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".
Why did you make it?
Many hobby projects 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.
- They typically only support one battery chemistry.
The BQ25186 is a modern, smarter way to power your small Li-Ion/Li-Polymer, Lithium Phosphate/LiFePO4 cell powered projects but sadly no breakouts were available from the usual places.
What makes it special?
- The BQ25186 does power path management
- The battery is not connected directly to the thing you're powering.
- As a resul the BQ25186 can charge the battery separately and whatever's in your project will be powered by the most appropriate source, either the battery or charging supply.
- Power path management includes a 'ship mode' which effectively switches the powered device off. So your device can fully 'shut down' via software.
- It is configurable for different cell chemistries over I²C, defaulting to very conservative LiPo cell settings.
- It is monitorable over I²C, with simple reporting of charge status.
- Connection for an external button that can serve as an on/off button to wake the powered device from 'ship mode'. The onboard button is there for convenience under testing.
- The output voltage when under charge is configurable: it's not designed to be your main voltage regulator but if, for example it's connected to solar panel, the low load voltage can be be multiples of the nominal voltage and this will bring it down to <5v or track the battery voltage as it charges.
Why you might not want this
- This is not a "fire and forget" LiPo charger, you really should be using it as intended and have whatever it's powering configuring and monitoring it over I²C. The default startup charging current is only 35mA.
- The USB connection is 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.
Gotchas
- The I²C pins are not fitted with pullups, as there's no regulated supply on the board to use for them. You should add appropriate pullup resistors (probably 4.7K) to the VCC of your project.
- As this board is expected mainly to be used in one-offs or 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 and connect a nominal 10k NTC thermistor between 'Button' and GND, as per the BQ25186 datasheet.
- Settings configured over I²C last only until power is completely removed by shutdown/ship mode/undervoltage lockout or battery removal. You should ensure you check and set them every time your device starts. The defaults are very conservative so this isn't inherently risky but is to be borne in mind.
- Sadly it doesn't give you the current battery voltage over I²C, which would have been handy.