A deeply discharged Odyssey AGM, ideally, wants 40 amps applied per 100Ah of capacity until 14.7v is reached, then hold 14.7v for 4 more hours then float at 13.6 to 13.8v.
Do you have a plug in charging source capable of this?
Few do, as few exist.
80% charged to 100% charged is not going to happen in less than 3.5 hours even if you achieve 14.7v at battery terminals and drive 3.5+ hours.
Expecting your vehicle's charging system to do an Ideal job , even if you drive for 3.5 hours, is unwise.
The drop to 12.7v for fuel savings only makes it worse, and the system, with two Odyssey batteries, which retain surface charge voltage higher and longer than a regular starting battery, will likely having them not only powering the vehicles electrical loads while underway, but also feeding into engine starting battery when it decides 12.7v is fine and dandy.
Seeing as how you are not even going to come close to being able to achieve ideal recharging via your vehicles charging system, I recommend accepting whatever your vehicle's voltage regulator allows, and then having a plug in charging source that can meet the 40 amps upto 14.7v, hold for 4 hours, per 100Ah of capacity requirement/recommendation.
It would be best to hook up this charger in their most depleted state.
You can of course get away with much less, and it will be philosophically 'just fine' for a period of time, but your 300$+ Odyssey AGMS will fail to live up to their potential.
I have cycled a Northstar AGM group27 to death at 6 years and 1200+ deep cycles and thousands of shallow cycles. Northstar has the same pure lead thin plate construction, and it indeed LOVES the high amp recharge to full.
Its voltage retention and engine cranking gusto is extremely obviously better after it gets a huge charge rate from a well depleted state and this is more and more apparent the more often it gets less than a huge charging rate and charged to less than full cycle after cycle.
I have fooled my 1989 engine computer with a 10ohm 50 watt resistor into thinking it is still attached to the externally regulated alternator, and no check engine light. I use a modified adjustable external voltage regulator whose voltage adjustment knob is on my Dashboard. I have one of the very few vehicles that can indeed fully charge a depleted battery to full in the minimum time safely possible but the 3.5 hours minimum, from 80% to 100% still applies.
Beware of idling to recharge. Underhood airflow and higher alternator fan rpm plays a huge part in alternator cooling, and Depleted Odysey AGMS can easily suck up 50+ amps each and fry your alternator, if it was told to deliver 14.2+v to the batteries.
The DC to Dc converter, unless it has an alternator temp sensor, can overheat the alternator even when the vehicle decides 12.7v is fine and dandy.
Any charging is of course better than no recharging of a depleted battery, but treating a pair of Odyssey AGMS correctly, to get them to live upto their potential, requires those high amp recharge parameters be met every few deep cycles .
