so it needs source protection close to both batteries. A fuse sized from amplifier marketing wattage or battery C-rate alone is not a sound design.
Voltage drop = current × resistance
At 500A, a total path resistance of 0.005 ohm causes a 2.5V drop. Measure losses under load before buying additional battery capacity.
Measure the Finished System Before Increasing Output
- Record charging voltage, alternator output and vehicle load at the intended RPM and temperature.
- Compare voltage at the bank and amplifier terminals during the same controlled load.
- Check whether voltage recovers between peaks or steadily declines over a session.
- Inspect terminals, fuse holders and cable connections for abnormal heating.
- For lithium, check individual cell voltages as well as total bank voltage.
- Stop if voltage leaves an approved range, a cell diverges or hardware heats abnormally.
If the bank holds voltage while the amplifier reading falls, investigate the current path. If both fall over time, examine charging contribution, state of charge and usable reserve. The car audio voltage-drop guide explains those measurements.
Battery and Alternator Sizing Questions
Is a 320A alternator enough for a 5,000W amplifier?
It depends on actual output at operating temperature and RPM, vehicle load and average audio demand. At 15.8V and 80% efficiency, full amplifier output needs about 396A before vehicle loads. The bank must cover any shortfall, and quieter periods must allow recovery.
Does a 1,000W daily system always need lithium?
No. Check the existing battery, alternator and wiring first. Lithium may suit reserve, weight or future-power requirements, but the amplifier rating alone does not require it.
Can more Ah compensate for an undersized alternator?
More usable capacity can delay discharge. If average demand continues to exceed charging input, state of charge will still fall. Reduce demand or provide sufficient charging and recovery time.
Why can the Bank Builder differ from a measured calculation?
The Evolution Lithium Bank Builder currently assumes 80% amplifier efficiency and treats 75% of the entered alternator rating as usable by the audio system. That is a simplified net contribution, not a measured hot-idle output. For a detailed budget, replace that allowance with actual alternator output minus vehicle load, rather than subtracting the vehicle load twice.
Choose a Bank from the Complete Electrical Budget
Bring together amplifier demand, available charging, battery current limits and usable reserve. Then confirm the voltage compatibility and current path before selecting the final bank. A recommendation based on wattage or Ah alone leaves too many of those questions unanswered.
Contact Evolution Lithium with your amplifier RMS ratings, final impedance, measured voltage, alternator details and intended music, demo or SPL use.
