Self-described "tinkerer and do-it-yourself-er" Chris Jones has taken a cheap inverter to bits, for good reason: to unlock an artificial restriction, which locked it to first-party branded battery packs.
"I noticed the Greenworks G-Max 40v inverter (IV40A00) was possible to pick up for much less [than rival devices], $30-$50 depending on where you look and if there was a sale," Jones explains. "So I picked one up just to mess around with, even though I did not have one of their batteries. To my disappointment I quickly found out that they actually have two versions and the latest version actually expects a special signal to enable the inverter."
Designed to improve the safety of the device by preventing the use of potentially dangerous out-of-spec battery packs or to boost the manufacturer's bottom line by forcing users to buy their own-brand batteries, depending on which angle you want to take, the restriction in the inverter's design made it unsuitable for Jones' existing pile of battery packs. Rather than send it back, though, Jones instead took the device to pieces to figure out if it was possible to lift the restriction.
"After taking the unit apart it was clearly apparent that this was going to be far more complicated than I originally thought," Jones admits. "My first thought was I could try to reverse engineer the communication protocol, but after probing around a bit this seemed like it might not even be needed, plus that would require me to add an additional device if I wanted to use other batteries. So I wanted to stick with KISS [Keep It Simple, Stupid] and find out how this thing worked."
What followed was a process of reverse-engineering, figuring out what connects to where and whether datasheets were available for any of the various chips dotted around the device. Finding a microcontroller which appeared to control the inverter's operating logic, Jones made a breakthrough: "The blue trace from the 'mystery' chip might be a pull down to activate the 8051 microcontroller which might handle the actual logic for managing the inverter. So I did what any hacker would do and I broke out the soldering iron and a resistor to see what might happen if I pulled the line low with a lower value resistor."
The answer: disabled the lockout, allowing the inverter to be switched on and off regardless of its power source. "After cleaning up my solder spots to remove any residual flux and putting down a generous glob of hot glue to make sure there was no stress put on the joints I reassembled the unit and it was good to go," Jones says. "I tested it with every battery I had that matched the voltage range and didn't see any issues."
The full project write-up is available on Jones' Hackaday.io page, but comes with a warning: "This is working with mains level voltages, charged capacitors and high current batteries that could likely injure or kill someone, this is not advised, and is very dangerous," Jones writes, "so I don't condone anyone to attempt this if you are not comfortable and understand the precautions."