Orphaned (expensive) batteries - options?

I had a 3 string 24 volt battery bank (4 Trojan L16H-AC batteries per string). The batteries are a bit less than 2 years old.
A recent maintenance check revealed that one battery in one string was dead (volts=4.1; SGs=0.000, 1.110, 1.225). Fortunately, I have several 12 volt circuits, and panels and a controller left over from an old 12 volt system, that would make use of two of the batteries.
That leaves 1 orphaned battery that is still in very good shape. I would hate to see it just sit and waste away. Are there any options for maintaining it? Can it (and whatever may be the odd battery out) be cycled in the 12 volt system (or the 24 volt system)? Is there a 6 volt charger, or other charge maintenance option, that can maintain a 435 AH battery? I haven't found anything yet but then I may not be searching correctly. Any thoughts greatly appreciated. Thanks.
Comments
Swapping in and out sounds like a PITA to me, but I suppose it could be done.
No hope for a warranty claim to replace the failed battery?
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
https://www.batterychargers.com/en/products/battery-chargers/fr01333-15a-6v12v-fully-automatic-battery-maintainer
Rancher
Hi NicaSol,
IMO the self-discharge rate of a 435-ish Ah battery would most probably exceed the capability of most Battery Tenders. Do not use them, but have not seen a tender that would manage a 6V battery.
If you did have an automotive 12V battery charger, you could place a power resistor in series with one of the output leads to keep the charge current limited. Such a resistor may be hard to find, with the appropriate resistance and power rating.
You might try an auto headlamp in series with the output of a 12V automotive charger, and measure the current to see if it is appropriate for the charger that you are using.
If you are planning to trying to tend this battery manually, you will need a multimeter (DMM), and some ammeter. A Clamp on DC ammeter should be fine for this task.
If you do not use an ammeter, you could use your DMM, measure the voltage drop across a resistor of a known value, to determine the current -- I=E/R. A headlamp would not have this a known R ...
R=Resistance, E=Voltage, and I=Current.
Good luck, Vic
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
I am available for custom hardware/firmware development