HI Nobby2,
If the battery fully charged is 11.9 volts, then it is faulty to start with. It should be at the very least reading over 12 volts.
When SLA battery's get to 9 volts, they are unusable. Never let a SLA battery go below 12 volts.
I'd take it back to Jaycar.
Cheers,
Damien.
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