1989 Johnson 70HP battery confusion from previous owner

I couldn’t find any direct answer reading some other posts but I just purchased an 1989 Bass Tracker and on it is a 89 Johnson 70HP motor. The starting battery looked a little rough and the guy I bought it from mentioned something about the cheap Atwood onboard charger not working for that battery, but I also thought the motor charges this battery anyways while running, but I’m a newbie to this. But I pulled it out to clean the terminals and noticed it’s the same exact battery as the one That controls the trolling motor. First time I’ve bought a boat like this so I don’t know much but the way I understood it there’s a cranking battery and a deep cycle battery to run electronics. The battery is an ever start 845MCA both say deep cycle power, the starting battery definitely looks a lot tougher than the other. I’d like to put a new cranking battery for the motor or at least sort out if it’s running the right batteries. It does start and run, only have had minor motor issues unrelated to the battery or electrical system.

I think EverStart is the brand sold at WalMart, correct? Those are the batteries that were in the 89 Tracker that I bought recently. Both of mine said Deep Cycle. They worked for the guy that I bought the boat from for over six years. Not bad for relatively cheap batteries from Wally World. Generally, cranking batteries can unload lots of amps for a short period of time........for starting engines. They aren't particularly good for slow drains like trolling motors would require. Conversely, deep cycle batteries aren't good for high amp dumps like engine starting, but are good for slow drains like a trolling motor.
Also, most boats will have the electronics hooked to the cranking battery.......probably because most electronics are run through the boat's fuse panel.......not hooked directly to the battery the way a trolling motor normally is. And in some boats the trolling motor is 24 or even 36 volt, which would require the electronics to be hooked to the 12 volt starting battery.
I've heard and read that the new Ever Start batteries are not quite as reliable as they were several years ago. That may or not be true. I just changed out my batteries for the trolling motor because I went 24 volt. Picked up two Optima deep cycle batteries for that. Still running the EverStart in the cranking/starting position.......the one that came with the boat. Not sure how long it will last as it is 7 yrs old now. Fished with it two wks ago. Lasted all day starting the 50 Johnson, powering two fish graphs, and running the live well pump some. Couldn't rev the outboard much as it was having a fuel pump problem, so pretty much just above idle whenever it was being used. Hooked it to a portable charger each evening and it was down to about 68 percent after all day use.

You want a marine starting battery for that motor and not maintenance free. The charging system on that motor is unregulated and will cook the electrolyte out. So get a battery that has the removeable covers so you can check it. Deep cycle batteries are not the greatest as starting batteries because they like to give up there power at a slower rate. This makes them great for trolling motors but engine starters require power quickly.

Ya the cranking battery is a maintenance free and been used in the boat for quite a while. I just saw on the side the deep cycle and the battery for the trolling motor was the same exact thing. But the boat doesn’t seem to have an issue so I was just confused as to if I go get a new battery, get relatively the same thing, but sounds as though I should get a non maintenance free specific cranking battery to start the motor. Just kind of tripped on how the boat doesn’t have a problem apparently running both those same deep cycle batteries with starting and running everything. For as long as it’s been running with that deep cycle battery I wonder if it’s a good idea to check my rectifier and what not. I’ve heard a maintenance free battery can fry those things which would lead to my loss of the re-charging system I think? Appreciate all the information I’m quite confused on this larger boat and just want to understand everything as best I can so I can take care of the thing!

You want the starting battery and the trolling battery to be separate. Your motor has a 5 or 6 amp charging system, and it only puts that out over 4000 rpm. Youre motor is really only capable of keeping up with the starting battery especially with tilt and trim on it.