Battery Question

[cjblovinlife]

96 380 Santego

I have two systems a house set up with two AGM‘s and a cranking set up with two lead acid batteries. Both systems have the 2nd battery wired in parallel but system 1 is isolated from system 2 on the + side unless of course someone puts the battery switch in the BOTH position. HOWEVER ALL the negatives/ engine grounds are tied together.

My question: If my house system call it #2 is low at 9V and #1 system is good at 12.7 . Without hooking up any charger they will EQUALIZE… How does that happen?? Is it through the negative side of the circuit somehow?

Thanks so much for your wisdom I’m still learning.. should I separate the negative/ grounds for each system?

[Midnightsun]

Not unless you have a battery switch that will combine the 2.

Normally the start banks are left separate to make sure you have enough juice to get the motors up and running. One common mistake that gets new boaters into trouble is combining start and house bank which can leave you stranded.

[bud37]

When you say hook up a charger, can you clarify.......would that be the charger is physically disconnected or you mean to turn the charger on.

Charge all up then disconnect your charger leads completely and see what happens. Remember there are parasitic loads hard wired to different batteries, like bilge pumps, CO monitors, radio preset maintain etc.

[cjblovinlife]

My switch does have a “both “ position but I never use it , house & cranking batteries are completely separate except for the grounds they are all on a bus for both systems I’ve been having issues since I bought the boat 2 months ago so i disconnected everything and started over . I have a noco genius 10x4 but it’s not connected at all I’ve been troubleshooting while putting everything back together adding one load at a time in hopes I might locate a load leaking voltage or a grounding issue. The problem exists with or with the hardwired bilge pump’s attached to the house batteries but that is the only load connected. So with absolutely nothing connected to the batteries ( which are charged and good 12.5 Volts. I also have a battery tester that induces a load and all batteries test good. ( at one point I thought I might have a battery with a bad cell - nope). When I start the engines connected to system #1 and switch at one position it starts and runs great but the voltage on the starboard engine fluctuates erratically and goes up to almost 17volts … (I tested both alternators (with only a multimeter) and each is putting out about 13.4 volts… When I stop and switch to the house #2 system the voltage drops to less than 9 and I’m assuming there is some type of undervoltage cutoff switch because the main DC panel in the salon shuts down … So I go turn the switch to #1 position and everything works well the voltage is 12.5 on #1 and still less than 9 v on the #2 system BUT I can watch the #2 system with voltmeter attached and it goes up about a volt every 2-3 minutes until both systems equalize and read 12.3 V…… After 10 minutes I can switch to #2 system and the panel lights up and works again!!!! I’m baffled if I separate the negatives on each system I suspect they won’t equalize but I haven’t done that yet . I thought I’d check with you guys before going further… I bought the book 12v bible and read it twice but still haven’t figured this out hopefully you guys can help- Thank you!!

[Viper]

Do you have a dual battery isolator for 2 alternators and 2-3 banks utilizing diodes? A faulty diode can leak back and essentially connect two banks together. Also, if you have a engine sync gauge, disconnect it and the module if applicable.

The panel shutting down due to low voltage is new, I don't think I've seen that one before but a previous owner may have installed a low voltage cutoff device, check behind the panel. When you say the panel shuts down, does the main breaker trip? Does the panel DC meter drop to zero? No DC loads work at all?

When you note battery voltages are you looking at ship's meters or a multimeter?

Do you have an inverter?

[bud37]

Source of the post I’m baffled if I separate the negatives on each system I suspect they won’t equalize but I haven’t done that yet

The DC grounds are all connected together on a boat.......there has to be a completed circuit for there to be power usage. Perhaps do some extensive continuity testing to find where the connection is on the positive side.

Clearly there seems to be a connection or leakage/ bad motor/isolators/inverters etc as has been mentioned ( eg. bilge pumps ), even those one, two, all switches have been known to be faulty but lets face it something is pulling one bank down under 9 volts ......did you test the banks at the batteries themselves.

What is using all that power or do you just have a bad battery, have you tested all your battery cell levels and individual Hydrometer readings to check for bad cells. I would do that to see if you have a good baseline.

[Midnightsun]

One needs to rule out any connection between the 2 banks as a connection between them would equalize the banks which translates into equal voltages on both banks.

How old is the house bank? House banks tends to be neglected by drawing it down too far when away from shore power causing an early demise over time. Right now it is being severely hurt by sitting at 9v when one considers 11.5v is 100% dead. I suspect you have a bad cell or bad battery on the house side, maybe even a direct internal short.

If connected to shore power, can you bring the battery SOC up to full? If so, then how long does it take to diminish the voltage once the battery charger is disconnected? Do you even have a charger? You do mention you have a NOCO 10 x 4 but it is not connected?

[cjblovinlife]

Thanks everyone that gives me a lot to think about and check on .

A few answers to questions posed.

No I do not have an isolator or ACR
All batteries test good under load, disconnected from the system using Schumacher tester.
I do have the noco charger but have chosen not to hook it back up yet. I charge the house batteries off line then put them back just trying to simplify and troubleshoot and didn’t want another variable.
The sync guauge is interesting I did not disconnect that ..
I don’t know how to truly test alternators or voltage regulators all I know is to test voltage at battery while it’s running hoping for 13-14v
On the DC panel I’ll look for added components the mains do not trip all panel lights go dim and lose power I guess I should put a meter on the mains when that happens and at the switch.. all lines have been traced there are no black boxes

[cjblovinlife]

Source of the post Do you have a dual battery isolator for 2 alternators and 2-3 banks utilizing diodes? A faulty diode can leak back and essentially connect two banks together. Also, if you have a engine sync gauge, disconnect it and the module if applicable.

The panel shutting down due to low voltage is new, I don't think I've seen that one before but a previous owner may have installed a low voltage cutoff device, check behind the panel. When you say the panel shuts down, does the main breaker trip? Does the panel DC meter drop to zero? No DC loads work at all?

When you note battery voltages are you looking at ship's meters or a multimeter?

Do you have an inverter?

I’m looking at the boats voltmeters and verifying with my multimeter.
I don’t have an inverter
The sync module is interesting I’ll check that. I was wondering if I should get an act or isolater but haven’t yet , actually I was trying to figure out if the Noco genius essentially has an act built in .. I would think it must but they don’t use that terminology

[Midnightsun]

My understanding is the NOCO is like having 4 small independent chargers which is why it can do separate chemistries on each output. 10a is not a very big unit at all for a house bank, actually rather minimal.

After you have charged the batteries and put them back, how long before it drops to unacceptable levels?

Is the fridge shut off? Could be the AC side is not functioning and it operates from DC only.