I can't figure what's going on

[Sternwake]

You say the oldest odyssey in your bank is from 2011. How old is the newest?

About how many total deep cycles have you gotten from them? Time means little. Batteries do age but the amount of cycles they last is much more telling than their age. One can keep a battery on float voltage for 12 years. That is not impressive, but 300 deep cycles every year for 12 years certainly would be.

There is no guarantee another battery, whose ideal recharge requirements could be less demanding, would last any longer in the same usage.

The different ages of batteries in your battery bank make me suspicious of imbalanced wiring along with the un-ideal nature of different age batteries in parallel

If the voltage only recently fell to alarming levels, you could have one battery with a shorted/dead cell, and the other 5 batteries are basically trying to charge that battery and power your remaining loads. I had one battery of 2 in parallel fail and one morning I expected to see 12.45 and saw 12.18 instead. Investigation revealed a shorted cell. The other battery lasted another 150 cycles covering the same loads.

YOu could try unparallelling the batteries and see if any instantly drops to the 10.5v range, of so remove it from service.

If you need new batteries, your current location basically dictates what you will wind up with. If you got over 800 deep cycles from the Odysseys, they did pretty good considering their less than ideal recharging.

I got a hankering for Lifeline AGM's, I think they are the top Dog deep cycle AGM. My Northstar has been impressive, so far, but it is only in the 150 deep cycle range so far.

Beef up your alternator cabling and ventilation, to get your bank to 14.7v as fast as possible when you run your engine. Paying for more copper now will save you money in lead later.

[jeepbluetj]

I'm a big fan of FLA (wet) cells. Except for spillage if you roll over. And I think GC2's are the best bang for buck of any battery. A 440 AH bank is less than $400 (in the US). And they charge well from solar.

Maintenance is minimal, look at the levels every now and then and dump in some distilled water. Of course, if you can't get to your batteries easily - that's a plus for AGMs. But as mentioned, AGMs need a very good charging regimen to last a long time.

[Ostinato]

Wiring from the alternators to the house bank is 0000 - about a 3% loss. Wiring between houseb batteries is atleast 000. I could add fans.

The two newest batteries are 2015. I did loose one of the 2011s due to a short while the boat was on the hard last winter. We were delayed in returning to Turkey due to health reasons so i have no clue as to exactly when. This whole thing could be due to that of course.

So do i replace just the 2011s if i stay with Odysessy or do I throw them all out?. I'm inclined to replace my starting batteries with the 2015s (over kill for sure) if I can fit them in. The starting batteries also date to 2011.

Total cycles on the 2011s in the house bank is probably less than 250 but that is educated guess based the meter.

[Sternwake]

New batteries paralleled with older and deeply cycled, degrade quickly.

But I would fully charge them individually at the 40% rate to 14.7v and terminate only when amps taper to 0.62, and then give them 24 or more hours to rest, and see what their voltage levels off to. They could be great starting batteries, even if overkill.

I would not add new and old batteries in parallel again as a house bank.

Sounds like you cannot beef up your alternator-battery cabling anymore, perhaps the alternator can be, but I suspect with 4/0 cabling, it already is a very capable unit.

Will your voltage regulator allow 14.7v?

[Ostinato]

The regulators are Balmar 612s. I will have to check. Nigel Calder (whose Boatowner's book put me on to Odyssey batteries) reports in an in an Ocean Navigator article that he set up a 614 so it did something like that. The 612s and 614s are very similar.