A push-pull inverter converts DC battery power to AC by using two switching devices (typically MOSFETs) to alternately drive current through the primary winding of a center-tapped transformer. For a 1500W continuous AC load, assuming 85% inverter efficiency, you must supply 1764W of DC power. On a 12V nominal system, this demands 147A of continuous current, requiring a minimum 200Ah LiFePO4 battery bank to maintain a safe 0.75C discharge rate and prevent severe voltage sag.
Push-Pull Inverter Topology: From Battery Bank to AC Load
The push-pull topology is a staple in low-voltage, high-current DC-AC conversion. Unlike a full-bridge inverter that requires four switches and complex high-side gate driving, a push-pull design uses only two low-side switches. This simplicity makes it popular in DIY 12V and 24V builds and older commercial modified-sine units. However, the trade-off is leakage inductance in the transformer, which generates high-voltage spikes that require robust RC snubber circuits to protect the MOSFETs.
To understand how power flows, here is the system block description from source to load:
- Source (Battery Bank): Provides raw DC voltage (12V, 24V, or 48V).
- Main DC Disconnect: A Class T fuse or DC breaker sized 125% above the maximum continuous DC draw, placed within 18 inches of the battery positive terminal.
- Push-Pull DC-DC Stage: Two MOSFETs alternately switch on and off, pulling current through opposite halves of a center-tapped transformer primary winding.
- Transformer Secondary & Rectification: In high-frequency designs, the stepped-up AC is rectified to high-voltage DC (e.g., 340V DC for 240V AC RMS). In low-frequency 50/60Hz designs, the secondary outputs AC directly.
- H-Bridge Unfolder (HF only): Converts the high-voltage DC bus into a 50/60Hz AC sine or modified-sine wave.
- AC Filter & Breaker Panel: LC filters smooth the waveform before it reaches the AC load panel and connected appliances.
Sizing Math: Battery Bank Configuration for a 1500W Load
Sizing the battery bank requires calculating the true DC draw, accounting for inverter efficiency, and applying Peukert’s law if using lead-acid chemistry.
The Math:
Target AC Load = 1500W
Assumed Push-Pull Inverter Efficiency = 85% (0.85)
Required DC Power = 1500W / 0.85 = 1764W
If you build a 12V system, the continuous DC draw is 1764W / 12V = 147A. According to NEC-style guidance (NEC 310.16, 75°C column), a 147A continuous load requires a conductor rated for at least 184A (147A × 1.25), meaning you need 1/0 AWG copper wire. If you step up to a 24V system, the draw drops to 73.5A, allowing you to use 4 AWG copper wire and significantly reducing I²R heating losses in the cables.
Series vs. Parallel Consequence for V and Ah
When configuring four 12V 100Ah batteries, your wiring topology dictates the system voltage and capacity:
- Series (4S): Voltages add, Ah remains the same. Result: 48V nominal, 100Ah total. (Best for high power, lowest current).
- Parallel (4P): Ah adds, voltage remains the same. Result: 12V nominal, 400Ah total. (Requires massive, expensive busbars and 4/0 AWG cables to handle 147A+ safely).
- Series-Parallel (2S2P): Result: 24V nominal, 200Ah total. (The optimal middle ground for a 1500W push-pull inverter).
Peukert’s Law and Depth-of-Discharge (DoD)
If you use flooded lead-acid (FLA) batteries, Peukert’s law severely penalizes high-current draws. With a Peukert exponent of 1.3, drawing 147A from a 100Ah FLA battery reduces its effective capacity to roughly 40Ah. You will experience massive voltage sag and premature cell death.
Lithium Iron Phosphate (LiFePO4) cells have a Peukert exponent near 1.05, meaning capacity remains stable regardless of draw. For a 24V 2S2P LiFePO4 bank (24V × 200Ah = 4800Wh), applying a standard 80% Depth-of-Discharge (DoD) limit yields 3840Wh of usable energy. At a 1764W draw rate, this provides approximately 2.1 hours of runtime.
| Parameter | Value | Notes |
|---|---|---|
| System Voltage | 24V Nominal (28.4V Max) | 2S configuration |
| Total Capacity | 200Ah / 4800Wh | 2P configuration |
| Continuous DC Draw | 73.5A | 1500W AC / 0.85 eff / 24V |
| Discharge C-Rate | 0.37C | Well within standard 1C limit |
| Wire Size (Battery to Inv) | 4 AWG THHN Copper | Based on 75°C column, 1.25x multiplier |
| Main Fuse Size | 100A Class T | Sized just above max continuous draw |
Charge/Discharge Limits and Lithium Safety Protocols
Operating a push-pull inverter near the limits of your battery bank requires strict adherence to charge and discharge C-rates. A standard 100Ah LiFePO4 cell typically has a maximum continuous discharge rating of 1C (100A) and a charge limit of 0.5C (50A). In our 2S2P 200Ah system, the maximum continuous discharge is 200A. Our 73.5A operational draw represents a 0.37C rate, leaving a comfortable thermal margin for surge loads like compressor startups.
When sizing the inverter/charger to replenish this bank, the charger must respect the 0.5C charge limit. For a 200Ah bank, the maximum charge current is 100A. A 24V 3000W inverter/charger with a configurable 80A AC charge profile is ideal, as it replenishes the bank at a safe 0.4C rate without tripping a standard 15A or 20A AC shore-power breaker.
| Scenario | Recommended Topology | Recommended Chemistry | Reasoning |
|---|---|---|---|
| 12V System, < 500W | Push-Pull | LiFePO4 | Low current keeps push-pull MOSFET stress manageable; LiFePO4 avoids Peukert losses. |
| 24V System, 1000W - 2000W | Push-Pull or Full-Bridge | LiFePO4 (2S) | Current drops to manageable levels; push-pull remains viable with proper snubbers. |
| 48V System, > 3000W | Full-Bridge | LiFePO4 (4S) or FLA | Push-pull requires MOSFETs with >100V Vds ratings and suffers high primary current; full-bridge is superior. |
For comprehensive wiring and overcurrent protection standards, always cross-reference your design with the National Electrical Code (NFPA 70), specifically Article 480 for storage batteries and Article 706 for energy storage systems.
Push-Pull Inverter Frequently Asked Questions
Why does my push-pull inverter blow MOSFETs on startup or under heavy load?
This is almost always caused by transformer leakage inductance. When one MOSFET turns off, the magnetic field in the primary winding collapses, generating a massive voltage spike (V = L × di/dt) that exceeds the MOSFET's drain-source breakdown voltage (Vds). To fix this, you must install an RC snubber network directly across the primary winding or across each MOSFET's drain and source. Calculate the snubber values based on your transformer's measured leakage inductance and the switching frequency.
Can I wire mismatched capacity batteries in parallel for a push-pull inverter?
No. Wiring mismatched cells (e.g., a 100Ah battery in parallel with a 50Ah battery) creates dangerous equalization currents. The higher-capacity or higher-state-of-charge battery will dump current into the weaker one, bypassing the BMS charge limits and risking lithium plating or thermal runaway. Always parallel identical cells with matched internal resistance and capacity, and use a BMS on every individual parallel string.
How do I size the inverter/charger for a 1500W continuous load?
Size the inverter's continuous rating at 125% of your maximum expected load to handle inefficiencies and ambient temperature derating. For a 1500W load, a 2000W continuous (4000W surge) inverter is required. For the integrated charger, size it based on your battery's maximum charge C-rate. If you have a 200Ah LiFePO4 bank with a 0.5C charge limit, the charger must not exceed 100A DC output. Ensure the AC input breaker can handle the combined load plus charging current (typically a 30A AC breaker for a 24V 2000W unit).
Is a push-pull topology better than a full-bridge for 48V systems?
No. Push-pull topologies excel in low-voltage (12V/24V), high-current applications because they only require two switches and simpler gate drive circuitry. However, in a 48V system, the primary current is lower, but the MOSFETs must withstand at least double the input voltage plus ringing (requiring 150V or 200V rated FETs). A full-bridge topology uses four switches, clamping the voltage stress on each switch to the input voltage, making it vastly superior and more efficient for 48V systems and power levels above 2000W.
