What is it?

This is a breakout board for the Texas Instruments BQ25186 "1A I²C-controlled linear battery charger with power path and solar input support".

This is a modern fully featured IC for charging single LiPo/LiFePO4 cells that integrates power path management, battery protection etc. for small devices.

Why did you make it?

Many hobby projects and prototypes rely on simple, cheap LiPo charger boards (eg. TP4056) but they have significant limitations.

• They are not designed to charge and power devices at the same time. Doing so is essentially unsafe.
• They provide no feedback on the charge status except for some LEDs you usually can't see once it's inside your project.
• The USB connector provides no USB data connection, only power.
• They will not work unmodified in small solar projects.

The BQ25186 is a modern, smarter way to charge your small LiPo/LiFePO4 cell powered projects but sadly no breakouts were available from the usual places. An official TI evaluation board exists but isn't appropriate for putting in a small one-off device of your own making.

What makes it special?

• 3-18V input voltage.
• Battery regulation voltage 3.5-4.65V in 10mV steps.
• Li-ion, Li-Poly, and LiFePO4 chemistry compatible.
• 5mA to 1A configurable fast charge current (but see gotchas).
• Up to 3A discharge current (but see gotchas).
• The BQ25186 is configurable and monitorable over I²C, no need for dedicated GPIO to control and monitor it. There are optional 'charge enable' and 'interrupt' pins exposed on the PCB header.
• The BQ25186 does power path management, ie. the battery is not connected directly in parallel with the thing you're powering. As a result this means the BQ25186 can charge the battery while whatever's in your project will be powered by the most appropriate source, either the battery, charging supply or in certain cases supplement the supply from the battery. This power path is configurable over I²C.
• Power path management also includes 'ship mode' and 'shutdown mode'. So your device can 'switch off' by configuring the BQ25186's I²C registers or the use of a button. Quiescent current is just 15nA in 'shutdown mode' or 3.2μA in ship mode.
• Connection for an external button that can wake the device from 'ship mode'. The onboard button is there for convenience under testing but you can connect an external one in parallel. The button will not wake it from 'shutdown mode', which is the price you pay for the lower quiescent current.
• Connecting a charging supply wakes the the device from both 'ship mode' or 'shutdown mode'.
• Battery undervoltage lockout is configurable at 2.0/2.2/2.4/2.8/3.0V.
• The output 'system' voltage when under charge is configurable to 'pass through', bring it down to 4.4-4.9V or track the battery voltage as it charges.
• Voltage input dynamic power management (VINDPM) means it can effectively charge from high impedance/low power charging sources such as solar panels. This reduces load on the supply as needed to continue charging.
• The onboard 'power good' LED can be controlled over I²C as a general purpose output, for example to show charge state.

Why you might not want this

• It is imperative you configure the charging settings appropriately for your battery otherwise there may be a battery fire! If you are not confident in your ability to do so, do not purchase or use this device.
• This is not a "configure and forget" LiPo/LiFePO4 charger, you must be using the BQ25186 as intended and have whatever it's powering configuring it over I²C on startup and ideally monitoring it thereafter. The default charging settings on a reset are for a 4.2V cell at 10mA.
• The USB connection is just legacy D-/D+ and not routed to be length/impedance matched. The assumption is it will be used for USB 2.0 CDC connections to upload firmware to your device or do basic control not High Speed USB.

Gotchas!

• The I²C pins are not fitted with pullups. You must add appropriate pullup resistors (probably 4.7K) to your device's logic level.
• As this board is expected mainly to be used in one-offs, prototypes on stripboard etc. the temperature sensor thermistor is bypassed with with a 10K resistor. To monitor enclosure/battery temperature you should desolder this resistor (see listing photos) and connect a suitable thermistor between 'Button' and GND, as per the BQ25186 datasheet.
• Charge settings are only retained by the BQ25186 so long as the cell is connected and above the undervoltage lockout or there is a usable external supply. Your device must check or configure it appropriately every time it starts up to cover this eventuality.
• Various of the charging parameters are related and you should read the BQ25186 datasheet carefully before setting them.
• The USB-C connector uses 5.1K resistors on the CC1/CC2 pins to trigger power delivery, it is not a full PD device. This should suffice in most use cases.
• The tracks to the pin headers are limited by the size of the board and copper fill areas are 'kept out' of the header footprint to enable easy soldering. If you're expecting to draw a larger current (>500mA) for extended periods use the solder pads which have more track area connecting them to the BQ25186.
• All the 'DC Power In' connectors/pads/headers including are connected in parallel with the expectation only one of them will be used. For example there is nothing to stop a DC jack connected to the solder pad "back-powering" the USB socket. Don't do this.
• All the 'GND' and 'B-' pads/headers including are connected in parallel.
• The 'power good' pin of the BQ25186 is only connected to the onboard LED, not the PCB header.
• The optional JST S2B-PH-SM4-TB battery connector is an exceptionally tight fit for the generic ones used on many cells, please only plug one in if you're ready for it to stay there. It is expected in most use cases cells will be soldered with flying leads to the pads or header.
• Sadly it doesn't report the current battery voltage over I²C, which would have been handy. I may build another breakout with an onboard I²C fuel gauge.

Pinout

Note: All GND and BAT- pins/pads and the GND of the USB connector are connected in parallel.

Solder pads

• GND - Ground connection.
• IN - DC Input Power Supply 3-18V.
• OUT - Power for your device. Default output is 4.5V but configurable.
• B+ - The 'positive' terminal of your battery
• B- - The 'negative' terminal of your battery

Headers

Software support

The BQ25186 is controlled by 14 I²C registers filled with bit masked values and flags. I have written an Arduino Library which gives you control of most of these and examples for likely use cases but you should refer to the datasheet for detailed configuration.