A Chance to Build

Thanks to an end-of-season clearance, I acquired the tool, two batteries, and a charger for about $250—$50 for the trimmer, $25 for the charger, and $85 each for the batteries. This investment aligns with the costs of other battery systems while offering the flexibility to swap out my 48V 5Ah (240Wh) batteries as needed. Having long envisioned this project, I saw this sale as an ideal opportunity to create a backup power system.

How It Works

With five batteries on hand, I can manage around 1kW, allowing for some downtime between cell swaps. During a power outage about a year ago, I powered our fridge for approximately 12 hours using a 48V 10Ah eBike battery without issues. Using a Kill-a-Watt meter, I discovered that our fridge consumes around 1kWh in a 24-hour period, significantly less if it remains closed. By connecting some emergency solar panels, I could likely charge one or two of the 5.0Ah batteries daily—it's not perfect, but it’s a start.

A Word of Caution

This project carries inherent risks and is not without hazards such as fire or electric shock. It also voids any warranties or responsibilities from Snapper or Briggs & Stratton. While I’m not a professional, I respect the power involved in this setup. This endeavor is purely personal, undertaken under my Right to Repair, and I advise against replicating it due to potential serious injury or damage.

Disassembling the Equipment

The initial phase involves dismantling the hedge trimmers to utilize the battery cradle for a household inverter. I'm specifically looking to access the battery cradle and replace the power switch. While this isn't the most efficient approach, it serves as a straightforward "drop-in" solution to power our fridge and freezers during emergencies. By removing the cradle, I can also take measurements and possibly 3D print my own connectors.

In the long run, I aim to integrate the charger and battery bay into a system that can easily switch between "charging" and "discharging" using a large switch. This will be a double-pole double-throw (DPDT) switch, ensuring that only one function operates at a time: either charging or powering devices, with an additional switch for main power—likely a standard light switch.

Teardown Process

I preserved the entire assembly but disconnected the motor for testing. I intended to use the temporary power switch until I confirmed it wouldn't overheat. During disassembly, I encountered star-head screws, but I managed to free the battery receptacle using a long thin flathead screwdriver. The plastic casing is sturdy, with ample support from the screws, particularly around the battery bay. I detached the battery from the motor, although I encountered some difficulty separating the motor from the blades for future projects. The controller appears to function as a power supply, and I'm eager to test its output voltage—hoping it maintains 48V.

Voltage Testing

Upon testing, I was pleased to confirm the system operates at 48V! However, I do have concerns regarding the wiring. The black wire is rated for 300V 16AWG, with varying sources suggesting it can handle anywhere from 10 to 17 Amps. Given that 10A at 48V equates to 480 Watts, I want to ensure that the wiring can handle the load. While my lawn mower operates efficiently, I’ll monitor the setup closely with an infrared meter to prevent overheating.

Budgeting Power

Revisiting my application, I estimate the fridge requires about 1000W over a 24-hour period. Although the compressor might increase this demand temporarily, the average usage aligns with approximately 42 Watts per hour. Including a 10% conversion loss from 48V DC to 120V AC, we arrive at around 50W per hour. This suggests one battery could power the fridge for about 9 hours at full discharge, but to avoid overdischarging, I would likely limit usage to around 6 hours.

Designing the System

I plan to construct the system from wood, with my wife’s carpentry skills contributing to a well-crafted design. To achieve a vintage "1940s Popular Mechanics" aesthetic, I might use tin ceiling material for the top and back to aid heat dissipation and protect internal components. A sketch of the design outlines how the project will come together.

Exploring New Materials

Venturing beyond my usual wood projects, I’ve decided to utilize T-slot aluminum scaffolding for this build. This material allows for flexibility in design while enabling me to incorporate wood or acrylic veneer for the finish. Initial test fits indicate I may need additional parts to support the back of the inverter.

Encountering Challenges

I initially connected the control board to the inverter, but it appears to draw too much current, causing the power light to blink. With no load, the system shows 48V, but under load, it drops to 18V and blinks. My suspicion is that the control board is designed to monitor inrush current, entering standby mode when it detects an issue. After removing the control board, the inverter operated correctly, but this raises safety concerns, as there’s no monitoring to prevent deep discharging or overheating.

Final Thoughts

I believe I've reached a good stopping point for now. I've ordered parts for a more robust enclosure and gained valuable insight into the battery system's operation. In emergencies, I now know I can power my fridge for a limited time.

Future Improvements

For enhanced safety, I plan to implement an Arduino-controlled switch to monitor battery temperature and disconnect power if it overheats. Additionally, I’ll add a circuit breaker or fuse to the power line and construct a permanent enclosure to protect the wiring and users.

I’m curious if there are standard components available to monitor the switch or if I can design a simple comparator circuit that triggers disconnection at a specific voltage threshold. I may explore this further and document my findings in a future article.