# Pocket Lab Power Supply: a 4S lithium battery bank that doubles as a bench supply

> BenMakesEverything's open hardware project turns a 4S lithium-ion pack into a portable, adjustable bench supply with USB-C/PD charging and an LED matrix readout. The catch is that the safety-critical parts live outside the board.

**BenMakesEverything/Pocket-Lab-Power-Supply** — A lab bench power supply, but pocket-sized and battery powered.

- Repository: https://github.com/BenMakesEverything/Pocket-Lab-Power-Supply
- Stars: 558 · Forks: 44
- Language: C++
- License: CERN-OHL-S-2.0
- Published: 2026-09-17 · Updated: 2026-09-17 · Language: en
- Canonical page: https://hysenlabs.com/projects/benmakeseverything-pocket-lab-power-supply

## What the Pocket Lab Power Supply actually replaces

A bench supply is a box that plugs into the wall and gives you a settable voltage and a settable current limit. The Pocket Lab Power Supply keeps the second half of that promise and drops the first. It is a 4S lithium-ion battery bank, charged over USB-C with Power Delivery, that exposes adjustable output voltage and an adjustable current limit on its output terminals. The README describes it as "a lab bench power supply, but pocket-sized and battery powered", and the repository is organised around that claim: /hardware holds the PCB, schematic, BOM, Gerbers and CAD, /firmware holds the MCU code and utilities, /docs holds a build guide, diagrams and notes.

The audience is narrow and worth naming. This is for someone who already owns a spot welder or buys pre-built 4S packs, who is comfortable reading a schematic before soldering, and who has a use case where mains power is missing: field debugging, a bench in a room with no free outlet, a quick check on a device that only needs a few watts. It is not a replacement for a mains-powered supply in a lab that needs 30 V and 5 A, and the README never claims it is.

## Architecture: BQ25792 charging, an MCU, and a display backpack

The charging side is built around the BQ25792, a buck-boost battery charger controller. The README is explicit that the PCB was designed for 4S packs and that the BQ25792 "will charge the pack up to 16.8V". Pin 20, labelled PROG on that part, is the component the README points at if you want a different cell configuration. That single sentence carries most of the design's risk: the board is configured for one pack topology, and the firmware and the charger share that assumption.

The user interface is an 8-character 5x7 LED matrix, one of the HCMS-2971 through HCMS-2975 parts listed in the parts list. It is not soldered to the main board. The parts list names a separate "backpack" PCB for the display module, and the repository splits accordingly. A rotary encoder with a printed knob handles input, and the parts list includes a plastic cap for the button. The README points to a flowchart image at docs/images/flowchart.png as the overview of how the main components connect, so the wiring topology is documented as a diagram rather than as prose.

One design decision stands out because it is stated in capitals in the README: the design does not include a battery management system. The pack must supply its own. That is a deliberate split. The board handles charging and regulation; the pack handles cell balancing, over-discharge and short-circuit protection. It also means the safety of the finished unit depends on a part the project does not sell, does not ship and, as of the README's update note, no longer links to.

## Installing it: there is no installer, only a build

There is no package to install and no binary to flash from a release page. The repository has no releases listed, and the README does not document a prebuilt firmware image. What it documents is a build: order the main PCB and the display backpack PCB, populate the BOM, print the body, and assemble. The build guide lives in /docs and the README calls that directory a work in progress.

Start by reading the parts list and the BOM directory before ordering anything, because the README removed the link to the 4S battery pack it originally used. The stated reason is a problem with the integrated BMS in one purchased unit, and the author notes that after investigation there was no problem with the circuit. If you want the exact sourcing the author used, that link is gone.

```bash
git clone https://github.com/BenMakesEverything/Pocket-Lab-Power-Supply.git
cd Pocket-Lab-Power-Supply
ls hardware/ firmware/ docs/
```

That gives you the three directories the README describes. hardware/ contains the PCB, schematic, BOM, Gerbers and CAD; firmware/ contains the MCU firmware and utilities; docs/ contains the build guide, diagrams, images and notes.

```bash
ls hardware/bom/
ls docs/
```

The first command shows the component BOM files the README links to as "Components BOM: here". The second shows what documentation actually exists today, which matters because the README says the FAQ and the build guide are both incomplete. Expect to fill gaps from the schematic and the flowchart rather than from a written step-by-step.

Before you apply power, the README's safety notice is the operative instruction. The board charges to 16.8 V, and the README states in capitals that another cell configuration requires changing that, that pin 20 PROG on the BQ25792 is the adjustment point, and that a pack without a BMS must not be used. Treat those three statements as the acceptance criteria for your build.

## Where this design can hurt you

The largest limitation is the one the README itself flags: no BMS on the board. A 4S pack without cell balancing will drift, and the first cell to hit the upper limit takes the damage. The README's warning is blunt, that using another cell configuration without changing the charge target will overcharge and damage the cells. Anyone who treats the board as a complete battery product has misread the project.

The second limitation is documentation maturity. The README states that the FAQ is a work in progress and that more will be added, and the build guide sits in a docs/ directory the README labels WIP. The overview of connections is an image, not text. For a project that involves mains-adjacent charging over USB-PD and a lithium pack, that is thin for a first-time builder, and it is the main reason this is not a beginner kit.

The third is calibration and accuracy. The README lists adjustable output voltage and current limit as features but does not state an accuracy figure, a resolution, a ripple specification or a maximum output current anywhere in the text provided. If your work needs a supply you can trust to a millivolt, this project gives you no number to check against. That is not a defect in the design, it is a gap in what the project publishes, and you should treat the readout as a convenience until you verify it against a meter you trust.

## How it compares with a USB-C PD trigger and a mains bench supply

The closest functional alternative is a USB-C PD trigger board paired with a PD-capable power bank. Both give you a portable DC source charged over USB-C. The difference is control. A PD trigger negotiates one of the fixed voltages the source advertises, and you get whatever current the source will give. The Pocket Lab Power Supply puts an adjustable regulator and a current limit between the pack and the terminals, which is the part that makes it a supply rather than a cable. It also means the output is not limited to the PD voltage ladder.

Against a conventional mains bench supply, the trade is obvious and one-directional. You lose the wall plug, the high voltage range and the high current capability. You gain portability and independence from an outlet. The README's framing is honest about which side of that trade it sits on, and the project does not pretend to cover bench work that needs sustained high current.

Within the same niche, there are other portable supply designs people search for by name, and the practical distinction to look for is whether the battery protection is integrated or assumed. This project assumes it. A design that integrates the BMS removes a sourcing decision from the builder, at the cost of a more complex board and a pack you cannot swap as freely. Neither approach is wrong, but the difference determines how much of the safety case you are personally responsible for.

## Licence, maintenance and what you take on

The project is licensed under CERN-OHL-S-2.0, the strongly reciprocal variant of the CERN Open Hardware Licence. The README states that unless otherwise stated, the licence covers the hardware design files, firmware, software utilities, documentation, images, diagrams, CAD files, manufacturing files and other project files in the repository. Commercial use is allowed under the terms of the licence. The project name, author name, logo, branding and media identity are not licensed for marketing or endorsement without permission. The full text is in LICENSE.txt, and the top-level entries include both LICENSE.md and README.md.

Strong reciprocity is the part that matters for anyone planning to sell boards. If you modify the design files and distribute the result, the licence's reciprocal terms apply to those modifications. That is a real constraint on a product built from this repository, and it is a question for a lawyer rather than for this article.

On maintenance, the last push to the default branch was on 2026-08-27. The repository is not archived. The README carries an active update note about the removed battery link, which is a sign of recent editing rather than a promise of a support schedule. There are no releases listed, so there is no version to upgrade from or pin to. Your upgrade path is pulling the main branch and diffing the hardware and firmware directories yourself. For a hardware project that is normal, but it means you own the diff review.

## Conclusion

Adopt it if you already build lithium packs, understand 4S charging, and want a pocket-sized adjustable supply you can carry to a bench that has no mains outlet. Do not adopt it if you want a finished product, if you cannot verify the BMS inside your pack, or if you need a supply that is certified for anything beyond your own bench. Before ordering boards, read the safety notice in the README, confirm your pack uses an XH2.54mm connector, and check the BOM under hardware/bom/ against what your distributor actually stocks.

## FAQ

### What is the Pocket Lab Power Supply used for?

It is a 4S lithium-ion battery bank that works as a portable lab bench power supply, with adjustable output voltage and current limit. The README describes it as a bench supply that is pocket-sized and battery powered, charged over USB-C with Power Delivery.

### Does the Pocket Lab Power Supply include a battery management system?

No. The README states in capitals that the design does not include a BMS and that you must use a pack that contains one. The board charges the pack up to 16.8V through a BQ25792.

### What battery pack does the Pocket Lab Power Supply need?

The README specifies a 4S 21700 battery with an integrated BMS and an XH2.54mm connector. The link to the pack the author originally used was removed after a purchased unit had issues with its integrated BMS.

### Can the Pocket Lab Power Supply charge a different cell configuration?

The PCB was designed for 4S packs. The README warns that using another cell configuration without changing the charge target will overcharge and damage the cells, and points to pin 20, PROG, on the BQ25792 as the adjustment point.

### What display does the Pocket Lab Power Supply use?

An 8-character 5x7 LED matrix, one of the HCMS-2971 through HCMS-2975 parts listed in the parts list. It mounts on a separate backpack PCB rather than the main board.

### Is the Pocket Lab Power Supply open source?

Yes, under CERN-OHL-S-2.0, the strongly reciprocal variant of the CERN Open Hardware Licence. The README states this covers the hardware design files, firmware, utilities, documentation, CAD and manufacturing files, and that commercial use is allowed under the licence terms.

## Sources

- [BenMakesEverything/Pocket-Lab-Power-Supply on GitHub](https://github.com/BenMakesEverything/Pocket-Lab-Power-Supply)
- [Issues](https://github.com/BenMakesEverything/Pocket-Lab-Power-Supply/issues)
- [License: CERN-OHL-S-2.0](https://github.com/BenMakesEverything/Pocket-Lab-Power-Supply/blob/main/LICENSE)
- [README](https://github.com/BenMakesEverything/Pocket-Lab-Power-Supply/blob/main/README.md)

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Hysen Labs editorial analysis, written from the project's own repository and release notes. Cite the canonical page: https://hysenlabs.com/projects/benmakeseverything-pocket-lab-power-supply