Dual battery hot-swap and I²C power monitor board

Overview

Kookabus-01 joins two lithium-ion packs into one uninterrupted supply for a robot or other mobile load. Each pack feeds an ideal-diode stage, so the higher-voltage pack carries the load and a pack can be unplugged and replaced while the other keeps the bus up. A high-side current and voltage sensor on the output reports bus voltage, current and power over I²C through two Qwiic/PiicoDev connectors. An LED beside each battery connector shows which pack is supplying the load.

The board is a power pass-through. It does not regulate, charge or balance, and it needs no configuration to swap packs: the ideal diodes run from the packs themselves. The sensor and LEDs draw their 3.3 V from the I²C host.

• Two XT60 battery inputs, one XT60 load output, common ground.
• 5S lithium-ion packs as the primary target. Every part is rated for 6S (25.2 V).
• 100 W peak, 40 W continuous. 6.7 A peak and 2.7 A continuous from a flat 15 V pack.
• Ideal-diode ORing. 20 mV regulated forward drop, no brownout on swap, reverse-current blocking.
• Reverse-polarity tolerant inputs. A reversed pack is blocked, not shorted.
• INA226 high-side monitor. 1.25 mV bus voltage LSB, 0.5 mA current LSB, 16-sample averaging.
• I²C at 3.3 V logic, four selectable addresses with 0x40 default, on-board pull-ups with a cut jumper.
• Two Qwiic/PiicoDev connectors for daisy-chaining, with ESD protection on SDA and SCL.
• One 10 A ATO blade fuse per pack, replaceable without tools.
• Bidirectional 33 V TVS on each input, 26 V TVS on the output.
• Active-pack indicator LEDs, red for A and yellow for B.
• Standby drain about 0.32 mA per pack with the host off.
• 64 × 62 mm, four M3 mounting holes, all SMD parts on the top side.

Applications

• Hot-swappable battery supply for ROS 2 and other mobile robots.
• Battery monitoring for Raspberry Pi, Jetson and microcontroller hosts.
• Bench supplies built from cordless-tool packs: Ryobi ONE+, Einhell Power X-Change and similar 5S packs on XT60 adapter leads.

Board revision 2, datasheet revision 1.0, 25 September 2026. Preliminary: values not marked as measured are from design calculations and component datasheets.

Specifications

Recommended operating conditions. Stay inside these for rated performance. Absolute limits are in the ratings section.

The board

J1, J2

BATT A and BATT B. Male XT60 pack inputs.

J3

LOAD. Female XT60 output, nose overhangs the right edge.

F1, F2

ATO blade fuse holder for each pack. Fit 10 A fuses before use.

U1 Q1, U2 Q2

Ideal-diode stage for each pack: MAX74700T controller and AO4262E 60 V FET.

C15

100 µF output capacitor on VLOAD, in parallel with 4.7 µF ceramic.

R1

5 mΩ 2512 shunt, Kelvin sensed.

U3

INA226 voltage, current and power monitor.

U4

MCP6002 comparator driving the two active-pack LEDs.

LED A, LED B

Red beside BATT A, yellow beside BATT B. The lit one is supplying the load.

J4, J5

JST SH 4-way I²C connectors, wired in parallel.

A0, A1

JP1 and JP2, INA226 address jumpers.

PU

JP3, cut to remove the 4.7 kΩ pull-ups.

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How the hot swap works

Only the pack with the higher voltage conducts. When the two packs are within about 20 mV of each other both stages conduct and share the load. When a pack is unplugged, the other pack's FET body diode conducts immediately and its controller then enhances the FET, so the bus never drops out. Current cannot flow back into a pack: a fully charged pack plugged in beside a flat one does not charge the flat one.

Nothing on the board needs to be told a swap is coming, and the hot swap works with nothing connected to J4 or J5. Only the LEDs and the I²C reading need the host's 3.3 V.

Typical connection

1. Fit a 10 A ATO fuse in F1 and F2.
2. Connect the robot's power input to LOAD with a male XT60 lead.
3. Connect the host's Qwiic or PiicoDev port to J4 with a 4-way JST SH cable. Use J5 to chain further I²C boards.
4. Plug the first pack into BATT A. Its LED lights once the host's 3.3 V is present.
5. Plug the second pack into BATT B. To swap packs later, unplug the one whose LED is off, replace it, and repeat for the other. The load never loses power as long as one pack stays connected.

Ratings

Absolute maximum ratings

Stresses beyond these values may cause permanent damage. These are limits, not operating conditions.

Electrical characteristics

Calculated at 25 °C from component datasheets unless stated. VBUS is the ORed node after the ideal diodes; VLOAD is the load connector after the shunt.

Power path

Monitor (INA226, U3)

Indicators and I²C

Connectors and jumpers

Power connectors

All three connectors share one ground. There is no isolation between the packs, the load and the I²C host.

I²C connectors

The pin order is the Qwiic and PiicoDev standard, so any Qwiic cable connects directly. J4 and J5 are wired in parallel for daisy-chaining. The host must supply 3.3 V on pin 2: the board has no regulator of its own, and the monitor, comparator and LEDs stay off until it does. The hot-swap function does not depend on the host.

Solder jumpers

I²C address selection

To change an address, cut the bridged trace on the GND side of the jumper, then bridge the centre pad to the VS side with solder. The table is also printed on the back of the board. Up to four Kookabus-01 boards can share one I²C bus.

Fuses

F1 (pack A) and F2 (pack B) are ATO/ATC blade fuse holders rated 30 A at 60 V. Fit a 10 A, 32 V ATO blade fuse in each (Littelfuse 0287010.PXCN or any equivalent). The fuses ship separately and must be fitted before use. A fuse on each pack, rather than one on the output, also protects against a short in that pack's input TVS, capacitor or FET. Blade fuses break more than 1 kA at 32 V, which a tool pack can deliver into a short.

I²C interface

The only I²C device on the board is the Texas Instruments INA226. Its register map applies unchanged; the values below are specific to this board's 5 mΩ shunt. Registers are 16 bits, most significant byte first.

Examples

from smbus2 import SMBus

ADDR = 0x40
with SMBus(1) as bus:
    def rd(reg):
        b = bus.read_i2c_block_data(ADDR, reg, 2)
        return (b[0] << 8) | b[1]
    def wr(reg, val):
        bus.write_i2c_block_data(ADDR, reg, [val >> 8, val & 0xFF])

    assert rd(0xFE) == 0x5449          # INA226 present
    wr(0x00, 0x4527)                   # 16 sample average, continuous
    wr(0x05, 2048)                     # 0.5 mA per LSB with the 5 mOhm shunt
    volts = rd(0x02) * 1.25e-3
    raw = rd(0x04)
    amps = (raw - 65536 if raw > 32767 else raw) * 0.5e-3
    watts = rd(0x03) * 12.5e-3
    print(f"{volts:.2f} V  {amps:.3f} A  {watts:.2f} W")

from machine import I2C, Pin

ADDR = 0x40
i2c = I2C(0, sda=Pin(8), scl=Pin(9), freq=400_000)   # Pico pins on a PiicoDev adapter

def rd(reg):
    b = i2c.readfrom_mem(ADDR, reg, 2)
    return (b[0] << 8) | b[1]
def wr(reg, val):
    i2c.writeto_mem(ADDR, reg, bytes([val >> 8, val & 0xFF]))

assert rd(0xFE) == 0x5449
wr(0x00, 0x4527)
wr(0x05, 2048)
volts = rd(0x02) * 1.25e-3
raw = rd(0x04)
amps = (raw - 65536 if raw > 32767 else raw) * 0.5e-3
watts = rd(0x03) * 12.5e-3
print(volts, "V", amps, "A", watts, "W")

#include <Wire.h>
const uint8_t ADDR = 0x40;

uint16_t rd(uint8_t reg) {
  Wire.beginTransmission(ADDR); Wire.write(reg); Wire.endTransmission(false);
  Wire.requestFrom(ADDR, (uint8_t)2);
  uint16_t v = Wire.read() << 8; return v | Wire.read();
}
void wr(uint8_t reg, uint16_t val) {
  Wire.beginTransmission(ADDR); Wire.write(reg);
  Wire.write(val >> 8); Wire.write(val & 0xFF); Wire.endTransmission();
}

void setup() {
  Wire.begin(); Serial.begin(115200);
  wr(0x00, 0x4527);          // 16 sample average, continuous
  wr(0x05, 2048);            // 0.5 mA per LSB
}
void loop() {
  float volts = rd(0x02) * 1.25e-3;
  float amps  = (int16_t)rd(0x04) * 0.5e-3;
  float watts = rd(0x03) * 12.5e-3;
  Serial.print(volts); Serial.print(" V  ");
  Serial.print(amps, 3); Serial.print(" A  ");
  Serial.print(watts); Serial.println(" W");
  delay(500);
}

On Linux the kernel ina2xx hwmon driver also supports the part. Declare it in the device tree with compatible = "ti,ina226" and shunt-resistor = <5000> (micro-ohms), and read voltage, current and power from sysfs.

State of charge

The bus voltage reads the live pack minus the series drop. For a percentage estimate, divide by the cell count and use a per-cell resting-voltage table rather than a straight line from 3.0 V to 4.2 V. Under load, correct for sag with the measured current and an estimate of pack resistance, which can be taken from the voltage step when the load changes. Integrating the current register gives a coulomb count for the pack that is currently supplying the load; the board cannot tell the host which pack that is, other than by the voltage reading itself.

Protection and safety

Mechanical

Connector positions

Board-local coordinates, origin at the top-left corner of the outline as viewed from the top, x to the right, y down.

Application notes

Host firmware checklist

• Write CALIBRATION = 2048 at every start-up, and CONFIG = 0x4527 if averaging is wanted.
• Poll bus voltage and current at a few hertz. Stop the load, or warn, when bus voltage falls to the pack's minimum (about 15 V for a 5S pack, 3.0 V per cell).
• Detect a swap as a step in bus voltage. The higher-voltage pack always carries the load, so the voltage reading always describes the pack in use.
• Disable regenerative braking in the motor controllers, or fit a brake resistor on the load side.

Using the board without a host

The hot swap works with nothing connected to J4 or J5. Only the LEDs and the I²C reading need the host's 3.3 V. For a bench check, any Qwiic host or a 3.3 V supply on pin 2 of J4 lights the LED of the conducting pack.

Thermal

At 40 W continuous the board dissipates about 0.18 W and barely warms. At the 100 W peak the pass FET of the conducting pack dissipates up to 0.47 W and rises about 30 °C above ambient in still air; the fuse and shunt each add about 0.3 W. No heatsink is required for the rated duty. Sustained operation above 7 A is outside the rating.

Ordering and files

• Datasheet, PDF, document revision 1.0Download
• Promo video, MP4Download
• KiCad 10 project, Gerbers, BOM and placement filesNot yet published

Revisions

Information on this page is believed to be accurate at the date of publication but is provided without warranty. Values marked as calculated have not been verified by measurement. Adventures in Silicon Pty Ltd accepts no liability for damage or injury arising from the use of this product, which connects to lithium-ion batteries capable of high fault currents. The user is responsible for fusing, pack protection, enclosure and compliance with local regulations. Product names are trademarks of their respective owners.