EPS Charger Module – BQ25306

Subsystem: EPS

Status: 🧪 Testing

Latest Release:

Version: v1r2.2
Date: 2026-01-2
KiCad: v9

Overview

This buck charger module is intended for use on the Build a CubeSat EPS v2 with a 2s2p battery pack for development and testing. Its primary use is for external charging from a 9V source (e.g. USB-C PD from the EPS Main Board). It can be used with solar cell strings that have a max. power point voltage of around 11V for a 2s battery pack or 5.5V for 1s respectively, but it does not provide a programmable MPPT setpoint. Under input current limiting, the charger dynamically tracks battery voltage via VINDPM, which typically pulls the solar input below Vmpp. The IC supports an input voltage range from 4.1V to 17V and a charge voltage range from 3.4V to 9V.

Please refer to the datasheet for details on the IC.

The BQ25306 features "Input Voltage Dynamic Power Management (VINDPM) tracking":

When the input current of the device exceeds the current capability of the power supply, the charger device regulates PMID voltage by reducing charge current to avoid crashing the input power supply. VINDPM dynamically tracks the battery voltage. The actual VINDPM is the higher of V INDPM_MIN and (1.044*VBAT + 125mV)."

— Datasheet Rev A, p. 18

This board can be tested using the Butter.CubeSat.Utils test board developed by Butter Robotics.

Pinout Diagram

PCB Specifications

Dimensions: 23 mm × 23 mm
Layer Count: 2
Panelization: No
Mounting: Castellated edges for modular integration
Stacking: Not applicable

Design Files

KiCad project: Located in the kicad9/ folder
Schematic: PDF
Pinout diagram: WebP
3D Render: WebP

Manufacturing Files

Release: eps-charger-module-bq25306-v1r2
BOM: Included
IBOM: Included
Instructions:
- Ordering: Included (PCBWay)
- Sourcing: Not included
- Assembly: Not included

Settings

This module is set up for use with a 2s2p battery pack and an input voltage of 9V from an external source, with which it will charge at 2A. It can easily be reconfigured for a 1s4p battery pack with a 5V input, and for lower or higher charging currents.

The table below show currently implemented settings and alternative.

Setting	Parameter	Formula	Resistor, resistor divider, or jumper	Default value	Alternative value/s
Charge voltage	VREG	VREG = 1.1(1+R1/R2)	Datasheet and schematic: R1 (top, BAT), FB (center), R2 (bottom, FB_GND)	8.33V: R1 657k, R2: 100k	8.17V: R1 634k, 8V: R1 626k (R2 fixed at 100k)
Charge current	ICHG	ICHG = 40000 / RICHG	Datasheet: RICHG, schematic: R3, R4	2A: R3 10k R4 10k	1A: R3 10k R4 30k2, 3A: R3 10k R4 3k3
Battery charge temperature range	TS	See datasheet rev A, p. 18	Datasheet: RT1 (top, REGN), TS (center), RT2 (bottom, GND); schematic: R5, TS, R6	0°C–45°C: R5 4k53, R6 23k2	See datasheet rev A, p. 17
Inductor selection	L	See datasheet rev A, p. 22		2uH2 (VVBUS_MAX > 6.2V)	1uH (VVBUS_MAX < 6.2V)
nEN pin behavior	POL	Float: nEN, ground: EN	R9	DNP: nEN	0: EN

NTC Thermistor Notes

The datasheet specifies a Semitec 103AT-2 thermistor for monitoring of battery temperature, but other simlar devices (10K, B25/85: 3435K), that make better contact with the batteries, may be considered. For example Semitec 103JT-025 or TEWA TT6-10KC8-9-25.

Safety Features

Thermal regulation and thermal shutdown

Input Under-Voltage Lockout (UVLO) and Over-Voltage Protection (OVP)

Battery overcharge protection

Safety timer for precharge and fast charge

Charge disabled if battery feedback pin FB is open or short

Cold/hot battery temperature protection (charges between 0°C and 45°C)

Fault report on STAT pin

Reverse discharge protection (automatic sleep mode when input voltage falls below battery voltage)

— Datasheet Rev A, p. 18

Status and Fault Reporting

Mode / State	Conditions	REGN LDO	Charging Enabled	STAT Pin Behavior
Disable Mode	Device disabled (EN = 1, POL floating)	OFF	NO	OPEN (Hi-Z)
HiZ Mode	Device enabled, VBUS < VBUS_UVLOZ	OFF	NO	OPEN (Hi-Z)
Sleep Mode	VBUS > VBUS_UVLOZ and VBUS < VBAT + VSLEEPZ ()	OFF	NO	OPEN (Hi-Z)
Charge Mode	VBUS > VBUS_LOW and VBUS > VBAT + VSLEEPZ; no faults; charge not terminated	ON	YES	LOW (short to GND)
Charge Termination	VBUS > VBUS_LOW and VBUS > VBAT + VSLEEPZ; charge complete; no faults	ON	NO	OPEN (Hi-Z)
Safety Timer Expired (VBAT > recharge threshold)	Safety timer expired after charge completion	ON	NO	OPEN (Hi-Z)
Fault Mode	1. VBUS overvoltage 2. TS hot/cold fault 3. battery overvoltage 4. IC thermal shutdown 5. safety timer expired with VBAT below recharge threshold 6. ICHG pin open/short 7. FB pin open/short	ON	NO	BLINKING at 1 Hz (50% duty)

Note: The STAT pin is open-drain only. “OPEN (Hi-Z)” indicates the internal transistor is off and the pin must be pulled up externally. The device never actively drives STAT high.

Design Notes and Limitations

The BQ25306 does not provide a programmable MPPT setpoint. When the available input current is insufficient, the charger reduces charge current and dynamically regulates the input voltage (PMID) as a function of battery voltage (VINDPM). As a result, solar strings with a Vmpp significantly above the battery voltage may not operate at their maximum power point.
The STAT pin is open-drain only and cannot actively drive a logic-high level. A HIGH (open) state may indicate charge completion, charger idle, sleep mode, or the device being disabled. The STAT pin alone is therefore insufficient to distinguish between these states without additional context (e.g. VBUS presence or battery voltage). R7 pulls STAT up to an external 3v3 supply (which also drives the LED).
This module is primarily intended for external charging from a regulated input source (e.g. 9V from the EPS main board). Solar charging is supported for development and testing purposes but is not optimized for maximum energy harvest.
Not flight-rated. For development and suborbital testing only.

License

CC BY-SA 4.0

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