Charging a zero'd-out AGM battery, what worked for me

[RDJ]

Hello all. I did not see the below published elsewhere on this forum; maybe many already know it, but it was news to me.

Apparently, AGM batteries' chargers have cut-outs that stop them trying to recharge bad batteries. Unfortunately, the same eletroniz gizmo stops them charging way-too-low-remnant-charge batteries which can’t produce enough voltage to disable this well-intentioned safety; even when the battery could recover from very low charge and provide years more service life.

Workaround:

disconnect the dud
leave off the charger for now
find another fully-charged 12V battery
hook up the dud to the fresh 'un, +ve to +ve, -ve to -ve (i.e. in parallel)
connect the charger to the fresh battery and then turn on the charger at the wall
charge the fresh battery - which will provide a signal to the charger that both batteries are OK to charge
charge for 1-2 hours - till the dud is back to about 10.5V
switch off charger
remove fresh battery
connect charger to formerly-dud battery which now has sufficient charge to trigger 'OK go ahead' from charger
charge overnight (from about 10.5 till about 12.5-12.8)


and yer done

saved me buying a new battery after The Virus stranded a bike 'o mine for 6 months in the US and then 2 months to ship home...

YMMV. Use at your own risk, etc., so forth.

[F5 Dave]

Or just add another load over the terminals usually does the trick. An old indicator worked fine for me then remove after a while. Does get a fair bit hot so use caution for cheapo fittings.