Latest PCB version is 2.1, read important updates at the bottom!

TL;DR link to electronics project: https://oshwlab.com/positron96/powertool-battery-protection

A year or so ago I bought a PARKSIDE jigsaw with 20V battery, with the intention to also use the battery in DIY projects. PARKSIDE is a brand of tools from LIDL chain store with decent quality for non-regular work and competitive prices. Turns out, this particular tool ecosystem does not include power control inside the batteries, cell terminals are simply routed to outside without any MOSFETs in between. This seems to be the common case where the Li-Ion product range evolves from NiMH era, where there protection has always been in the power tool itself. Interestingly, cheap no-name batteries from China usually have protection built-in, though the reliability of those is questionable. So to use the PARKSIDE battery in DIY projects, external protection is desirable.

I believe there are cheap 5s protection boards on aliexpress, but all I could find were BMS modules with per-cell balancing connections, intended to be installed into the battery itself. (2026.11 update: found one: https://nl.aliexpress.com/item/1005010043780355.html, look for "18V/20V/21V Lithium Battery Protection Board 15V Over-Discharge Under-Voltage Low-Voltage Protection")

Also, one feature of this battery pack is that it has a pin that not only provides temperature reading, but also undervoltage indication. I didn’t want to duplicate this logic inside and outside the battery. So with these considerations in mind, I ended up designing my own protection PCB.

Requirements

  • Disconnects load from the battery,

  • Conducts some serious current (this is intentionally vague, as I will use it in low-power situations, so this requirement is mostly for self-education)

  • Uses fault information provided by battery pack.

  • Temperature reading not necessary (presumably overheating will be reported by battery itself)

  • Keep it simple, no MCU, firmwares and stuff.

Principle

To put it simply, inside the battery, "T" pin is connected to ground via a 10k thermistor and a transistor.

In the event of under-voltage or over-current (I don’t actually know if battery handles over-current situation, but there seems to be a shunt/fuse inside) the transistor goes off, and the pin becomes floating. Under normal conditions the transistor is on and the pin has around 10k pulldown to ground.

Simplified schematic of T pin inside battery

External circuitry (at least on the PARKSIDE soldering iron) is providing 5V to that pin via 10k pullup and measures the voltage. If it’s around 2.5V, all is ok, otherwise there is a fault.

There is one quirk though, in a form of some kind of a power saving feature in the MCU of the battery. if it does not sense voltage on the pin, it will turn the MOSFET off. My first schematic was providing around 2 volts to the pin, and the battery shut the pin off after couple seconds.

This video from bigclivedotcom has schematics of a Parkside soldering iron that plugs onto such battery, so it helped a lot while designing the project.

The design

PCB schematic and layout (v2.0)

Project electronics page: https://oshwlab.com/positron96/powertool-battery-protection

There is 2 stage power supply on the PCB, 12V and 5V, both via LDOs. Since this is the part that’s always connected to the battery, LDOs are selected to have a low quiescent current (HT75xx). 5V is fed to the T pin through 10k resistor. The voltage is fed to a negative input of a comparator (L393), positive input of the comparator is supplied with reference of around 4V from a voltage divider. Strictly speaking, the comparator cannot handle rail-to-rail inputs, it needs to be powered with higher voltage, and this is where 12V comes into to play. So the output of the comparator is 0V if there is a fault, and pulled to 12V in normal conditions. The output is fed to gate of the low-side power MOSFET.

Having a 12V rail also allows to use a non-logic level MOSFETs (which tend to have lower Rdson), but I had an IRLML (a logic level MOSFET) and used those for the first iteration of the board. While researching I found DP4080 MOSFET in my XY-SK120X PSU. It’s available at LCSC at low price.

Also from LCSC come blade connectors for the battery, SKU is C39833014. They suit the design perfectly.

Blade connectors and PCB backside

First version of the design (not presented here) had single power [1], no comparator, and directly fed voltage from "T" pin to an inverting BJT, and then to the gate of the mosfet. This didn’t account for power saving feature mentioned above, so the battery shut off the output after couple seconds. Version 2.0 is described above and works around this issue.

Current handling considerations

To handle high currents, the PCB has space for 3 parallel MOSFETs, uses copper pours for high-current paths and there are bare copper areas to add solder bars if needed.

Solder bars for higher current (on the 1st PCB version)

I don’t have proper devices to overload the battery, cannot measure the temperature of the PCB, so I can only hope that the PCB can handle some 10 amps. The highest power test I did was to power a Pinecil with it, at peak it consumed 45W (so around 3A), everything remained cold to touch.

I have no experience designing high-current PCBs, this is my first one, so any suggestions are much appreciated. In any way, it does not seem like I will use it for high current projects.

2025.09 Update

I’ve checked how the circuit works in overdischarge conditions, and it works! Battery indicates overdischarge fault at around 13V (2.6V per cell), which shuts down the protection module.

2025.10.13 Update

After trying to use the PCB to charge a laptop (using a 60W USB-PD DC-DC converter), I found out that the battery performs some weird tricks on the "T" pin. Turns out, during normal operation, battery MCU disconnecs "T" pin for a fraction of a second regularly, couple times a second. A disconnected "T" pin is treated by my board as a "battery says it’s dead" (which is true for undervoltage situation), which causes it to disconnect the power.

I didn’t notice it before as I didn’t put a lot of load to the board, and there was a USB-PD capacitor at the output, which smoothed drops of voltage. When charging the laptop, the capacitor gets emptied quickly, so I noticed that power disconnects regularly.

Why battery does that I don’t know, but this happens with both batteries I have.

To account for these disconnects of "T" pin, I added a low-pass RC filter at the input of the comparator. Now MOSFET shuts down only when "T" pin is disconnected for longer periods, should be around 1s.

Updated PCB schematic (v2.1)

PCB is now at version 2.1.

1. 5V was produced by 78L05 regulator. 2 of them died from unknown reasons, before I switched to 2nd version