Originally Posted by Rusty123
Please comment or point out any errors in my math or logic.
Unfortunately the data set is based on unrealistic white coat, pencil protector laboratory geekedom that simply does not translate well at all to the real world.. I wish it did because it would mean marine
batteries would be lasting coastal cruisers 20+ years, and we know that does not happen, ever...
I have never once seen a battery
in a marine
application get anywhere near the lab rated cycles
are done in a lab and conducted one after the other; discharge, charge, discharge charge all at a controlled temp, optimum charge rates, absorption voltages are higher than most boat owners will accept, and with all kinds of lab geekery going on to "ensure" these batteries hit the absolute max cycles possible. No boat owner I know of can ever hit the same parameters they use in the lab cycle after cycle.....
Unfortunately for us this type of data never translates into the real world. Temps are different we never discharge then immediately re-charge to 100% etc. etc. on and on. Also none of the manufacturers tend to do their cycle testing identically except for that it is done in a lab
, which definitely does not ever mimic how we use these batteries in the real world,and especially not in boats.
We also can't forget that it was not very long ago a particular AGM
maker claimed 80% DOD was possible with their product in the marine world... They claimed this, like you, to try and show that their approach was cost competitive.. What happened? It 150% backfired on them. 80% DOD on boats was killing these batteries in 8-24 months as a cost of $800.00+++++ per bank. The sulfation was murdering these high tech, high cost AGM
batteries yet their lab data never predicted this, only real trials in the real world could. Folks who bought them and paid huge money
were NOT happy. Folks like me who bought these claims got burned hard and also were NOT happy.
Today the same exact company that told us 80% DOD in AGM batteries was a better deal than FLA, and doable, is now recommending 50% DOD as the typical safe discharge level for the best cycle life / $$$ equation. Go figure that they now are in concert with Trojan, Deka, Superior, US Battery, Exide, Rolls etc. etc. on the depth of discharge recommendation
s being not more than 50% for optimal cycles to $$$...
The real world
factor throws a monkey wrench into the cost analysis...
Picture yourself driving through a 500 foot bug storm (read 50% DOD). After the 500 feet you can still see out the window enough to drive. You turn on the windshield wipers and get a fair amount of the bugs off, but not all. These 500 foot bug storms repeat over and over and the ones you did not get off the first time remain until the next bug storm. New bugs add to the accumulating visibility issues, over time, until you eventually can't see at all..
Now picture yourself driving through an 800 foot bug storm (read 80% DOD). As you come out the other side the window is totally occluded and you can't see at all. You turn on the wipers and get rid of some but not all, because you started with more bugs. These 800 foot bug storms repeat over and over and the ones you dd not get off the first time remain. New bugs add to the visibility issues until you eventually can't see at all.
Occluded visibility happens much more rapidly in the 800 foot storm than the 500 foot bug storms.
This is similar to what sulfation is like based on depth of discharge. At 80% DOD more material is converted and covering the plates. When you re-charge you never re-convert it all, not at 80% DOD, not at 50% DOD not even at 30% DOD. The difference is that the shallower the cycles the less you have to reconvert and the less sulfation build up you have. The deeper the discharge the faster the sulfation kills the battery. In the real world of marine cycling applications this is MUCH faster than in the lab.
The more material you need to re-convert, there is a lot more at 80% DOD than 50% DOD, the more permanent sulfation remains built up after each cycle. Add on-top of this the non-lab environment
and 80% DOD becomes a lot worse than 50% DOD because you have a lot more material to re-convert.. Because you never really get to full this only compounds the issue and exacerbates the decline of the bank in the "real world".... Because of this the plates simply sulfate faster at deeper DOD's in the real world
than they do in the labs.
225 AH Bank - At 76Ah / day the average load is about 3.2A. When corrected for Peukert on a 225Ah bank the effective load is about 2.3A over 24 hours. On a 225Ah bank with a 1.27 Peukert and 76Ah's per day your effective bank size is roughly 316Ah. So per day your DOD would really be about 75% SOC when corrected for Peukert.
450Ah Bank - At 76Ah / day the average load is about 3.2A. When corrected for Peukert on a 450Ah bank the effective load is about 1.9A over 24 hours. On a 450Ah bank with a 1.27 Peukert and 76Ah's used per day your effective bank size is roughly 760Ah. So per day your DOD would really be about a 10% DOD, when corrected for Peukert.
The bigger bank wins when you correct for Peukert unless the bank has a very low Peukert exponent like LiFePO4
. You can't not correct for Peukert when trying to show a cost efficiency for a smaller bank, in a boat, when deep cycling.
This throws a monkey wrench into the cost analysis....
Voltage sag can be a problem in FLA batteries. At your loads this should not be an issue on a 225Ah bank at 50% DOD but could become an issue at 80% DOD for certain equipment
. The drop off in ability for an LA bank to hold voltage diminishes pretty quickly once past about 60% DOD and also when sulfated. They will simply sulfate faster at 80% DOD cycling than at shallower rates.
The 90% "knee" is based on a charge rate of 10-13% of "C" and is how they charge golf cart batteries. No boat owner wants to charge at 10% of "C" unless tied to a dock
with ample time on their side.. You will be throwing 20-25% of "C" at an FLA bank on a boat in bulk if you want to be at all efficient. At 20-25% of "C" you will hit absorption considerably earlier. Throw solar
at a bank at 5% of "C" and you won't be hitting absorption until the mid to high 90's, throw 25% of "C" at the bank and you hit absorption far earlier...
Absorption voltage, as related to SOC, is entirely current dependent
as to where you achieve it in the state of charge. At 20-25% of "C" charging
you will be hitting absorption at 70 - 80% SOC..
This also throws a monkey wrench into the cost analysis....
No one has ever said "Don't exceed 50% DOD". What pretty much every battery maker suggests for typical DOD's in a cycling application is 50%. This considers cycle life to costs in a cycling application.. You certainly can cycle to 80% DOD, and it is not going to end the day, but each cycle there will result in more sulfation than the cycles to 50% DOD do especially if not recharging to 100% quickly thereafter.. Heck cycling to just 70% SOC results in even more cycle life so the bigger bank always wins because the cycle depth is shallower and the resulting sulfation is less meaning longer cycle life.. Shallower cycles result in less sulfation, which results in longer cycle life...
These are Trojan's recommendations:
"Discharging batteries is entirely a function of your particular application. However, below is list of helpful items:
1. Shallow discharges will result in a longer battery life.
2. 50% (or less) discharges are recommended."
At the price
of Costco batteries none of this really matters. It sounds as if you will mostly be at a dock
and only discharging to 80% DOD occasionally. When Peukert corrected, even less often. Recharge to full as often as you can, equalize and suck up the hit in cycle life and all is good.
My biggest concern with deep discharges to the 80% range is always voltage sag on critical items like nav lights, radar
pumps etc.... Less critical items such as fridge compressors and windlasses can also suffer..
Trying and justify this, with lab data, on a cost per Ah basis is not reasonably or easily extrapolated out into use on actual boats... There is no real advantage
in discharging to 80% DOD in the real world
Just use the batteries and replace them when they need it. $160.00 in the whole scheme of boats is a drop in the proverbial bucket. Even if you need to replace the batts every 2-3 years it is still likely less costly than re-vamping the entire boat. If you want a 450Ah bank do it. If you don't, then don't do it.... Either way you will survive and neither will be the end of the world..
It does seem to me that you are the kind of guy who would sleep better at night with the larger bank, rather than with the smaller bank, but that is just my observation from reading many of your posts....