deep cycle aux battery

A psychic is a person who claims to have an ability to perceive information hidden from the normal senses through extrasensory perception.

I'm one and I perceive that it has dropped a cell.

BrendanI4,

What model is your car?

When your measuring the voltage are you measuring it on the battery terminal posts
or the battery terminal clamps?

What brand of battery?

How do you know the battery is fully charged?

How is the battery normally charged?

Leigh

I would say it is not charging properly....12v is not enough. Maybe your charger???

brand /type. ??size

Brendan, a fully charged battery should be around 12.8 V, not 12 V. I agree with those who say the battery is not being charged. When are you going to answer the questions posed by the others so we can give you some real answers?

Brendan it can have a lot to do also with how far have you discharged it and how long did you leave it in a low charge state. Deep cycle batteries should not be taken below 50% Soc

Which would equate to about 12.3 to 12.4v so if your battery is reading 12v then it could be as low as 25% soc. As others have said check your charger.

It is not unusual for batteries to have a very short life indeed.....no matter what the brand or type of battery if it is cycled too deep or too often it will die in short order.

Some batteries, a single realy deep cycle can kill them.

cheers

Brendan
You have good quality batteries, good enough. There is always better. And people fall for the trap of buying the latest, greatest batteries, AGM, whatever.
Charge (lack of, mainly, but not only) and heat kills batteries. Run them way low, or cook them with engine heat, and kiss them goodbye. dying in front of your eyes. Deep cycle along with the rest. You say you have solar, but do you park it outside often? What wattage, what controller, PWM or MPPT?
Yes, cells do just die, and yes that's a warranty issue. But have you had similar issues before? I know a lot of people who have had batteries failing for years, and move on each time to a "better" battery each time.
There is a lot of good advice in the replies but one thing that stands out is the question about when you measured the voltage? During charge, or how long afterwards? About 14.1 volt during (the latter stages of) charge, and it drops down to 12.8 if unused, and in good condition, once the charge is removed (say 20 minutes later). But with load connected, it will drop a lot faster. But don't expect to measure the battery voltage when its wired in place, load and the charge system will affect the measurement
All those figures are approximate, because the internal resistance of the battery changes as it takes charge.
Make sure you give the vehicle regular runs. Don't leave on a substantial load such as a fridge unless you need it. Solar doesn't do much in the shed. Buy yourself a small cheap smart charger, say a Ctek 5 amp smartcharger, and wire in the loom that comes with it such that you can quickly plug in the charger when you get home. Its only 2 wires onto the battery.
The smartcharger indicates the state of charge with led's. When you are 100% sure the battery is fully charged for most of its life, then you can consider what battery is best. Low charge will kill the best of them.
But with certainty, your current battery is dead. Yes, it can be that you got a dud, it happens. But only very occasionally. Think back, did you ever run it flat? Real flat? That time out with your mates?
Replace it, and look at charge system vs your usage
Do I sound like a bloke who has spent a lot of money making bad decisions re batteries in the past ha ha?
Good luck