[Minnewaska]

Seems like you diagnosed the problem. I was going to question the quality of the multi-meter as well or its connection to the bank.

I'm more interested in how one taps 12v off a 24v two batt series??

 

[Minnewaska]

I swear that diagram would suggest the panel was 24v as well. While not advised, some do take the series voltage entirely off the last batt in the series.

It's just better charging and use balance, if you take neg from one end and pos from the other, like your trolling motor does.

Actually, how did you get your meter to show 12v? Did you disconnect the series first? Are you sure the jumper doesn't go pos to pos and you really have a 12v trolling motor?

 

[Minnewaska]

When I test the solar panel I get around 18v, so it's definitely a 12v solar panel.

Why would a 12v panel put out 18v?

I'm getting the 12v battery reading by putting the multi-meter right on the terminals of the battery. It makes sense that I'm getting 12v readings there, because without the trolling motor being on it's an open circuit. I think.

Not to me. Once you connected them in series, it should be 24v. We need an expert here, I get a headache.

But there is clearly something more to it, if you test the panel and it shows 12v (or more likely something in the 13s). But I don't see how, from that diagram.

 

[Minnewaska]

Got it now. Serial connections just don't sink in for some reason. Probably because I've never needed to rig one.