Consumers can purchase assembled PCBs, such as the one pictured above, or complete product enclosed in a relatively nice aluminium extruded housing.
According to the silkscreen of versions pictured on-line, different variants are available including:
- 3S Li-Ion
- 4S Li-Ion
- 4S LiFePo4
- 12V Lead Acid
I’m still old-school and wanted to purchase a 12V lead acid version, but couldn’t find any available for sale.
And like many similar electronics products from the above mentioned on-line retailers, documentation can be sparse.
As the product appeared to have a very well laid out printed circuit board, I decided to purchase one to pull apart and modify. In the process, I documented the design below.
The 5 to 28V input voltage range supports 36 cell solar panels (sometimes known as a ‘12V panel’). These panels are characterised as having an open circuit voltage (VOC) output of approximately 22V and a voltage at maximum power point (VMP) of around 17V.
Two PCB options are available – one supporting an output of 5A and the other supporting 10A.
The BQ24650 supports a battery voltage of 2.1V to 26V. However, as a synchronous buck topology, the input voltage must be higher than the output/battery voltage. As VMP is set as 17.2 volts, the battery voltage must be set under this.
Hence this is the reason why only 3S and 4S options exist. Lithium Ion chemistry has the highest cell voltage of 4.2V thus a 4S pack will have a maximum voltage of 16.8V – just squeezing in under the 17.2V max.
PCB Design & Schematics
The good news is the PCB designators closely follow the designators used in the Texas Instruments datasheet. Where extra components have been added, they have been given an incremental designator following the last designator used in the datasheet. This made it easier to find the MPPSET and output voltage dividers.
MPPSET – Maximum Power Point Set Point
The BQ24650 doesn’t track the maximum power point like more sophisticated designs. Instead, it tries to maintain a constant voltage – the Maximum Power Point (MPP), preset via R3/R4. The BQ24650 will reduce the charge current to try to maintain the maximum power point.
VMPPSET is equal to 1.2V x [1 + R3/R4].
The board has a 200k resistor fitted for R3 (code 30D) and a 15k resistor (code 18C) fitted for R4. This sets the MPP voltage at 17.2V which should be reasonably close for most 36 cell solar panels.
The output current is set via RSR , an 8 milliohm shunt resistor. The maximum charge current is set via:
ICHARGE = 40mV / RSR
This sets the current at 5A. This same shunt resistor also sets the termination current to 1/10 of 500mA. Once the voltage reaches the target and the current falls to below 500mA, charging will complete.
R1 and R2 are used to set the output/battery voltage as per the datasheet, however their positions are swapped. To help cost down, a third resistor R13 has been added in series with R2 to allow for cheaper and more common E24 series resistors to be used.
The battery voltage can be set using:
VBAT = 2.1V x [1 + R1/(R2+R13)]
The board includes two LEDs – A red D1 and a green D2. These LEDs report the status of the charger:
|Charge suspended, overvoltage detected, sleep mode, battery absent
The charger can be powered by either the solar panel/input or via the battery – the body diode of the high side FET will conduct, effectively connecting the battery to VCC.
However, the voltage on VCC must exceed the under-voltage lockout of 3.86V and the voltage on VCC must exceed the voltage present on SRN (connected to the battery) before charging will initiate.
If the voltage on VCC is under SRN, the device will enter Sleep mode (<15uA) to prevent draining the battery.
This cheap and popular BQ24650 based battery charger is a relatively well designed and extensible charger. The output voltage can be reconfigured by changing R1/R2/R13.