I’ve seen lithium batteries that are 24 volts but come in a single case as a single battery, if you don’t have room for 3 12v trolling motor batteries is it possible to use 1 of these 24v batteries in series with a 12v lithium to make a 36v system? Or do all the batteries have to be the same voltage to run them in series like that. 

Why not just use a 36 volt lithium?

It feels wrong to put a 12 volt and a 24 volt lifePo4 lithium battery in series but I can’t think of a reason why it would not be okay.  To get the best results you would want the batteries to use the same cells internally.  I’ve never seen a manufacturer that listed what cells their batteries contain so that would be difficult.  You would also want the BMSs in both batteries to have the same current limits for optimum results.  

 

You would need a 12 volt and 24 volt charger which would be more inconvenient than a single 36 volt charger.

Good points! It was more of a curiosity than anything. I’m not trying do it, just wondering if it was possible. Thanks