Battery power board

18V Battery Power Board

A reusable power board that turns a Makita 18V LXT battery into protected, switched power for your projects: the raw battery rail plus regulated 12 V, 5 V and 3.3 V, with a built-in low-voltage cutoff that protects the battery.

Designed in KiCad 10. Two layers, 56 × 62 mm, M3 mounting holes.

Not affiliated with or endorsed by Makita. "Makita" and "LXT" are trademarks of their respective owner.

Features

• Battery protection: a cutoff switches everything off at about 16.3 V and back on at about 17.5 V, so a 5S Li-ion pack is never over-discharged. The battery's own signal pin is not needed.

• Main fuse: replaceable 7.5 A mini blade fuse, plus a TVS diode clamping input spikes.

• On/off switch input: a 2-pin header for an external switch. A solder jumper (JP1) turns the board permanently on when no switch is fitted.

• Four outputs:

  Rail	Source	Rating
  VBAT	Switched battery, own 2.5 A polyfuse	15.8–21 V, ~2 A
  12 V	RY9430 synchronous buck	2 A
  5 V	K7805-2000R3 module	2 A
  3.3 V	K7803-2000R3 module	2 A

• High-current output: an XT30 connector on the switched battery rail for loads above 2 A. Protected by the main fuse only (7.5 A).

• Battery sense output: a filtered divider giving about 3.0 V at 21 V, safe for any MCU's ADC.

• Status LEDs: one per rail (VBAT, 12 V, 5 V, 3.3 V).

Power path

Battery ─ J1/J2 ─ F1 (7.5 A) ─┬─ TVS + bulk cap
                              └─ Cutoff (ATL431 + P-MOSFET) ─ VBAT_SW ─┬─ F2 (2.5 A) ─ VBAT out (J5)
                                                                       ├─ 12 V buck ──── J5
                                                                       ├─ 5 V module ─── J6
                                                                       ├─ 3.3 V module ─ J6
                                                                       ├─ XT30 (J7)
                                                                       └─ Sense divider ─ J4

The cutoff and the switch both act on the same MOSFET, so turning the board off (or a low battery) disconnects every output.

Connectors

All signal and power connectors are JST PH 2.0. Pin 1 is + everywhere.

J5 — HV (5-pin PH)

J6 — LV (4-pin PH)

J4 — Battery sense (2-pin PH): pin 1 VBAT_SENSE, pin 2 GND. Battery voltage = ADC reading × 6.92.

J3 — On/off switch (2-pin PH): closing the switch turns the board on. It carries only microamps, so any small switch works. Bridge JP1 instead if you don't use a switch.

J7 — XT30 (female): switched battery voltage, fused by F1 only. Polarity is marked on the silkscreen.

J1 / J2 — Battery wire pads: BAT+ and BAT− with strain-relief holes, for 1.5 mm² wire.

Power and ground pins are doubled on J5 and J6 to share the current, since PH contacts are rated about 2 A each. The two connectors have different pin counts, so the HV and LV cables can't be swapped.

Harness convention

Each project uses a fan-out cable from J5/J6 to small device connectors:

• 3-pin PH (+, NC, −): VBAT or 12 V
• 2-pin PH (+, −): 5 V or 3.3 V

Use "same-direction" PH cables only (pin 1 to pin 1).

Building one

Ordering

• PCB: Gerbers and drill files are in exports/. 2 layers, 1.6 mm, 1 oz copper, tented vias.
• Parts: see the BOM. LCSC part numbers are in the KiCad symbols' "LCSC Part" field. The 7.5 A blade fuse is not in the KiCad BOM; order it separately.

Assembly

Everything is hand-solderable with an iron and a hot-air station.

1. U2 (RY9430) first: its exposed pad must be soldered. Tin it thinly, then reflow the chip with hot air.
2. Remaining SMD parts: the electrolytic and the LEDs are best done with the iron.
3. Through-hole parts last: fuse holder, connectors, XT30, then the K78 modules.
4. Clean the flux with isopropyl alcohol, especially around U1 and R1–R3. Residue on that high-impedance node can shift the cutoff voltage.

First power-up

1. With no fuse and no battery, check with a multimeter for shorts between GND and each rail.
2. Insert the fuse, connect the battery, and measure every output before connecting any load.

Notes and safety

• J1 and J2 sit before the fuse. Route and secure the battery wires so they can't chafe.
• The XT30 output is protected only by the 7.5 A main fuse. Size your wiring accordingly.
• Pulsed loads (e.g. stepper or motor drivers) should have their own bulk capacitor at the driver.
• The board draws a small standby current even when switched off. Disconnect it from the battery for long-term storage.

Repository layout

*.kicad_pro / *.kicad_sch / *.kicad_pcb   KiCad project
libraries/                                Project-local symbols, footprints and 3D models
exports/                                  Gerbers, drill files and BOM per revision

License

This hardware design is licensed under the CERN Open Hardware Licence Version 2 – Permissive (CERN-OHL-P-2.0). See LICENSE.

© 2026 Malcolm Nihlén