There's very little energy stored in a battery below 10.5v, some AGM say 11.1v at rest, is considered 100% discharged.
Huge difference if it was drawn below 10.5v 7 weeks ago or the day before, in terms of potential recovery.
Getting it battery to its full (unknown) potential remaining capacity, requires a true full charge. Not just old school charger A, then intelli- maintainer B, until green light flashes.
Might as well just rip out your speedometer and odometer and just drive until somebody tells you have arrived from an unknown starting point.
I'll assume a 100 amps hour group 27.
Hold it at 14.2 to 14.4v until amps taper to 0.5 or less, then float at 13.4v for 24 hours plus.
DeKa says 30 charging amps max, per 100 ah of capacity.
I greatly exceed this, often.
My dekas have a 5/2017 due code.
In deep cycle duty, no less than 20 charging amps and charge to true full as often as is possible, is the only way to get good + service life from agms.
Starting duty can fall anywhere from unintentional deep cycle and partial recharges, and whining humans, to a long life lived at 99%+ charged, and humans who then think AGMs are amazingly invulnerable to abuse, which they most assuredly are not.
Whenever I notice less voltage held than expected from my gc-2 6v Deka intimidators, when powering either a 180 amps starter for 2 seconds, or a 5 amp load for 10 hours, i intentionally drain the battery to ~50% or less, over 6 to 10 hours, then immediately hit them with 40+ amps, bring them to 14.4v for 30 minutes, lower to 14.1v, and wait for amps to taper to 0.5 or less.
This is no less than a 7 hour recharging process, even with a charger with 100 amp potential.
Each subsequent deep cycle, or engine start, the voltage maintained is significantly improved.
The deeper the discharge, the more important the recharge becomes, in maintaining both capacity, and cranking amp potential, and the potential speed of subsequent recharges/charge acceptance.
Deep cycled AGMs are tickled to a premature demise with low and slow charging currents. It is also important to achieve a true full charge, often.
The ONLY way to determine true full charge on an agm, is by holding the battery at absorption voltage, Not float, and watching the ammeter.
When it cannot accept more than 0.5 amps at absorption voltage, it is in the 99% charged range.
That last 1% can take 15 minutes, or 6 hours and how long it takes also indicates battery health.
But without an ammeter to see flow, much less a voltmeter to see electrical pressure, all one can hope for is the green light from an extra specially marketed charger/maintainer, and the amount of fanboys on the internet praising its abilities, without a clue.
