1. Pack voltage, before anything else

An inverter is built for a specific input voltage range: 12, 24 or 48 V nominal. This is not a label detail. A “48 V” lithium pack actually swings between roughly 40 V when empty and 58 V fully charged, depending on chemistry and the number of cells in series. The inverter must accept that whole range, not just the nominal figure.

48 V became the standard for stationary storage for a physical reason: at equal power, doubling the voltage halves the current and cuts resistive losses fourfold. A 1 kW inverter at 12 V draws over 80 A at its input; the same one at 48 V draws about 21 A. Wiring, connectors and heating are in a different league.

2. Continuous power and peak power

Two numbers appear on every datasheet, and the second one misleads. Continuous power is what the inverter sustains indefinitely. Peak power lasts only seconds, long enough to start a motor.

A 150 W fridge can draw 800 W as its compressor starts. A pump or a compressor, more still. Sizing on continuous power alone guarantees cut-outs at the worst moment.

Watch the commercial arithmetic: some sheets print peak power large and continuous power small. If only one figure is given, assume it is the flattering one.

3. Pure sine wave, not “modified sine”

Cheap inverters produce a modified square wave, sold as “modified sine”. A heating element tolerates it. Almost nothing else does.

  • Motors and compressors: overheating, noise, poorer efficiency, shorter life.
  • Switch-mode power supplies (computers, routers, chargers): erratic behaviour or refusal to start.
  • Medical devices and sensitive electronics: avoid entirely.

For real household use, pure sine wave is not a comfort option, it is a prerequisite.

4. Protections, on the battery side and the output side

This is what datasheets detail least, and what determines the safety of the installation. An inverter connected to a lithium pack must at minimum provide:

  • Low-voltage cut-out, so cells are never discharged below their limit — a deep discharge permanently damages a lithium cell.
  • High-voltage cut-out, in case the charger misbehaves.
  • Thermal protection and current limiting.
  • Output short-circuit protection.

These do not replace the pack's BMS: they add to it. The BMS protects the cells, the inverter protects itself. Both are needed, and we explain the first in what a BMS does.

5. What second life changes

A second-life pack behaves differently from a new one in one specific way: its cells do not share exactly the same history, so their voltages diverge faster at the top and bottom of the range. The usable voltage window is, in practice, slightly narrower.

Two practical consequences. First, prefer an inverter whose low-voltage cut-out is adjustable rather than fixed, so it can match the pack's real limits. Second, a second-life pack copes poorly with violent, repeated current surges: slightly oversizing the inverter spares the cells.

Why we published ours as open source

The 48 V inverter market is dominated by products whose design choices nobody can check: not the schematic, not the actual protections, not the sizing. For home storage fed by a battery able to deliver hundreds of amps, that opacity seems hard to defend.

So we published cirBATTverter, our 48 V to 230 V inverter, under the CERN-OHL-S licence: KiCad sources, build documentation and our own technical review, including the faults we have not fixed.

To be clear: it is a prototype under review, not a kit ready to assemble. Several points must be corrected before anything is built, and they are listed in the open. We would rather publish an honest project than a showcase — and any documented contribution moves it forward. Details on the project page.