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Buy a low battery voltage alarm. Common in boat and battery shops or on ebay.
Some smart chargers just will not charge a really flat battery.
What I would do is use a lab supply set to a voltage limit of 14.4V and a current limit of 0.9A (capacity/10) and see what happens to the voltage over a few hours. Maybe you could borrow a suitable supply.
If the voltage rises, keep supply at 14.4 until the current drops to about 90mA. Then set voltage limit to 13.6 and leave for a day.
Don't try charging at over capacity/10.
Even if charging works, the battery will never regain full capacity and will have a shortened life.
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