Originally Posted by sailinglegend
I think you should do the C20 test on each battery individually to see if they are all the same. That way the loads are much more controllable with lights and even maybe the inverter on small AC loads.
I'd be interested in the results as I don't know anybody who has managed a full 20 hours monitoring the loads. If a battery has lost a lot of capacity then it may reach 10.5v after only 10 hours. If that happens I assume you have to do the test again with a lower C20 current based on a reduced estimate of the actual capacity of the battery. If it still doesn't last 20 hours then you do it again with an even lower load.
Your battery capacity is based on the "as new" C20 rating. When it falls below 80% of capacity, by industry standards, it is dead. AGM
makers claim 50% of new rating, but in my experience any time they dip below 70% of rated as new capacity
they tend to fall off the cliff rather rapidly and become quite unpredictable. Same goes for good quality flooded batteries.
There is really no need to identify a new 20 hour rate on used batteries unless you are going to be doing some very specific use or testing with them. A C/20 test, at factory discharge rate, tells you all of what you need to know in terms of your batteries health
I'll pass some batteries at 75% of new ah capacity, yet fail others. All this is simply based on my experience with a particular type of battery. Many batteries will continue to perform house duties below 80% of new but once they drop to 70% or so, in my experience, their predictability becomes poor and the batteries simply don't charge or discharge they way they should.
Of course all of this is dependent upon the particular application & use
. I have some AGM
banks out there that I let go another year as low as 65% of new capacity based on use, load, bank size
etc. I also have some good quality deep cycle flooded banks I would let do the same. No real simple
I let one customer talk me into letting him sail to Bermuda
with AGM's that tested at 73% of new. He threw the "blah, blah, blah 50% is dead for AGM's"
at me. I knew better for his intended use
that this was a bad idea at 73%.. I tried hard to talk him out of it but in the end it was his boat and his choice.
Sadly it bit him in the a$$ pretty hard and his cost of replacement AGM's down there was astronomical compared to what I had just quoted him here in Maine
, according to him more than double
.. No simple answer.... I would have let a coastal cruiser keep those batts another season but for off-shore.... Oops....
While you could get very picky and try to identify the batteries new, used condition, 20 hour rate, as I have had to do for some testing I have conducted, you are going far beyond what is necessary for what you want to know.
If you really want to get that picky, for an Ah/Coulomb counter, then you'd also want to identify the new Peukert and capacity based on varying temp, unless you can control battery temp to 75-80F all the time.
I've yet to see an accurately calibrated Ah counter, that matched actual tested bank capacity, and don't expect to see one any time soon. Is this bad? Not really as I still find owners with battery monitors still get longer bank life, on average
I always note the SOC and -Ah screens before removing batteries for Ah capacity testing. The closest I see is within 10% and I have seen them over 80% off.... One capacity test at factory C/20 is really quite adequate, for what you need, and will get an Ah counter far closer than it previously was. This is also far better than perhaps 99.8999% of what most boaters actually know about their banks current physical capacity..
Any battery that can't supply 50% of the original factory Ah capacity rating during a C/20 load test, at 77F (75-80F is fine), is really scrap lead...