I have a cabin with a single large solar panel, with a controller and a couple of large 6 V batteries powering an inverter. I have attached a voltmeter permanently to the battery terminals in hopes that I can learn something about this system. There is a bit of a mystery yet as I notice that the battery voltage goes up when the sun is shining on the solar panel and then drops rapidly when the clouds interfere. It's surprising that the battery voltage gains and drops so rapidly. Am I really measuring the battery capacity or the voltage of the current coming out of the controller or some combination?

 

A solar panel's output voltage is typically constant despite sun intensity. The current output, however, varies significantly according to the suns brilliance.

See this document for more details:

http://cds.linear.com/docs/LT Magazine/LTMag-V19N4-04-LT3652-JimDrew.pdf

Austin

 

You have to disconnect the solar to get the battery voltage properly. Solar controllers with voltmeters, disconnect the solar panel for a few milliseconds to sample the battery voltage then display. then do that every second or so.

 

Thank you, that's very interesting. I suppose that there isn't any simple device that would provide the interrupt and measure voltage to satisfy my need to know my battery's health.

 

you could use a switch on the solar line with the exact setup you have now.

When you want to know the battery voltage, flip the switch, read the voltage, then flip it back. A light swich will do if its rated for or above the output of the panel.

Opening the switch will disconnect the panel giving you battery only reading to the DMM connected to the battery terminal.
Then closeing the switch re-connects solar.

nice and easy.

 

Neat. I'll do it.

Thanks.

 

Also be aware that the voltage of lead-acid batteries can change over time after charging. At higher charging rates, the recommendation is to wait a couple of hours after charging to measure the voltage. Then in conjunction with knowing the battery temperature, you can estimate the state of the battery's charge by measuring the voltage (search the posts, as the formula and a table have been posted elsewhere).