Adding starter batter to already all electric setup

Hey guys,
I recently got my first tinny, a 1994 Alumacraft Jon Crappie Jr (1436 size). It is currently all electric with a 24v 48lb bow-mounted trolling motor and an old humminbird depth finder. I recently bought a 9.9 Honda electric start and need to wire it up. My current setup already has 2 group 27 deep cycle batteries and they are heavy! Per the manual, my outboard is made to accept a 12v signal.

So here is what confuses me... If my trolling motor is 24v then how could the depth finder be hooked up the same circuit (assuming the depth finder is 12v, and why wouldn't it be?)? He had the setup done pretty nicely and I cannot actually see any wires besides the leads that connect to the batteries (1 battery has two sets of leads and another has one- That's gotta be the key here).

Either way, I will crack open the decking and see what is going on. But it appears there must be a way the depth finder is getting a 12v signal, while the tm is getting 24v. And if so, I want to tap into the 12v for my outboard starter.

From what I read online the main reasons you should not do this are 1) interference messing with depth finder and 2) potential to drain starting battery.

But my depth finder is already on the same circuit and works fine. Also, it is a 9.9hp motor, so it should crank fairly easily even if the batteries are drained. And if they are drained, I can pull start is easily... So why not? I could save money, time, weight, and space by not having a 3rd battery...

Thoughts?

To get 24 volts your two batteries are wired in SERIES and the TM is wired to the + of one battery, and the - of the other.

But... for 12 volt systems using this setup you wire to the + and - terminals of ONE of the two batteries.

Try it, measure the + on one battery, and the - on the second, you will see ~24v.

Then measure just the + and - of ONE battery and you will see ~ 12 volts.

Since I run all electric if I ever ran a gas engine again, I'd run a separate battery for it.

On my boat (Tracker 1648) I run a 24V, 74# Minn Kota Maxxum using 2 Everstart group 27 batteries. I also pull of each battery independently for 12V for stereo, bilge pump x 2, LED nav lights, LED interior lights, sonar, LED light bar, 12V plug x 2 and starter for my '94 Johnson 50/35 jet. Zero issues with things setup the way I have them in my boat. Although this is not ideal since you're not draining the batteries in series equally which is supposed to be bad for their health. But I have not noticed any ill effects of this. I had zero desire and limited room to add a 3rd battery to my setup. The real disadvantage to this is the potential to run the batteries dead with the trolling motor, which I have done once while fishing for 8+ hours running the trolling motor hard. I keep a portable jump starter in my boat for that reason and it works well. With your ability to pull start I wouldn't hesitate to pull off one of your batteries for the starter on your 9.9hp outboard.

Thanks for the input guys. I just needed some confirmation on my thoughts... I am not going to add a 3rd battery. I do not think it is worth it, even if a 3rd starting battery is 'ideal'. I ahve gone fishing 6-7hours tm only and did not get close to draining them. With an outboard I will not even be using the tm at full power (as a means to cross lakes etc.) so my battery use should be even less than it is now. Thanks!