Which one is my starting battery?

Sorry for the novice question, been a minute since I've done some of my own maintenence. Dealer batteries were identical, but I am putting in a cranking and deep cycle. Which battery is the starting battery? There is one with just the normal cables and the charger cables and the other has the same plus a heavy gauge ground and an additional ground. I couldn't decide if that would be from the engine or the amps. I tried to test the numerical positions assuming 1 is the cranking battery, but with one disconnected, I think the circuit is broken, so I am posting instead of connecting extra battery configurations. Thanks in advance.

So my wife had to hop on a PTA call, so I ended up with more tinker time. As I suspected after I posted, the heavier gauge ground was just the jumper from the other battery, and the other "new" ground was from the amps. I was able to test battery positions once the grounds were all in circuit and before I connected the second battery positive terminal. I had correctly guessed battery 1 and 2 positions and they are both in their new homes, cranking on 1 and deep cycle on 2. That being said, I'll leave this up for any other helpful tips

I could be wrong, but with the 2 boats I have, I had always been told that 1 or 2 would start the boat. I was told to alternate the numbers every time you use the boat in order to keep a good charge on each one. I could be wrong, but if you do it this way, if one battery dies etc. you will have a back up. This happened to me this winter in FL. My flat boat trolling motor wore out one battery for some reason. I switched the number, started right up, went home and threw a charger on the other. This is my understanding. I am sure others have comments as well.

I'll use 1 or 2 to start the boat, then switch to BOTH while under power. This charges both. Then if we stop to play and swim, I'll switch to one battery or the other, to ensure we have a full battery to start the boat.

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Originally Posted by SSurfer321

I'll use 1 or 2 to start the boat, then switch to BOTH while under power. This charges both. Then if we stop to play and swim, I'll switch to one battery or the other, to ensure we have a full battery to start the boat.

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Sounds right on the new boats. I think the advice from SC is to operate the boat in #1&2 (when running) and have it switched to #1 or #2 when not operating.

Thus, if you are starting, you could use #1 position, #2 position, or #1&2 position - just do not switch of OFF when the boat is running.

For the charging and the alternator:
#1 position and the alternator only charges that battery
#2 position and the alternator only charges that battery
#1&2 position and the alternator charges both batteries

I always use position 1&2 (combined), and have switched to either 1 or 2 a couple of times when floating or listening to the stereo while docked. But I think my boat will crank on either 1 or 2 by itself. Though I have not purposefully tried it that I remember.

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Originally Posted by HFarr

I always use position 1&2 (combined), and have switched to either 1 or 2 a couple of times when floating or listening to the stereo while docked. But I think my boat will crank on either 1 or 2 by itself. Though I have not purposefully tried it that I remember.

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Same here. There might be a few times where I know we will be anchored in a cove for a long time, then I will switch to #2 when playing music.

Most of the time we are there a short time, so I do not bother. I also feel the switch should be easier to access - instead of telling someone to move or moving bags off the seats.

We run combined the full time, never had an issue with it draining down, both batts are identical. If you're swapping out to two different battery types, you can't leave them combined while the engine isn't running as one will drain the other and will eventually damage the other battery.

I run 1&2 at all times and have never had a problem.