The trick to "fool" the newer chargers to recognize a completely dead battery is to hook a charged battery to the terminals and begin the charging process. Once the battery comes up to the minimal voltage amount the charger will take over and you can remove the auxiliary battery.

The old rectifier type chargers just slam the juice in there with no feedback required but no regulation of current. They are rough on AGM batteries and can quickly ruin them.

If your battery voltage drops quickly under a load then you have a dead or shorted cell. No amount or type of charging will amend this. Replacement time.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now