New AGM batteries - what am I doing wrong?

[Adelie]

Also, AGMs are not a good choice for sailboat service unless you motor a lot.
They don’t like:
A. PSoC service (Partial State of Charge), ie not being fully recharged almost every cycle. Firefly/Carbon Foam AGMs are resistant to this but cost a lot more.
B. Low-C bulk charging. They should be recharged at 0.2C minimum (20Amps for every 100Ahr capacity, so 60Amps for a 300Ahr bank), 0.3-0.4C would be better. That means you need a high output alternator and probably a serpentine belt to drive it and a smart regulator. Even Firefly’s need high-C charging or cycle life will suffer.

For those that don’t know bulk charging is when the battery bank will accept as much amperage as the alternator can put out. Generally this is from about 50% state of charge up to around 75%. For FLAs and Gels this should be 0.13C or less. Voltage will be below the target voltage of about 14v but will slowly rise until it gets to target at which point bulk charging ends and the regulator limits voltage to avoid damaging the battery. This is call absorption charging and amperage slowly tapers off.