To run a standard 1/2 HP (approx. 600W input) 120V AC motor off a battery bank, you need a 12V 100Ah LiFePO4 battery, a 1200W pure sine wave inverter with a 2400W surge rating, and 2 AWG copper cabling protected by a 150A Class T fuse. This combination handles the continuous draw and the massive inrush current of an inductive motor without tripping the battery management system (BMS) or starving the windings.
Sizing battery powered motors is not as simple as matching wattage. Motors are inductive loads that demand high surge currents at startup, and battery chemistry dictates how much of its rated capacity you can actually use under heavy draw. Below is the exact engineering path to build this system.
System Block: From Battery Terminals to Motor Shaft
A reliable DC-to-AC motor drive requires a strict sequence of components. Skipping the protection or conversion stages will result in voltage sag, melted terminals, or a dead inverter.
- Source: 12V 100Ah LiFePO4 Battery (provides stable 13.2V under load).
- Protection: 150A Class T Fuse (mounted within 7 inches of the positive terminal to protect against dead-short fires).
- Conduction: 2 AWG stranded copper welding cable (keeps voltage drop under 1% at 50A over a 3-foot run).
- Conversion: 1200W Pure Sine Wave Inverter (converts 13.2V DC to 120V AC, handling inductive phase angles).
- Load: 1/2 HP 120V AC Induction Motor (the physical work output).
Sizing Math: Peukert’s Law, C-Rates, and Inverter Efficiency
Let’s calculate the exact DC current required. A 1/2 HP motor produces 373W of mechanical work. Assuming a typical 75% motor efficiency and a 90% inverter efficiency, the DC electrical input required is:
373W / 0.75 (motor) / 0.90 (inverter) = 552W DC Input
At a nominal LiFePO4 operating voltage of 13.2V, the continuous current draw is 41.8A (552W / 13.2V). We will design for a 45A continuous baseline.
The Peukert Penalty
If you attempt to power this battery powered motor with a 100Ah Flooded Lead-Acid (FLA) battery, you will fail. Peukert's Law dictates that as discharge current increases, the effective capacity of lead-acid batteries drops exponentially. A 100Ah FLA battery rated at the 20-hour rate (5A draw) has a Peukert exponent of roughly 1.3. When pulled at 45A, its effective capacity shrinks to about 65Ah. You would hit a 50% depth-of-discharge (DoD) cutoff in under 45 minutes.
Lithium Iron Phosphate (LiFePO4) chemistry has a Peukert exponent near 1.05. A 100Ah LiFePO4 battery pulled at 45A will deliver roughly 95Ah of usable capacity. According to LiFePO4 chemistry data, this flat discharge curve is what makes lithium mandatory for high-draw motor applications.
Series vs. Parallel: Voltage, Amp-Hours, and Cell Matching
When scaling battery powered motors for larger loads (like a 1HP lathe or a winch), you must decide whether to wire batteries in series or parallel. The consequences for voltage (V) and amp-hours (Ah) are absolute:
| Configuration | Voltage Consequence | Amp-Hour (Ah) Consequence | Primary Use Case |
|---|---|---|---|
| Series | Doubles (12V + 12V = 24V) | Remains Same (100Ah) | Halves DC current draw; allows smaller wire gauges and higher-power inverters (e.g., 2000W+). |
| Parallel | Remains Same (12V) | Doubles (100Ah + 100Ah = 200Ah) | Extends runtime at the same voltage; maintains 12V accessory compatibility. |
The Golden Rule of Parallel Banks: Never parallel mismatched cells, batteries of different ages, or different chemistries. If you parallel a new 100Ah battery with an older 100Ah battery that has higher internal resistance, the newer battery will force current into the older one during discharge, causing severe overheating and BMS failure. If you need more capacity, buy a single larger battery (e.g., a 200Ah or 300Ah unit) rather than paralleling smaller ones.
Inverter and Charger Sizing for Inductive Loads
Sizing the inverter for a motor requires looking past the continuous running wattage and focusing on the Locked Rotor Amps (LRA) or inrush current. When an AC motor starts, the rotor is stationary, and the windings act almost like a dead short until the magnetic field builds and back-EMF is generated.
A 1/2 HP motor drawing 552W continuously will typically pull 3 to 5 times its running current for 1 to 2 seconds during startup. That means a surge demand of roughly 2200W to 2700W. If your inverter's surge rating is only 1500W, it will fault and shut down the moment you flip the motor switch.
Charger Sizing Limits
Once the motor stops, you must recharge the bank. LiFePO4 batteries accept charge much faster than lead-acid, but you must respect the manufacturer's C-rate limits. The standard maximum charge rate is 0.5C. For a 100Ah battery, this means a maximum charge current of 50A. Pairing the battery with a 12V 50A AC-to-DC smart charger (like a Victron Blue Smart IP22) will safely recharge the bank from 20% to 100% in roughly 1.5 hours without degrading the cell chemistry.
The Decision Tree: Final Component Selection
Use this decision path to finalize your exact bill of materials for a 1/2 HP battery powered motor setup. Do not downgrade the wire gauge or fuse class to save money; DC arcs at 12V are notoriously difficult to extinguish.
| Decision Point | Condition / Requirement | Concrete Selection |
|---|---|---|
| Battery Chemistry | Must handle 45A draw without Peukert capacity loss; 80%+ DoD. | LiFePO4 (Lithium Iron Phosphate) |
| Battery Capacity | Must supply 45A for minimum 1.5 hours; 0.5C max charge rate. | 12V 100Ah (e.g., LiTime 12V 100Ah Plus with built-in 100A BMS) |
| Inverter Type | Must output clean AC for inductive windings; handle 2500W surge. | Pure Sine Wave, 1200W Continuous / 2400W Surge (e.g., Victron Phoenix 12/1200) |
| DC Wiring | Must carry 50A continuous + 100A surge with <3% voltage drop over 6ft total loop. | 2 AWG Stranded Copper Welding Cable |
| Overcurrent Protection | Must interrupt 10,000A+ fault current at 12V DC without sustaining an arc. | 150A Class T Fuse with block (e.g., Blue Sea Systems 5191) |
By following this exact specification, your battery powered motor will start reliably under load, run at full torque without voltage sag, and recharge safely without degrading the battery bank. For further reading on DC system safety and renewable integration standards, refer to the U.S. Department of Energy's solar and storage guidelines, which outline best practices for battery enclosures and ventilation in DIY power systems.
