12v sealed lead acid battery has low volts

[jonovid]

I have a 12v 1.2Ah sealed deep-cycle lead acid battery that now has 8 volts.
charging has not fixed the problem, the battery still holds 8 volts
its like 2 calls are non-existent     electrolyte levels?
the battery is 3 ys old and is charged one a week for 12 hrs.

[ejeffrey]

It certainly sounds dead.  Two of the cells might have shorted out.

You say it is charged 1/week for 12 hours.  Are you deep discharging it between then?  Are you sure it is actually being charged?

[Gregg]

The best way to test it is to check the voltage with a load.  Something like an automotive brake light bulb that draws a couple of amps would work well.  If the voltage drops to almost nothing, there is an open circuit either within a cell or between cells.  If it maintains something over 7 volts, one cell is shorted internally; probably from plates warping and contacting each other.

[IanMacdonald]

In my experience, if a SLA won't hold charge it is knackered. Attempts to revive them are rarely successful. Small sizes are cheap to replace and not worth the effort anyway.

[jonovid]

yes it looks knackered  did have it under 1 amp load. the voltage  sits at around 8V 
the problem only showed, up after a deep and full discharge when I left the equipment on.
with deep cycle batteries they do not say how many deep cycles you get. no guarantees you get the full nine lives.
-  thanks

[tautech]

the problem only showed, up after a deep and full discharge when I left the equipment on.
with deep cycle batteries they do not say how many deep cycles you get. no guarantees you get the full nine lives.
-  thanks

SLA's particularly don't like that, less so with a bit of age on them.
I've seen figures for 1000 charge cycles although you'd want to treat with with kid gloves in order to get that.

I use 12V 7A ones for night time pest spotlighting and even if you run them til the light dims repeatedly you're gunna shorten their life.
Once they start only holding a short charge, they're stuffed. You can piss around steady cycling/recycling them and desulfating them but you're only 

[Gyro]

...
the problem only showed, up after a deep and full discharge when I left the equipment on.
with deep cycle batteries they do not say how many deep cycles you get. no guarantees you get the full nine lives.
-  thanks

In your case it sounds like a combination of sitting fully discharged for a significant (I'm guessing) period of time and discharging below 1.8V per cell. A situation that is pretty much guaranteed to cause irreversible sulphation and drying out. They will tolerate full discharge but then require immediate recharging.