A full bridge inverter—commonly referred to as an H-bridge topology—uses four switching devices (typically MOSFETs for high-frequency designs or IGBTs for heavier industrial loads) to alternate DC polarity across a transformer or directly to an LC filter. By rapidly switching diagonal pairs (Q1/Q4, then Q2/Q3) using Sinusoidal Pulse Width Modulation (SPWM), the circuit synthesizes a clean 50/60Hz AC sine wave from a DC source. For off-grid solar and backup energy storage, 48V full bridge architectures are the undisputed standard for 3kW to 10kW systems. Operating at 48V nominal halves the DC current compared to 24V systems, drastically reducing I²R copper losses and allowing the use of reasonably sized conductors and busbars.
System Block Architecture: Source to Load
To understand where the full bridge sits in your power path, trace the energy from the chemical storage to the AC appliance. A properly engineered 48V system follows this strict sequence:
- Battery Bank (48V Nominal): The DC source, typically 16S LiFePO4 (51.2V nominal) or 4S Lead-Acid (48V nominal).
- DC Disconnect & Class T Fuse: A manual isolation switch paired with a high-interrupt-capacity Class T fuse (e.g., 200A to 400A) placed within 18 inches of the battery positive terminal to protect against catastrophic short circuits.
- Inverter DC Bus Capacitors: Bulk electrolytic capacitors inside the inverter that absorb high-frequency ripple current and stabilize the DC bus voltage during sudden load transients.
- Full Bridge Switching Stage (H-Bridge): The four power semiconductors that chop the DC into high-frequency AC.
- High-Frequency Transformer & LC Filter: Steps up the voltage (if isolated) and smooths the SPWM square pulses into a pure sine wave.
- AC Output Breaker Panel: Distributes the 120/240V split-phase or 230V single-phase AC to branch circuits.
For an inverter/charger unit, this path is bidirectional. When AC grid or generator power is present, a front-end rectifier converts the AC to DC, and a buck/charge-controller stage regulates the voltage to safely charge the battery bank while simultaneously passing AC through an internal transfer switch to the loads.
Battery Bank Topology and Charge/Discharge Limits
How you wire your cells or pre-packaged batteries dictates the system voltage and amp-hour (Ah) capacity. The fundamental rule is: series connections add voltage while keeping Ah constant; parallel connections add Ah while keeping voltage constant.
| Configuration | Voltage Effect | Capacity (Ah) Effect | Primary Risk / Consequence |
|---|---|---|---|
| Series (e.g., 4x 12V 100Ah) | Adds (12V x 4 = 48V) | Remains 100Ah | A single failed-open cell kills the entire string. Requires a multi-bank BMS or individual cell monitoring. |
| Parallel (e.g., 4x 48V 100Ah) | Remains 48V (51.2V LiFePO4) | Adds (100Ah x 4 = 400Ah) | Circulating currents if voltages are mismatched during connection. Requires identical cell chemistry, age, and capacity. |
| Series-Parallel | Adds then Adds | Adds then Adds | Complex balancing. Generally avoided in modern DIY builds in favor of pre-assembled 48V server-rack batteries in parallel. |
Once topology is set, you must respect the chemistry's charge and discharge limits, defined by the C-rate (a ratio of current to capacity) and Depth of Discharge (DoD).
- LiFePO4 (Lithium Iron Phosphate): Continuous discharge limit is typically 1C (e.g., 100A from a 100Ah battery). Charge limit is 0.5C to 1C. Usable DoD is 80% to 90%.
- Flooded Lead-Acid (FLA): Continuous discharge should not exceed 0.25C. Charge limit is 0.2C. Usable DoD is strictly 50% to prevent sulfation and premature death.
Sizing Math: Inverter and Battery Capacity
Let's size a system for a realistic off-grid cabin load: 4,000W continuous with a 6,000W surge (for starting a well pump or compressor), running for 4 hours between solar charge cycles.
1. Inverter/Charger Sizing
Inverters are not 100% efficient. A high-quality 48V full bridge inverter operates at roughly 90% efficiency at continuous load, dropping to 85% during heavy surge events.
- Continuous DC Draw: 4,000W / 0.90 (efficiency) = 4,444W. At a nominal 51.2V (LiFePO4), this is 86.8 Amps.
- Surge DC Draw: 6,000W / 0.85 (surge efficiency) = 7,058W. At 48V (voltage sag under heavy load), this is 147 Amps.
- Selection: Choose a 5,000W continuous / 10,000W surge 48V full bridge inverter (e.g., Schneider Conext XW Pro or Victron Quattro 48/5000). This provides a 25% thermal overhead.
- Charger Sizing: If the inverter/charger has a 100A internal AC charger, it will pull roughly 5,500W from a 240V generator (23A AC input) to push 100A into the 48V battery bank.
2. Battery Bank Sizing and Peukert's Law
Total energy required: 4,000W × 4 hours = 16,000Wh.
For LiFePO4:
16,000Wh / 51.2V = 312.5Ah required. Factoring in an 80% DoD limit to preserve cycle life: 312.5Ah / 0.80 = 390Ah total bank capacity. Solution: Four 48V 100Ah server-rack batteries in parallel.
For Lead-Acid (The Peukert Penalty):
Lead-acid capacity is rated at a 20-hour discharge rate (C/20). If you draw current faster, the usable capacity shrinks dramatically due to Peukert's Law, calculated as $T = \frac{C}{I^k}$ (where $k$ is typically 1.2 to 1.3 for FLA).
If you draw 86.8A from a bank rated for 400Ah, you aren't getting 400Ah. With a Peukert exponent of 1.25, a 400Ah bank subjected to an 86.8A draw will effectively yield only about 260Ah before voltage collapse. Furthermore, you can only use 50% of that (130Ah usable). To get 312.5Ah of usable lead-acid capacity at this high discharge rate, you would need a massive 1,200Ah lead-acid bank (twelve 2V 1200Ah industrial cells in series). This illustrates exactly why 48V full bridge systems have universally shifted to lithium chemistry.
| Component | Specification | Notes |
|---|---|---|
| Inverter | 48V, 5000W Cont / 10000W Surge | Full bridge SPWM topology, pure sine wave |
| LiFePO4 Bank | 48V (16S), 400Ah Total | 4x 100Ah modules in parallel, 80% DoD |
| DC Overcurrent | 250A Class T Fuse | Sized 125% above max continuous DC draw (86.8A x 1.25 = 108A min, sized up for surge) |
| DC Cabling | 2/0 AWG THHN or Welding Cable | Rated for 195A (75°C column), keeps voltage drop under 1% at 5 feet |
Full Bridge Inverter FAQ
Full bridge inverter vs half bridge: which is better for solar?
A full bridge (H-bridge) is vastly superior for solar and off-grid energy storage systems above 1kW. A half-bridge uses only two switches and a split-capacitor voltage divider, which limits its output voltage swing to half the DC bus voltage and requires massive, expensive bulk capacitors to handle the ripple current. Full bridge topologies utilize the full DC bus voltage, allowing for smaller magnetics (transformers/inductors), higher efficiency, and much better handling of the heavy surge currents required to start AC compressors and well pumps.
Why does my full bridge inverter blow DC fuses on startup?
This is almost always caused by inrush current charging the inverter's internal DC bus capacitors. When you close the DC disconnect, the uncharged capacitors look like a dead short circuit, drawing hundreds of amps for a few milliseconds. If your fuse is a fast-acting type (like an ANL or standard automotive fuse), it will blow instantly. Always use a Class T fuse, which has a high interrupt rating and a time-delay curve designed to tolerate capacitor inrush. Additionally, many high-end inverters require a pre-charge circuit (a momentary push-button with a power resistor) to slowly charge the capacitors before the main contactor closes.
Can I use a 24V battery bank with a 48V full bridge inverter?
No. The DC bus voltage dictates the peak AC output voltage of the H-bridge. A 48V inverter expects a DC bus of roughly 50V to 58V to synthesize a 120V RMS (170V peak) AC sine wave via its internal transformer or boost stage. If you feed it 24V, the under-voltage protection (UVP) circuit will immediately fault and shut down the unit to prevent the MOSFETs from drawing double the current and overheating. Always match the battery bank nominal voltage to the inverter's specified DC input range.
