Hence, you can get above average starting performance from a smaller sized bank than with wet cells, but you need a heap more AGM to get the same performance as a deep-cycle in a house bank application...

I have a low maintenance N200 flooded battery as my start battery, but I have it charged from the 60 amp alternator on the 2nd motor.

The charging profile for flooded or AGM batteries is very similar, they both want bulk at 14.4 volts, absorption at 14.2 and float around 13.5, however you would need to understand the charging capabilities of your setup and I have no knowledge at all of the charging performance of outboards.

If you do go for AGM for your house battery then definitely ensure you get a battery that can be equalized given that you won't have easy access to shore based power you will need to try and equalize every six months at least. Some AGM battery manufacturers still actively discourage equalizing their batteries.

The charging required for an AGM battery is the same as a flooded battery to all intents and purposes. They are closer in charging profile to a standard automotive flooded battery than a deep cycle flooded battery. ie bulk at 14.4, absorption at 14.2 -14.4 and float at 13.5. I would check with the manufacturer re charging your AGM at 14.8 bulk Busted, as most do not recommend going over 14.6. Both AGM deep cycle and Flooded deep cycle suffer from sulphation if they are not fully charged in each cycle or are left sitting below fully charged for any length of time. This can be reduced by ensuring each discharge/charge cycle brings the battery back to a minimum of 80% of capacity, this can be difficult in every day use. 

Both AGM and Flooded deep cycle batteries can use a conditioning or equalisation cycle to help both desulphate the battery and equalise the charge state of each of the six 2 volt cells that make up the battery. The equalisation process for an AGM battery should be checked with each battery manufacturer (some of the "lesser" battery manufacturers recommend against it, don't buy their batteries for long term use!) but is basically to run a very low amperage, 15.6 volt charge into the battery for about 8 hours ONLY after the battery has been fully charged in the normal way. It is this function that a normal entry level battery charger will not do, and that is the only reason you need a different or more expensive battery charger for AGM batteries.

AGM batteries have a lower internal resistance than a standard flooded battery and therefore can accept charge at a far higher rate than the flooded battery can and for a longer time deeper into the charging cycle. A typical flooded battery can only accept up to 25% of it's stated capacity ie a 200 hour battery can accept charging at up to 50 amps, go above this and you just generate heat. Deep cycle batteries like the Lifeline are rated to the full amp hour capacity of the battery, ie. you can charge a 200 amp hour battery at 200 amps and thus get amp hours you have used back into the battery a lot lot faster, therefore getting it back above the 80% mark a lot more easily.

There are AGM start and AGM deep cycle batteries and you shouldn't use a deep cycle battery as a start battery or vice versa.

AGM batteries have been used as start batteries in cars at least since the Mazda Miata (MX-5) was launched in the States in 1989. They are becoming the battery of choice for high end cars that require the ability to support a whole lot of electronic extras, check out the Optima range of batteries if you want to go to the recognised class leader in these types of batteries in the states (and you don't mind paying for it!)

Contrary to Busted's comments above, AGM deep cycle batteries (like the Lifeline range) are built to provide current at whatever amp range you want (within reason and the size of the bank!) and at stable voltage for long periods of time. They maintain their voltage way deeper into the discharge cycle and will at say 50% discharge be providing more voltage than an equivalent sized and rated flooded battery will. They are also able top provide more cycles to a deeper discharge state than a standard flooded battery, in fact Lifeline claim their batteries have a cycle life of 1000 cycles to 50% discharge where most other standard batteries are in the 300 to 450 cycles area.

AGM batteries also have a very low self discharge rate (2% per month) so can sit, preferably fully charged (like all batteries) for longer periods of time without requiring a "trickle charge" to keep them alive. 

BUT AGM batteries are a lot more expensive than the standard flooded equivalent.

To get back to your original questions Bossco, looking at the Odyssey , I would have said it is more suited as a start battery and would look for a specific deep cycle battery for your house loads. 

Disclaimer: No, I don't work for any battery company, nor do I supply or install batteries. I have just got tired of continually replacing my standard flooded lead acid deep cycle house batteries every 2.5 years or so and have spent about 15 months researching what I should do to improve the power use and charging regime on my 36 foot launch. So what have I done? I have replaced a single 240 amp hour N240 deep cycle flooded battery with 2 x 255 amp hour Lifeline Deep Cycle batteries. I have replaced my 15 year old BEP regulator with a Balmar MC-614 so I can program and monitor the rechargeing and I have replaced an 80 amp alternator with a 150 amp Balmar alternator (the biggest I could fit in the mounting space avialable. I have increased the amp hour capacity as I have added electronics (including sat TV and PVR) to the boat as well.

I know that LiIon has now become more readily available and the price, although still expensive, is slowly coming down, but I don't want to delve into that issue, just update what my experience has been with the setup I installed, which was 2 x Lifeline 255Ah AGM batteries, Balmar MC614 Regulator and Balmar 150 amp alternator for the house system on my 12m boat.

It is now just coming up to 11 years from installation and I am still using the same 2 batteries. They are now probably down to about 80/85% of their original capacity/performance, however, they still take a charge well and are perfectly adequate for 1 to 2 days between charges while out and about. I remain convinced of my original comments about these batteries specifically and AGM batteries in general. I will probably be looking to replace them next winter (2024) which will mean I have got 12 years out of them as compared to the 2.5 to 3 years I was getting out of standard flooded batteries.

The MC614 has also lasted well and only gave up a couple of days ago with some sort of internal dead short that blew the supply fuse at the alternator. I carry a spare so that has now been installed.

The alternator has been replaced once 3 years ago not because it failed but because the earth terminal was broken off by lifting and rotating the Volvo 200hp motor without removing the connected cable! No, it wasn't me!

What would I do if I was doing the same thing now (as I will be next year)?

Not certain about the issues with LiIon and battery management systems that they involve so my current thoughts are that I will just replace the Lifeline AGM batteries and look forward to another 12 years.

Lifepo4 batteries are still an evolving tech in the marine environment in my opinion. I am unconvinced of the safety/reliability of the so called "drop-in" solutions, although these have improved a bit recently. Also the disconnect and load dropping characteristics mean planning for redundancy in the case of failure if you are away from the marina for an extended period adds considerably to the expense and size of bank/battery management required.

But, as I said, that rabbit hole is yet to be explored!