Voltage is electrical pressure.
Will a charger seeking 14.2v be able to fully recharge a healthy battery. Yes. it will just take longer than one seeking to bring and hold the battery at 14.7v.
Now an aged battery, with a charing source seeking only 14.2, will it just take longer, well yes, but 14.2 might never be ale to fully recharge it either, higher electrical pressure required to top it up.
odyssey AGM batteries, are thin plate pure lead, TPPL, construction, they have HUGE CCA figures and can also be deeply cycled.
When they are deeply cycled, Odyssey recommends the charging source be able to provide NO LESS than 40 amps per 100Ah of battery capacity( the PC2150 is a group 31 at 100AH capacity, 1150CCA).
It is important to not folow the traditional LOW and slow trickle charge mentality on a deeply dischrged AGM, If one is looking to keep it performing as well as possible for as long as possible, and considering th rather large price tag of these AGMs, I would hope that someone would do what is required to meet their charging requirements.
NOw that is on a deply cycled battery. An Odyssey AGM as a starting battery, that is never discharged very far, the electical pressures are much lss important. Do not fear the fact that an alternator only goes as high as 14.2v where Odyssey says 14.7. if the battery is never discharged much the amperage reqired to intiially seek then hold 14.2 vs 14.7 will not be significant.
There are Very few charging sources which will meet Odyssey's recharge regimen on their own group 31. This battery acording to their own recomendations whe ndischarged to 50% or deper needs NO less than a 40 amp charger applied until battery voltage at the terminals reached 14.7. 14.7v is then to be held for 4 more hours. during this 4 hours amperage required to maintain 14.7v will taper to around 0.5 amps, and at this point the battery should be fully charged. but it might tke 6 hours for amperage to taper to 0.5% of the 100AH group 31 capacity.
THE ONLY way to determine full charge on an AGM battery is by bringing the battery to absorption voltage and holding it there until amps taper to 0.5% of the 20hr Ah capacity of the battery. If you cannot see the amperage, if your charger does not allow you to hold the voltage at 14.7v as long as required, well yhe battery is simply NOT fully charged unless amps required to maintain mid 14.0v range tapers to 0.5% of capacity, and 14.7v assumes a battery tempreature of 77F. Colder temps require higher voltages, hotter require lower.
AGM is the battery marketers newest favorite, as the consuming public believes them to be superior batteries, and they can be, but they are not immune to the abuse which degrades flooded lead acid batteries.
Most people shop on price, and AGMs have been falling in price significantly in the last few years. Does the cheapo AGM compared to a TPPL AGM like Odyssey or Northstar. no not even close. A low $$ AG will have slightly elevated CCA figures in most cases, they will have lower self discharge, and they should not offgass and cause terminal corrosion from being bathed in a sulfuric acid mist that occurs when a flooded battery is being held in the 14.0+ range. They should also be able to start the vehicle at a lower state of charge compared to the same group size flooded battery.
but they are not imune to chronic undercharging, and if they are intentionally deeply cycled, they really need a higher amp chargeing source than most garage chargers can provide, and getting them truly full is even more important.
Even the lesser$$ AGMS will benefit from higher recharging amperage, but osr lower $$ AGM batteries say to not exceed a 30% charge rate, whereas Odyssey says no less than 40%. Lifeline AGM, who make pretty much the best Deepcycle AGM batteries say No less than 20% when deeply cycled, and there are dozens of off griders who bought these batteries, feed them only low and slow solar only rechres and petulantly stop their feet when the battery capacity has declined much further than expected on their top of the line expensive AGM batteries.
Engine starting batteries have a cakewalk of a job in comparison to a daily deeply cycled battery, and their acceptable charging voltages and the amperage required to reach those voltages, and the durations at which higher voltages are held, are much less imortant than on a daily deeply cycled battery.
But ALL lead acid batteries always want to be fully recharged, and kept cool.
Truly fully rechrging a battery requires a lot more time than people expect, and this is whe nit is held at higher voltages.
A battery Nearly fully charged does not require many amps to be held at higher voltage, and many vehicles today are intentionally keeping the battery discharged low enough that it can accept higher voltages, when the voltage regulator tells the alternator to make enough amperage to bring system voltage up higher. The VRs basically ask for higher voltage when letting off the gas or braking, and this can somewhat help MPGs, but at the cost of battery longevity, as the battery always ants to be fully charged.
Such vehicle batteries would still benefit from being regularly top charged by a plug in charging source, but few of them are going to seek high engough voltage fr long enough to actually top charge the battery, as each battery will be different and change as it ages and the different temperatures it is when being charged, and the green light full chrge indicator on so called 'smart' chargers' is simply wrong, and mocks the end user who believes it. But ignorance is bliss, and the equipment and knowledge and effort required to actually have a battery reach a true full charge might not be worth the expenditure, compared to simply replacing the battery more often.
All lead acid batteries want to be returned to full charge ASAP, and it takes no less than 3.5 hours to go from 80% charged to 100% charged. it does not matter if you have a 300 amp alternator. The battery accepts only as much amperage it wants at the electrical pressure reaching the battery terminals and room temperature batteries should not spend much time at voltages above 14.8v.
Since this thread is about full charge resting voltage, my group 27 Northstar AGM rests at 13.06v, at 5 years of age and over 1000 deep cycles. It would NOT rest above 12.84v until I discharged it to 50% and then fed it 25+ amps until amps tapered to 0.45 at 14.7v, which is 0.5% of its 90Ah rated capacity. There is NO way this battery would still be going if I could not hold 14.7v until amps tapered to this low level.
All my charging sources, I can manually control the voltage, and all of them have digital ammeters so I can see how long to hold these high absorption voltages as I try hard to reach a true 100% state of charge after each discharge, whether it be the extremely minor anount required to start my engine, or the 25 Ah of cpacity required to run a small 12vDC fridge overnight in 80F ambients.
And regarding short trip driving, I did an experiment.
When this Northstar AGM battery was newer, if I held it at 14.7v for long enough, amps would taper to 0.0x amps. x as my ammeters cannot accurately read amperage below this. So when this battery was absolutely chock full, and I would start me engine, an my alternator would be commanded to seek 14.7v. So when amps tapered back to 0.0x at 14.7v, that energy whuch had been used to start the engine had been replaced. How long did this take? 45 seconds. 45 seconds of 14.7v replaced the energy into the battery that was used to start the engine. of couse not every vehicle seeks 14.7v after engine starting, and almost no vehicle is starting from a truly 100% charged battery, but the time it takes to return that which the starter motor required to start the engine, well it takes much less time than most people will tell you, especially on a modern fuel injected engine in mild ambient temperatures.
But few vehicle carging systems care about battery longevity, and do not recharge it fully ASAP. Parasitic draws with the engine off are higher, and some of the vehices today are using the alternator and starter as a very inefficient quasi regenerative braking system, which is very hard on the battery.
No lead acid battery is imune to living at a shronically undercharged state. Those deep cycling them and dep cycle battery makers are well aware of the damage incurred by living their life at a partial state of charge, and they are now incorporating carbon in to th plate paste in attempts to reduce the build up of capacity reducing hardened sulfation. One of the makers of these Carbon foam AGMbatteries is Firefly Oasis, and they come in a group 31 size for about 450$
