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But I think AGM batteries sit at a slightly higher voltage. I have seen a few charts online but I am wondering if someone has a reliable chart that they know works. Today I used my 80AH AGM deep cycle for the 1st time. I charged 5 6s 3300's (from 30%) and my resting voltage is at 12.68. I think the resting voltage at full charge was around 12.85.
I have a number of 12V AGM batteries that I have fully charged and the resting voltage (measured several days after charging was completed with no discharge) ranged from 12.96 to 13.3V. I used a 9 stage 15A smart charger to charge the batteries. The batteries are all less than 18 months old, seldom discharged and not to less than 12V.
Float does not charge the battery--It just keeps up with "self discharge" of the battery (0.1% or less for AGM), and also supplies current to your daytime loads. It sounds like your charge controller is reaching ~29.2 volts--The specifications for your battery (two in series).
Presumably, the numbers are based on the minimum rated state of charge of the battery/cell (i.e., low state of charge, battery will freeze at "less cold" temperatures). AGM are supposed to withstand some freezing, but I would not make a habit of it (most things do not do well when water/electrolyte freezes and thaws--Expansion and contraction).
Re: charging AGM batteries in cold climates you could, but temperature compensation of the voltage needs to be there. a cold battery will need a higher voltage and just how much should be in the battery specs for your particular battery. once charging starts it will start to warm the battery up and the temp compensation will change as it goes up.
Re: charge rates for agm batteries only way to be 100% sure is to contact them. that particular battery can be subjected to alternator charging so it may have a higher rating, but don't count on it unless you hear it from them. odds are the alternator would not sustain higher than 30% for very long before the charge rate would cut back so i'm not sure if they allow for higher bursts of charge ...
Looking a SOC chart for the AGM's I chose these settings: Low battery shutdown - 23.5v (~35%) Reconnect and alarm - 23.9v (~40%) Charge detect - - - - 24.2v (~55%) For Victron, if LOBCO is triggered 3 times in a row, the inverter will remain off until this voltage is reached, it will then wake up and resume inverting. My fear is the spread ...
Avoid spending much more than 1 day below ~75% state of charge--The longer a Lead Acid battery spends below ~75% state of charge, the more it will "sulfate" (fluffy lead sulfate crystallizes--and is no longer part of the battery chemistry). Estimate battery bank/cell state of charge with no loads/charging current after a several hour "rest".
My guess, anytime the batteries are below ~23.0 volts (11.5 volts for a 12 volt bank) under moderate loads--They should be recharged--They are getting into the 50% or less state of charge zone--which can limit battery life to the 500-1,000 cycle range or so (3 years * 365 days = 1,095 cycles)...
Lead Acid batteries tend to be "complicated" (temperature, aging, state of charge, internal resistance, etc.). Li Ion batteries tend to be "simpler" (very low internal resistance on "good cells"--So State of Charge tends to follow battery voltage better and no temperature offsets, low current offsets).-Bill "a few thoughts" B.