Weird battery wiring

[Carverlvr2]

Ok, I’ve been able to figure out and address most of the “previous Owner” changes but this one still has me baffled. A PO had a damn nice JL Audio stereo installed and was clearly professionally done complete with two 5ch amps, subs, and two very expensive Stinger AGM batteries cleanly wired (and fused) in parallel as its own bank so as not to pull down the others. Here’s the puzzler that I’m hoping the electricians here can explain. They ran very heavy gauge Pos and Neg cables from the Stinger batteries to the engine room to charge off the main charger (and I’m guessing alternators) but ran the Neg cable to the Neg terminal on the House battery and the Pos cable (fused at the battery) to the Pos terminal on the Port starter battery. You can see the cable and fuse in second pic. Why the split across the two charging banks/batts? I hope my drawing illustrates what I’m describing. And for additional info, the main charger is a 3 bank with 1 each to the 2 starting and 1 house batteries. Genny has its own.

[Carverlvr2]

Sorry photos hadn’t uploaded yet.

[Rocketman]

I think they were trying to isolate the stereo batteries so they would not be in parallel to the house or starting battery and pull them down as well. Unfortunately, I believe all your negatives are tied together so you have in fact put the batteries in parallel to the port starting battery.
They should have tied the negative to the overall ground bus bar located somewhere.
They should have tied the positive at the isolator located somewhere on the boat that allows charging but will not pull from one battery to another.
Look for a blue box that all your positives are tied to. That will be your isolator.

[Midnightsun]

Looking at the diagram you now have one big bank consisting of Your 2 stereo batteries and you port battery. From what I can see, they are not independent banks meaning if you run your stereo for a very long time you will also deplete your port starting battery.

All negative wires share a common ground so hooking it to any battery fills this requirement.

[Carverlvr2]

Thank you for clarifying that! That’s what I was thinking too! They may as well just put both Pos and Neg cables to the same battery because they’re all grounded together anyway. And yes, really should have gone to an isolator or just a separate charger on them. But at this point I think it makes more sense to change out the existing house batt for an AGM and parallel the Stingers to it and make one big house bank. I have to assume the Stingers are several years old. Any thoughts on mixing them with a new AGM or am I looking at swapping all three for new?

[Midnightsun]

Never mix old with new. Personally I do not like AGM. Expensive and have let me down on more than one occasion. I would go with 3 new lead acid batteries if it were me.

[Alanna Mo Cree]

Best practice is to change all batteries in a bank at the same time. There should be a sticker or punch card or something on each battery with its manufacture date so you can get an idea of how old they are. Sometimes those stickers are hard to find on an installed battery. I use a Sharpie to write down the install date on the battery somewhere I can easily see it whenever I change them.

Something else to look into is to whether those power cables are marine cable. I think Streetwires sells install kits with either copper clad aluminum or oxygen free copper cables. I think both could corrode in a marine environment, but oxygen free would be better. Tinned copper is typically used for marine wiring. It might not be a big deal in Illinois though. Here in Florida we get all kinds of electrical gremlins from wire corrosion, and from what I've seen they are some of the hardest issues to troubleshoot.

[Carverlvr2]

As you stated, the “proper way” to do it is same type, capacity, and age, and I’m working to get it all there but the reality is that bank has mixed type (FLA and AGM), mixed capacity (88ah and 105x2ah), and 2 yrs old vs possibly 10yrs on the Stingers, and all possibly being charged on a Gel charge profile (yet to be confirmed)….yet, it all seems to work. How well and for how much longer, time will tell. Does make me wonder how critical the ideal combo really is though.