As a starting battery you won't go wrong with it, but for real deep cycle use it's hard to go past LiFePO4. At half the weight of lead acid, and twice the actual capacity available at the same nominal size it's probably better from a cost perspective. They also maintain a flatter voltage curve over discharge. 

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!

also you have made the effort to install top quality components and are obviously clued up on how to treat your charging and energy storage system to avoid harm

it should come as no surprise that you have received good service from it

I think the biggest attraction with Lifepo4 batteries is their ability to take a high current charge all the way to full - rather than tapper off to a trickle for the last few hours like AGMs - really good if youre away from mains power for a period and not necessarily doing long engine runs for charging

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.

 

Some were replaced with AGM some with the same wet cell, and without changing anything else the wet cells lasted longer than AGM.  Change (upgrade) the charging system and you get more lifespan out of both types of batteries.  In all but a handful of cases the issue was with useage cycle and charging system performance not the type of battery.

 

Me main point is/was, if you use a start rated AGM in a house application you'll kill it in a very short space of time - much faster than if you used a wet cell starting batt.  AGM is very good for what it's good at, fitting more starting capacity into a limited space is one major gain and there are others.

 

They might have a lower internal resistance, but once they reach a reduced charge point they will completely discharge in a very short space of time even in a completely disconnected bank (1st hand knowledge there, embarrasment of no starty starty - and that info isn't in any propaganda I have seen).  Because of this they will ALWAYS require maintenance charge, and from a completely discharged level you are supposed to use a specialist charger to avoid battery damage and cookup.

 

As well, AGM was designed as a military option to prevent battery distruction from being tipped over or casing damaged, it's not a miracle technology.  For the average punter, wet cell with a charging system should give you longer than 4 years and if you get 7 you are doing extremely well.  If the setup fails in less than 3 you have a battery killing issue which is what SC described above.

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 was told to never deep discharge an AGM as you will never be able to properly recharge it without the proper voltages (i.e. you need the correct conditioning charger).   I have my setup charging at 14.8v to the AGM start banks, and the batts are due for a conditioning charge cycle now.

 

My house bank is two 6v deep cycle in series for 12v, which gives a much more stable voltage at smaller discharge currents, for a longer period of time.

 

In minor use applications (which is what most small boat owners are using them for) you can get away with using whatever battery you like.  However, use the battery to it's capacity and you will get punished for using the wrong type in the wrong application.

 

Everyone knows not to use a deep cycle as a start battery, and it's the same with AGM they are high load short duration UNLESS you are talking about a hybrid construction battery but then there are other issues with those...  The basic thing is a deep cycle battery bank, properly configured will outperform an AGM battery in house bank-type applications which is a discharge current for periods longer than 36hours. 

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.