howzit...
this one is right up my alley, so thought i'd throw-in here (boatguy 25+yrs, abyc elect cert; battery charging/monitoring systems Specialist)
have sold/installed Lifeline's for over 20years, here's what experience i can offer:
1st off: AGM batteries REQUIRE full recharge regularly - maybe not every discharge cycle, but pert near every time.
and 3phase charging is the ONLY way to go (bulk, absorption, float) - some automotive alt-regulators will do this, but not most.
bulk/absorb charge volts is also TEMPERATURE DEPENDENT and Lifeline (all batts actually) charging regimen is rated at 77degF with a top voltage (Lifelines) at this temp of 14.3 - also note: this is battery temp NOT ambient air temp.
its a VERY good idea to never run em above 14.2 (and make damn sure certain you are using a known-calibrated volt meter) unless its significantly colder than the 70's, with LOWER VOLTS as temp of the battery gets above 77; ie: at 90, you want top volts to be no more than 14.0 to 14.1 abs MAX
they can be charged at upwards of 100% of their capacity with a battery TEMPERATURE SENSING charger, but a safe/recommended level is 25%.
'thermal runaway' is a known issue with AGM batts, so its always a good idea to check the temp of them when charging at high rates.
especially with older batts - and esp with multiple batts in parallel or series/parallel configs - esp with older batts, where 1 cell in 1 batt could go bad and cause the charger to put out more current than the bank requires, esp at float phase - the typical current req'd to float is appx 1/10th of 1% of the capacity of the bank - ie: a 100amphour battery, new, fully recharged would require only 0.1amps; a 400ah bank would typically req appx 0.4amps (less than 1/2 amp)
its also a very good idea, once one has spent the money to buy AGM (or gel) batts to invest in an amphour meter, to monitor amps/volts +amphours consumed - that way you always know where they are, charge-wise - and always able to monitor how many amps they require to maintain float voltage.
reason for this: when they are new (and all batts share this phenom) they dont req more than 1/10 of 1% to float - but as they age, the internal resistance increases, requiring more amps to maintain a given float voltage - at some point, like when 1 cell (in 1 batt) starts to go bad, the current req'd to float increases - if you have multiple batts in the bank, this one bad cell will cause the charger to output more amps than the rest of the cells/bank require, causing the other cells/batts to start overheating - then 'thermal runaway' happens - and if you have a BIG CHARGER (esp non-temp regulated), on a BIG BANK ?
meltdown WILL ensue - as Riptide has just learned the hard way.
and just wanted to add:
all this should not be a reason to shy away from AGM batteries - since once past the pain of scratching the check to buy them, they really are THE way to go - esp if using solarPV to recharge, as they require appx 20-25% less charging (amps or time) to refill em;
ie: a typical lead-acid batt req's appx 135% of drawdown to fully recharge
vs appx 115% to refill an AGM
= less charge time = particularly important when using PV (or on a cruising sailboat, fuel burn)
and like anything else, TIME = $$$ (and more watts of PV)
also - when using PV - always use a regulator. - for something like vans/RV's with say, a couple/300watts of PV, i'm partial to what used to be known/sold as the Trace C-series regulators (or 'controllers' as they are referred to in the PV sector) - they are now owned/sold by Xantrex, nka Schneider Electric
have used these myself for over 15years, they are a true 3phase regulator, employing the bulk,absorb,float chg regimen.