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It's generally accepted that AGM batteries shouldn't be discharged by more than 50%, (which equates to 11.95v). If left heavily discharged for a long period, it may be impossible to recharge it fully. There are 'hacks' you can try to recover it, for example charging it in parallel with another, healthy, AGM battery. But, if you've any doubts about the battery's health, you're probably better off just replacing it.
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