Dual boat batteries - one way down on water

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Upstate NY
My new to me boat has a house and starting battery and battery switches and AC battery charger in addition to the engine alternator. Unsure if it has battery combiner or isolator.

Both batteries showed around 3 V. One was way way down on water.

So before spring I need to see if I can get the batteries up to usable condition with desulfating. Or buy one or two new batteries.

Then to determine if AC charger is not working properly and finally if engine alternator is working OK.

Batteries with very low voltage and low in battery acid with plates exposed do not give me hope for getting batteries back to working condition.

Am I missing something to check?
 

Donald

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Upstate NY
How old are the batteries? New batteries are always good . Time on the water is to precious to try and save $$$$.
Hard to tell. I see a punch out from 2010 to 2019 with no year punched out. Looked clean, not greasy or dirty. But it's a boat, not a car.

One is starting and the other deep cycle.

I have both a conductive and carbon pile load battery testers. So they need to pass all the tests before I will use them in the spring.
 

Donald

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Upstate NY
The deep cycle was down at least a cup of water over 6 cells. Maybe more.

New rig is 2011 Chaparral Signature 270. Volvo Penta 5.7L 300 hp duoprop

Prior owner was not a "engine" kind of guy so he could have done things to effect how/when the batteries were charged. Its got 2 or 3 battery switches. Its also possible the AC charger is defective and cooked a battery. If its a good AC charger it will have a seperate circuit for each battery. Unsure if it does.
 

Donald

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Well the starting battery that was not low on water seems to be 100% now. The one low on water is still less than 50% capacity.
 
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california
A battery that is both low on water and drained to 3 volts, has little hope of regaining usable capacity no matter how much effort one puts into trying to charge it.

And despite the sticker proudly proclaiming so, very few actual deep cycle flooded 12v batteries exist.

One can deep cycle a starting battery.
One can put a deep cycle sticker on a starting battery.

It does not make it a deep cycle battery.


A true capacity test is quite involved, and its highly unlikely any aged lead acid starting battery drained to 3 volts can be returned to 100% of its original capacity.

Even if it were relatively new, quickly drained to 3v, and prompty and fully recharged by a charger capable of bringing it to and holding it at absorption voltage for as long as is required to maximize specific gravity, it would not have 100% of its original capacity.
 

Donald

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I agree that its unlikely the battery that was very low on water will be usable again. However I have nothing to loose by letting the BatteryMinder try to rejuvenate the battery with it desulfation circuit. I would not buy a new battery until spring regardless.
 
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New rig is 2011 Chaparral Signature 270. Volvo Penta 5.7L 300 hp duoprop
Very nice, the Duoprop makes slow handling and docking a pleasure instead of a nightmare. I would think you really need to find out about the battery switch. Ideally you could charge both batteries from the alternator (lowest voltage first) while underway or combine them to boost a low voltage cranking battery. I hope the charger and the alternator check out as operating properly.
 
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california
Drained to 3v, its likely there was a continuous parasitic draw on them, ready to kill the next batteries unless its found and eliminated or its kept on a maintenance charger with more than enough output to offset it the whole time the boat is not in use.
 

Donald

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Very nice, the Duoprop makes slow handling and docking a pleasure instead of a nightmare. I would think you really need to find out about the battery switch. Ideally you could charge both batteries from the alternator (lowest voltage first) while underway or combine them to boost a low voltage cranking battery. I hope the charger and the alternator check out as operating properly.
My old Cruisers Inc 24' sport cruiser had dual batteries (starting and house) and a battery isolator. No battery switches. The alternator was connected to a battery isolator which in turn had a seperate connection to each battery to charge each one separately. Good concept except the battery had its SENSE wire (to determine how much to charge) connected to the starting battery and the isolator imposed a .7V drop of the charging voltage to each battery. The isolator is basically 2 silicon diodes connected as one way valves allowing alternator to battery flow but not battery to battery.
 

Donald

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25,041
Location
Upstate NY
Drained to 3v, its likely there was a continuous parasitic draw on them, ready to kill the next batteries unless its found and eliminated or its kept on a maintenance charger with more than enough output to offset it the whole time the boat is not in use.
The prior owner had the boat docked and plugged into shore power. Or the engine running. What I came across was after the boat was hauled and unconnected to shore power.

But I clearly will be on the lookout for issues in the spring.
 
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