Battery - one of a pair appears to be failing same as last time

[Momac]

Two 12V 110Ah batteries connected together. 12Volt system. These are the domestic batteries but also start the starboard engine.
A shunt on the positive terminal clamp on one battery is for measurement of current and voltage (I think).

Not getting expected performance 24 hrs stopped and voltage is low . 48 hrs should be achievable . One battery appears weaker than the other. Its the battery with the shunt . Batteries new June 2016.
The odd thing is this same thing happened before with the previous batteries .

How can one battery become weak or fail and not the other ? It seems like one battery is not being charged properly but how can that be when the two batteries are connected together.

 

[BruceK]

Even though my batteries are wired together properly with matching length connector leads etc I managed to add a temporary cockpit canvas light directly cabled to the batteries for our cruise last year. Because the ends with crocodile clips could not spread across the width of the bank I just put them across one battery. It was enough to cause that battery to flatten more than the remaining two and drag them down. Hard to fathom but proved quite a costly error as trying to charge them back up on battery charger also caused the alternator to fail. Leccy takes the path of least resistance. If the system isn't exactly matched then the batteries wont share load evenly. That's where I'd start looking.

 

[Croftie]

How are your 2 batteries connected? Seems there is a prefared way of connecting them in parallel to keep them in balance.
https://caravanchronicles.com/guides/how-to-connect-two-batteries-in-parallel/

 

[BartW]

perhaps you have the negative cable also on the same baterry that has the positive cable with the shunt ?
if so, the loads have more resistance to the other battery. (one battery gets more load)

so for best ballancing the load on the 2 batts:

pos connection to the loads is on one battery,
neg connection to the loads is on the OTHER battery,
interconnection between the two battery's are obviously cables of good size, and make good connection

edit: drawings in previous post explains what I mean ;-)
posts have crossed

 

[BruceK]

perhaps you have the negative cable also on the same baterry that has the positive cable with the shunt ?
if so, the loads have more resistance to the other battery. (one battery gets more load)

so for best ballancing the load on the 2 batts:

pos connection to the loads is on one battery,
neg connection to the loads is on the OTHER battery,
interconnection between the two battery's are obviously cables of good size, and make good connection

and are all of equal length

 

[BartW]

and are all of equal length

correct,
but in my experience,
when the size of the cables is adequate,
the connection from the cables to the terminals is equally or even more important,
(a bit more resistance if not correctly done...)

 

[Momac]

Thank you for the information on the correct way to connect two batteries etc.
I will need to study my set up to see if it is correct or not. But I do think the shunt and the earth are both on the same battery - which appears incorrect . It certainly means there is potentially a good explanation why one battery of the pair appears to be weaker than the other and repeating past experience.

 

[Momac]

I have read the caravan chronicles article again again and sketched up my batteries from memory (I am not at the boat).
I even think the fix may be possible by reversing the positive cable .