95 bass tracker
2 batteries in back with a whatcha ma call it that plugs in to charge them.
Battery age is unknown

Since my last trip 2 weeks ago it has developed a problem with holding a charge.

I can plug it in and it charges. But if unplugged the power drains.

I have one battery with only a garmin graph connected.

The primary battery is the cranking battery. Plus it has some other things connected that I have no clue what they are..lights , bilge etc...

There is a cable on the neg and pos connecting the 2 batteries.

Troubleshooting done.
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I took the negative cable off of the cranking battery. I put the testing light on the cable and then touched negative terminal. It did not light up. so there is no unwanted draw.

I took everything off of the cranking battery and put in the other battery. I had very little power.
I took it all off and put it all on the cranking battery again (with the battery disconnected) and it seemed to have good power.

Is it possible that if 2 batteries are linked together and the one that isn't the cranking battery could go bad and cause this?

make sense?

Why not just have the batteries checked?

Agreed have the battery’s tested. Sound like one is bad for sure..

I will take them to autozone to get tested tomorrow..

Time for a new set of batteries

I remember a few years ago I replaced all of my batteries at once. It's over $800 for me to do so. I have 5 batteries in my boat.
Afterwards I had problems with my trolling motor. Couldn't be the batteries since I just replaced them.

Bought new plug
Replaced connections
Clean sealed wires to the terminals X2
Replaced the wireless control batteries X2
Replaced the wireless control
Took the head off of the trolling motor off and inspected

Fast forward a couple months (because I can only do this stuff on the weekends, then trips to the lake to test)
Took the batteries out to have them tested - 2 new batteries were bad

Even with new batteries, don't do like me.
Forever now, my step one will be test the batteries.

I took one battery out and resolved the issue.I reckon one battery went bad and since they were connected it made the other battery lose charge?
I bought a new battery today to connect to the good battery.

what all do yall connect to the cranking battery?

when I bought the boat one battery had the trolling motor and fish finder on it. The other was the cranking battery and had other wires going to it.I assume that is the bilge, aerator etc...

No, you don't know how to test for a drain. A 12V test lamp is not meant to be used the way you used it, so your conclusions are not based on accurate measurements/fact. A 12V test lamp is not designed to test for current flow - it will test that you have a voltage differential between two points, not current flow. You need an ammeter to test for an unexpected constant drain on the battery.

No sane electrically-knowledgeable person will advise you to connect two batteries in parallel. This will guarantee that when one battery fails, the other will be discharged by the failed battery, and you will have TWO dead batteries in short order. If you want a Primary battery and a Secondary (back-up) battery, connect them using a battery management switch, which should be kept on "#1 or #2, never the "Both" connection.

If you have a trolling motor, the battery/batteries for it should never be attached to the boat's Primary battery.

...and a rewire so that both batteries are not in constant parallel arrangement - as this is what guarantees one battery will kill the other.

I would get the garmin over on the cranking battery. Could get interference and low voltage as trolling battery is drained.

You seem to have missed that he really only has a "cranking" electrical system. The POS and NEG of the two batteries are each connected from what he says. This means he has them wired in parallel. Any interference on one battery's terminals will be present on the other battery's terminals also. He's got a mess started by someone clueless.

What I have at this point is a charging bay and one battery .What I had before this issue was the charging bay and 2 batteries connected with cables .
I assume the batteries were connected together so when plugged in both batteries would charge together.

Before this issue the trolling motor and fish/depth finder was connected to the non cranking battery. The cranking battery had other wires going to it besides just cranking cables.

What I have now is everything but the trolling motor going to the cranking battery.

For the trolling motor I put another battery up front in a compartment and connected it to it. so it is now out of the loop.

I bought a marine deep cycle battery yesterday and i plan on putting it in the back to replace the one that went bad. I should probably connect the Garmin to it.
From what I am reading it is not good to connect them together. I can leave them unconnected but when I plug it in it will just charge the cranking battery.
What I noticed last week was when i had it plugged in for a while the inline fuse was hot..Is it supposed to get hot?

I am most likely just going to have to go get it all checked out

Unfortunately, I lost a long reply I had typed out because my Internet connection went down.

Bottom line: For a small alum. boat with a 12V trolling motor, you should should NOT need 3 batteries. Both batteries should be in the rear, where they are close to a dual bank charger. One should be dedicated to the trolling motor, with the only other wires connected being the +/- from a port on a charger. Everything else should be on the other battery, aka "Primary Battery" that some call "cranking battery" when it really does cranking + all boat accessories. That battery should also have a pair of leads from a charger connected to it.

You previously described that the two batteries in the rear were connected POS-POS and NEG-NEG if I understood correctly. This should never be the case. But, you must have two battery chargers (or a dual bank charger) to insure both get charged. Some more info on your charger(s) would be helpful. BTW, it's not wise to extend battery charger leads to reach a remote battery as the voltage drop in the extended wires won't allow the battery to be charged as well as on the original length leads. Your boat probably doesn't run worth a toot on-plane with the TM battery up front (where it should not be).