Designing a modular 4V tool ecosystem with swappable 18650 batteries (and the engineering mistakes I made along the way).

Hi all, sorry for the long post.

As an independent product designer and engineer, I build prototypes and tear down samples all the time. I use small 4V tools heavily for this—like 4V electric screwdrivers, hot glue guns, and rotary tools.

I've noticed that 99% of these tools use built-in batteries (usually a single 18650 cell). Sometimes when you pull one out of the drawer, it's dead, or it dies right in the middle of a job. You always have to stop and plug the whole tool in to charge (and usually you can't even use it while it's charging). Worst of all, built-in batteries often stop holding a charge unexpectedly, turning a perfectly good motor and gearbox into e-waste. If you look at Amazon reviews for countless 4V mini tools, endless comments point out this exact issue, wasting time and money.

Modular 12V and 20V battery platforms basically solve all of these problems.

Looking back at history, I've also been fortunate to see major brands try to solve this: From Black & Decker/Craftsman VersaPak and Skil Flexi-Charge, to Ryobi Tek4—most of them failed back then due to the insufficient performance/lifespan of NiCd/early Li-ion batteries and lagging BMS technology. In recent years, Ryobi finally released an upgraded version of the Tek4: Ryobi USB Lithium, putting the Type-C port directly on the battery pack and eliminating the bulky charger, which finally gained some market recognition.

However, I don't know why, even with the advancement of lithium battery technology, Ryobi's new USB Lithium tool lineup—whether it's the electric screwdriver or the 3/8" electric ratchet—has compromised torque performance compared to the older Tek4. That's fine for casual daily use, but for people like me who actually use tools for serious work or focus on making/building, it's clearly not quite enough. So I decided to develop a new 4V interchangeable battery platform from scratch, keeping all the portability advantages of independent battery packs without sacrificing torque performance and precise control.

Take the electric screwdriver as an example. I redesigned one, and above is my development journey.

• Pistol-grip shape + torque adjustment + speed regulation + LED ring light + independent 4V battery pack
• Magnetic 1/4" chuck—upgradeable later to a magnetic + quick-release locking chuck
• Speed: 120 RPM / 200 RPM, Torque range: 0.5 ~ 6.0 Nm (Hard torque)

What do you guys think of this setup?

Although it looks like a simple and conventional design, I ran into some of technical problems:

https://reddit.com/link/1ve9ubm/video/2dunb6o125hh1/player

1. Initially, I wanted to use a single 21700 cell for longer runtimes, but it turned out to be the wrong choice for 4V tools (as you can see from my previous posts). Compared to an 18650, the extra runtime gained from a 21700 just wasn't worth it given the swappable battery format, bringing a much bigger downside instead: a 30% increase in size and weight. In small 4V tools, size is everything. This wrong initial decision cost me half a year of effort, forcing me to completely redo the tools, battery packs, and protection boards from scratch. As an R&D engineer, it was a massive setback, but also a huge takeaway.
2. To achieve high and low speeds, I searched every supplier and couldn't find a speed-switching mechanism small enough. So I decided to DIY my own switch control assembly: using a vertical toggle switch for forward and reverse, and two micro-plate switches to achieve a 2-speed shift using the difference in trigger stroke timing. This way, it doesn't strip small screws easily when tightening them delicately, while still ensuring efficiency when fast assembly or disassembly is needed. (The sequential triggering of the micro-switches requires the trigger to travel very precisely in a straight line. The current 3D printing tolerance is quite large and doesn't trigger smoothly yet, so I've added guide plates back into the 3D model drawings).

Overall: I want to redesign a series of small 4V power tools using a high-discharge single 18650 lithium battery pack as a platform to drive a lineup of tools, helping prototype builders, DIYers, and makers like myself focus on creating without being interrupted or bothered by tool power and runtime issues.

What suggestions can you give me? Thanks.