I've got two solar panels set up in series as shown on the below schema. I connect positive on panel A to positive on battery, and negative on panel B to negative on battery. The two panels produce 9V each, and 18V in series on a good sunny day. That's what my multimeter says.
If I disconnect cabling between panels and battery, my multimeter shows that the panels produce about 15V (a bit cloudy today). However, when I connect the cables between panels and battery, my multimeter shows about 12.2V. I believe this is the battery voltage.
Why is it that when I connect my multimeter at point A and B it shows 12.2V (when solar panels are connected to the battery). As you can see on the schema, there's a diode in front of point B. Shouldn't that diode make sure that current from the battery not flows back to the panels?
If this is how it is supposed to work, how do I check the voltage my panels produce when the panels are connected to the battery.
Best Answer
When you connect the voltmeter between points A and B, you're connecting it directly across the battery, so if the solar panels put out less than the battery voltage the diode will be reverse biased and will, in effect, disconnect the battery from discharging into the solar panels and you'll be measuring only the battery voltage.
On the other hand, if the voltage from the solar panels is higher than the battery voltage and the drop across the diode, the solar panels will force current into the battery, charging it.
However, because the impedance of the battery is so low, it'll drag the voltage of the solar panels down close to the battery voltage, even though the solar panels will be continually pumping current into the battery.
As time goes by and the battery becomes more fully charged, you'll notice that its voltage will rise, but never to your solar panels' open-circuit full-sun value, because the battery chemistry won't allow it.
The proper way to monitor your battery's charging is to measure the voltage across it and the current into it, and never let either rise above the manufacturer's recommendations.