By using this same sequence you can set up a 36 volt system. If both batteries have 20 ah then this series connection is 20 ah.

Charging 24v Battery With 12v Alternator And Isolator Motor
How to hook up a 24 volt battery system. Be careful not to let the cables get mixed up. 24 volt 24v 24vdc all mean the same thing in this article. The 24 volt charger is connected to the negative on battery 1 and positive on battery 2 inputting 24 volts to the series battery bank. Divide the batteries into two sets and secure them to your project. How to wire two 12 volt batteries to make 24 volts please nate. Most 24 volt systems use 2 or 4 12 volt batteries.
Disconnect the battery charge each battery. Loosen the two terminals on the circuit breaker supplied with the trolling motor. 24 volt wiring repeat the process for connecting the black ground wire to the first boat battery. 24 and 36 volt wiring diagrams trolling motor wiring diagrams while small and medium trolling motors use a single 12v marine battery larger trolling motors use larger 24v and 36v systems and require 2 or 3 marine batteries accordingly. Second battery auxiliary battery house battery dual battery system second battery system and combinations of the above terms. Every system is different.
The charger on the left side of the image is a 24 volt charger. Hook up the cables from your machine to the respegtive terminals on the two batteries that will. Do not connect 3 twelve volt batteries to run a 24 volt trolling motor. Common ground earth all mean the same thing. On freshwater fishing boats the most common use for either a 24 or 36 volt battery system is to power a trolling motor. Run one cable from negitive terminal of one battery to the positive terminal on the other battery.
Turn the systems main switch to the off position. Repeat on the second set. Connect the positive terminals together on the first set with a short piece of wire. You will be using the negative terminal on one battery and the positive terminal on the second one to create the 24v system. What do i mean by this. Batteries used in this manner must be identical.
They need to be put back in the same spot.