Is my battery bank dead from sitting idle for a year?

I have a set of 12v Sun Xtender sealed Lead Acid batteries in my battery bank wired together in parallel pairs (24v). They had been working fine until I went away for over a year and they sat inactive. The voltage now reads as about 7-8v per pair and they don't seem to want to recharge now they are reconnected to the solar array. I have read that you should never let batteries go below 50% but it seems they are well below that.
Are they mostly likely dead but retrievable, or temporarily dead and somehow retrievable?
Thanks in advance for any kind advice.
Are they mostly likely dead but retrievable, or temporarily dead and somehow retrievable?
Thanks in advance for any kind advice.
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Most charge controllers won't try to charge a battery that is that low. I suggest putting them on a dumb charger, individually on something like a car charger. Be aware that they may have limited capacity at first, so watch the voltage! Once it gets up to 13-14 volts, I'd pull them off. See what voltage they are holding, then try them back on the solar charger. Go to SunExtender's web site, I think they have a system to do something like an equalizing. Where you hold the voltage at a high level for an extended period of time.
If you are lucky enough Marc might see this thread and give you some details. I can't imagine them getting to that low a voltage without having some type of load on them.
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
https://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101F-Lifeline-Technical-Manual-Final-5-06-19.pdf
Upwards of 3 volts per cell (18 volts @ 12 volt battery)--Should not be done to any other brand/model of lead acid battery.
-Bill
1. The battery type is PVX-1080T
2. The standing voltage after a while is 9.9V
3. The first three digits are 013, so I'm guessing this means they are nine years old
4, I have a total of eight identical batteries
5. There is no sign of bulging on the sides or around the terminals.
6. I'm located in Kyoto, Japan.
I have found the recommended procedure in the manual for deep discharge recovery and am wondering how to do a constant current charge