How much battery power / capacity
It looks like I will VERY soon need to replace four of my Trojan T-105 6 Volt golf cart type batteries that are 8 yeras old and do not keep the charge any more. 13.4V to 12.3V with all lights and stereo running for two hours? I can not see the capacity rating but according to salesperson new Trojan T-105 has 225 AmpH capacity.
I do have to confess that although batteries are 8 years old i did neglect them for a while and last October found them drying out having to fill in almosta quart of distill water in each which might have caused the drop in performance.
I use my boat mainly for daysail and sometime for a long weekend, using stereo, lights and nav instruments. No big consumers onboard. What sort of capacity you would recommend?
The batteries aren't probably in all that bad a shape. A fully charge 12 VDC bank, even one made up of four T105 GC batteries, in two sets of two, will only be 12.6-12.7 VDC at 100% charge. Depending on what kind of lights and stereo you have, it could very well be that you drop them from 12.6/7 to 12.3 volts in two hours.
Letting them dry out was probably not very good for them. What I would recommend you do before biting the bullet and buying new batteries, is to get a good hydrometer and charge the batteries fully, then disconnect the cables and let them sit for an hour or so... then check the individual cells to see what the specific gravity is of each cell. If they're all about the same and come up as fully charged... They're probably okay. It might be that one of the batteries has a bad cell, and that would bring all four down rather quickly.
I would try to put an Equalizing charge on them before giving up...but a pair of T105's WILL provide 225AH's at 12Volts. This means you have 115 ah's or so to work with before you need to recharge. (12.2V at rest.)
"All lights and stereo running" is hard to make a judgement about...but let us assume 3 amps for the stereo and 7 amps for a few lights and you have 20 amp hours used in 2 hours...not nearly enough to pull your battery down to 12.3 if it was in good shape.
But...it does seem like a pair of T-105's is sufficient for your weekending if refrigeration or other big amp users are not on board. I'd keep it simple and replace with the same if you can't revive the old ones.
I agree with Cam that it would be worthwhile equalizing the batteries first to see what improvement might occur. I also agree that if/when you decide to replace them, it probably makes most sense to replace them in-kind, i.e., T-105s. They can produce enough for your needs.
However, I would caution you about drawing too many conclusions based on voltage decay alone. In particular, older batteries which have not been equalized and which have not been exercised (drawn down to 50% or so, and fully recharged a few times) may exhibit exactly the type of behavior you're seeing. What's instructive is that they have a great deal more energy left after reaching the 12.3 volts under a modest 10-20A load. See, for example, what this 225AH gelled bank did with only a 10A load. http://gallery.wdsg.com/Misc-Stuff-S...PhaseII?full=1
It reached 12.3V after about 180mins, then took another couple of hours to reach 12.0v and a lot longer to get to 11.5 volts under load. The curves for T-105s under load were very similar, and were uniformly at lower voltages (gelled and AGM batteries produce slightly higher voltages than do flooded batteries).
Another data point: I have six T-105s on my boat in a single bank which are 2 years old. They have been well treated, kept at 13.2V floating with a Victron charger while at dockside, and never discharged below 12.2v under load and never run dry. Yet with only a very modest load they will drop from 13.2v to 12.3 or 12.4 volts in just 20-30 minutes. This is because they haven't been exercised since September and haven't been equalized. I expect (and hope) that as the spring cruising season gets underway and the batteries get exercised they will show much better "staying power". This has always been the case in the past (I've used T-105s for over a decade).
That said, at eight years your T-105s have most likely exceeded their useful life. And, yes, letting them run dry will do damage rather quickly. I'd therefore consider replacing them before you do any serious sailing.
Hope this helps a bit.
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