Autonomo Solar Charging Problem

Hi, I have recently bought two Autonomo (Rev 4b) units for monitoring remote tanks.

With a grove shield and Grove Ultrasonic Ranger together with an XRF Wireless Module communicating with a Raspberry PI already equipped with a Slice of Radio GPIO shield, these appear to be incredible bits of kit, with an impressively low power consumption.

My problem in testing comes with trying to keep the 1200mAh batteries charged using solar power only (I’m using a 1W 5.5V 170mA 100x80 panel also supplied by SODAQ.)

I allowed the LiPO battery to discharge overnight to about 80% capacity. When the sun came up the Autonomo charging light comes on but at the first sign of bright sunshine, the solar charge controller appears to cut charging to the battery and does not restart when light levels fall back to normal.

If I disconnect the solar panel and reconnect, charging resumes until the next period of bright sun when it stops again. I have swapped my autonomo board and both devices exhibit the same characteristic, as does swapping solar panels.

I am assuming the charge current is exceeding 500mA which is why it is cutting out. Is there anything I can do to stop this from happening, or to reset the charge controller using software control?

In case this may be causing issues, I am setting BEE_VCC and VCC_SW to low when ultrasonic measurements and data transmission are not required to try to increase battery life.

Any suggestions would be gratefully received. Thank you.

OK, it may be a charging restriction set by the battery voltage. Having reset the Autonomo yesterday and allowed it to run overnight, the lower voltage on the battery this morning appears to be allowing the solar charge controller to charge the cell with fewer interruptions (still not completely without turning off in bright sunshine, but it is at least resetting itself now).

If anyone has any technical details about the solar charge controller used, I would be pleased to hear what they are.

Thank you

According to the datasheet for the Autonomo, the LiPo charger is the MCP73831 from Microchip.

Perhaps you might find the details of the chip’s behaviour in it’s datasheet.

Out of curiosity, how are you calculating the Level%?

Thanks for that Gabriel. I’ll take a look.

To work out the % remaining I ran a full discharge of the LiPo battery through the Autonomo recording the battery voltage at 1 minute intervals to a csv file using a Raspberry Pi.

There is a short period at the end of the discharge when it drops away very quickly before the Autonomo shuts off. Ignoring this period of time, the voltage drops at a reasonably constant rate.

Fitting a linear formula to the data gave me the following correlation for a 1200mAh battery:

% remaining = (0.0021356*Batt_mV)-7.7873)*100.

Where this formula predicts more than 100% it gets rounded down and when it predicts less than 0 the Autonomo sets the % remaining to 0. It is really rough but enough for me to get an idea of how the charge level is varying with time or if I need to replace the battery.