Thanks for the quick reply. I have a few more questions.
1) Do you mean the output voltage level of solar panel is always equal to the battery voltage so we can read batter level to get the voltage reading?
2) If I want to add the ammeter, should I cut the battery cable to insert it or cut the solar panel cable to insert it?
3) Here, can I separately measure the current harvested from the solar panel and the current from the battery?
4) I know that solar panel itself cannot power the sensor node. Then, during sunny, daylight
hours, will the solar panel charges the battery and also powers the sensor nodes? Or the battery is charged by the solar panel and also power the sensor node at the same time?
5) I found that solar panel cannot charge the battery to reach 100% level. The battery level reach 95% at noon and then cannot go up even though there is sun insulation during afternoon. Is it normal? Can you let me know why?
you may monitor the battery charge level and have a reference of the balance in energy consumed and gathered, but that won't give you an accurate measurement.
You will need a current sensor to monitor both the current harvested from the solar panel and the current from (or into) the battery. You may use a non invasive sensor (a DC current clamp) or an invasive sensor, such as an integrated circuit sensor, that will be cheaper but will need of an electronic adaptation stage.