Solar panels...6 or 12 volt ?

So we have 7 feeders on our place all with solar panels.
over the years they have been moved around, motors changed, batteries changed, solar panels replaced, ect.
All of or our timers will work on 6 or 12 volt.
BUT....the solar panels are an unknown mixture of 6 and 12 volt to the extent that none of us on the place can remember which is what.
There has to be a way to test the solar panels and find out which are 12v and which are 6v.
Is it as simple as reading the panels voltage output on the wire leads with a volt meter?
Every time we go there's at least one battery crapped out, sometimes a couple, and I can't help but think it's due to mix matched battery/solar panel voltage.
Need to know a sure fire way to tell which solar panels are 6v and which are 12v so we can make sure to use the right battery with it.

Ohm meter with low voltage dc option

You need a volt meter. Set it on DC then measure. The 6V panels will probably show around 9-11 volts, the 12V panels will show around 16-19 volts. Be sure to test with the solar panel disconnected from everything.

Then use a sharpie to mark the inside of the controller. You could also just count the cells on the solar panel. I believe that each cell is 0.5V so the 6V panels will have 1/2 the number of cells as the 12V.

Make sure you have positive on positive and negative on negative. So you will need to figure out which wire is positive from the solar panel and connect to positive on battery. Same for the negative. Above is correct for the Solar panel out put voltage

A 12v panel will have 36 cells wired in series while a 6v will have 18. You can just count them. Otherwise a multimeter will tell you. Point it at the sun and measure disconnected. 12v will be 18-23v while a 6v will be 9-11v. This number varies wildly because it will be lower on a hot day vs a cold day.

Thanks for the info all.
Never thought of counting the cells.

It can be tricky if it’s a panel with the a cover that has all those lenses molded into it. They can also make it look like 36 cells but if you look really close you can see the cells are soldered down the center of the cell, which is difficult to see. The fool proof way is a multimeter, full sun and no battery attached to the panel.