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.