I would not bother with an AGM for an exclusive starting battery, especially if it is going to be cycled and only see the voltage the vehicle's voltage regulator allows for the likely short duration the engine is actually running.
Having to use an AGM just because the house bank is AGM is freaking asinine, in my opinion. Yes we have all read this recommendation for 20+ years on the intrernet and it is repeated ad nauseam so much so that it it taken as fact without any qusestion as to its veracity.
Perhaps if they were going to be discharged together that makes sense, but charged together, by the vehicles voltage regulation, It will be too low once the alternator warms up anyway, for either battery, flooded or AGM. The battery wioll accpet as much or as little as it wants at the voltage the vehicle allows, and this in 99,5% of vehicles is not going to be too high for too long to be dangerous to either flooded or AGM. It will likely be too low for way too short a duration for either battery, in the absence of other charging sources.
The advantage of AGM for engine starting is higher CCA, and no terminal corrosion, but these johnson control batteries have CCA figures barely any bigger than the flooded starting batteries. so that leaves the lack of terminal corrosion, which is easily negated with a little application of grease over the installed terminals.
If one is going to discharge the battery to 20% SOC, then the AGm will stand a better chance of starting the engine than a flooded. how often is that going to be an advantage?
12.84v might very well have been the max full voltage when resting fully charged for that JOhnson controls AGM battery.
The Odyssey or Northstar batteries are excellent, when one actually needs a very heavy duty and capable dual purpose battery. They have extremely high CCA figures, hold high voltages with huge loads on them, and can accept obscene amperage from high amperage charging sources, upto the 80% state of charge level when all lead acid batteries start limiting what they can accept and are likely no faster from 80% to 100% than flooded starting batteries.
But if deeply discharged they require these huge charging amperages, and with out them they are not going to be happy, nor live up to their potential and certainly be a poor choice considering their price.
I have not owned an Odyssey, but I do own a Northstar group 27, and the other morning I was down 70AH from its 90Ah capacity. it easily started my engine and my Wattmeter, which records minimum voltage, did not dip below 9.78v cranking my engine. This battery turns 5 years old in November and has about 800 deep cycles on it and a large percentage of those well below 50% SOC.
After starting my engine it was accepting 97 amps from my alternator, and after 3 miles of slow driving then 7 miles of highway driving it was still accepting 83 amps.
Does anyone need this ability from their engine starting battery?
Not if they have house batteries. I use my Northstar for both house and engine and its capability is required, and impressive. If I had a dedicated house battery, which I could and have in the past, then the engine starting battery never ever gets discharged, and by the time its capacity degrades to the point of iffyness, 7 years have passed.
High $$ AGMs are not immune to abuse. if one is notoriously hard on engine starting batteries this insinuates they do nto drive often and parasitic loads will draw the battery to low levels. An AGM is not immune to this, it is likely more susceptible to being in a state of less than fully charged compared to the cheapest startinig flooded battery available, so it makes even less sense to spend th eextra money for a high $$ AGm battery used solely as an engine starting battery, that is depending on the vehicles charging system and its **** voltage regulation to return the battery to full.
As far as ordering AGMs online, well who knows how long they have been sitting on the shelf, and UPS and FEdex drivers and anybody else having to unload a 80LB battery is not going to handle it tenderly. Expect the corners of the battery to be crushed, the battery to be old, likely presulfated, and likely damaged so some degree, in shipping from that free shipping seller.
Anyone with a house battery bank should be able to use that house bank capacity to easily start the engine, except perhaps in North Dakota in January.
Nobody with a House battery bank requires a High $$ AGM dedicated starting battery, as ALL loads should be on the house batteries, and the only discharging should be the parasitic loads of the vehicle on the engine battery, and the self discharge of the battery itself.
AGMs too have less self discharge than Flooded, but the high$$ ones like Lifeline Northstar and Odyssey ahave the super low self discharge, the budget AGMS like deka intimidator often relabelled as duracells or the Chinese UPG batteries, are not much better than Flooded batteries in terms of self discharge, so that leaves their main advantage being lack of terminal corrosion, which is easily remedied by a healthy dollop of high temp grease on the battery terminal clamps after they are tightened on the battery posts.
If one is going to let a vehicle sit, then an AGM should have less self discharge, but it is NOT immune to parasitic loads, it is not going to magically recharge faster to full when the engine is started and run for X amount of time, and there is Zero reason to thinK an AGM is going to last any longer in this scenario.
An AGM battery is NOt iummune to abuse, and it is not magical, and unless the vehicle itself, came with an AGM battery under a seat inside the vehicle then the only reason to go for an AGM is terminal corrosion, easily negated, and less self discharge, which is not much a factor if there are parasitic draws on the battery.
Higher CCA, well look at the CCA figures of more budget friendly AGMS compared to their flooded counterparts. Very little advantage there.
Now if one were going to regularly insure their starter AGM was top charged, not going to be subjected to parasitic draws of engine computer and door locks, and actually needed to start the engine with well depleted house and engine batteries in North Dakota in january, then have at it.
otherwise get whatever flooded starting battery that is most easily replaced if it should fail within the warranty due to defect.
No battery without defect will fail within warranty if it is properly recharged and not allowed to slowly discharge to low levels anyway. It requires so very little of a battery's ability to actually start a modern fuel injected enginne.
I would absolutely hate to be a battery retailer, as the general public is so freaking inept, and so freaking entitled, and so Ignorant of what a lead acid battery requires, and how incapable their vehicle's charging system really is, in its ability to keep a battery healthy.
