Quote:
Originally Posted by RossWilliams
Well, after looking around the internet, its obvious I was wrong to believe there was a consistent standard for cycles that assumes 100%. Which makes me a lot less certain of the impact of lower average DOD on the total power produced. I remain doubtful that it is really flat, since that contradicts repeated statements elsewhere. But it wouldn't be the first time that commonly accepted wisdom was wrong. The claim for a "dramatic" difference in the life of the battery seems a gross exaggeration if all it means is that if you use less power each time the power will last longer in proportion to the power used. That makes the number of cycles at any moderate DOD essentially just a function of the power used. Which, I understand, is the claim made here.
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What you are saying is very close to what a bunch of us said as we collected all this data a couple of years ago. It is rather unbelievable that so much of the "accepted" information appeared to be less than accurate. It took a long time to come to grips with what appeared to be actually happening. Claims like the "dramatic" shortening of life don't help understanding of what is really happening, and were probably written be \Marketing rather than Engineering.
You are also correct that the curves are not flat, as the last set of graphs I put up show, except for the Trojan T105 one. My guess on that one is that it probably would be flatter than the other curves because it is a battery that is purpose built to be good for deep discharges in golf carts, but not totally flat as shown. I think Trojan probably just left off the higher peaks to make the whole thing simpler for those that were doing power balance calculations before specing battery banks.
The Lifeline chart does show less life at 80% compared to 50% DOD in total energy, but it is in the 21% range that was calculated earlier in this discussion, so certainly not as severe as the old rules would imply, so you certainly are correct on that. Most batteries that we looked at looked very similar are not flat curves.
The emphasis on the fact that the discharge DODs, for the most part, can be averaged to get a good approximation of cycle life is a very important thing for everyone to remember. Before all this came up, we routinely would have folks telling everyone that even a couple times of going under 50% would severely shorten battery life, which is certainly not backed up by the data. I think the averaging can be a very good tool for folks to use when trying to decide just how much battery bank they really need to do all the things they want.
I think a lot of the battery issues and questions would be pretty easily seen and understood by users if all RVs came with a shunt based battery monitor, properly setup. Once you know what to watch for, you can determine if your charging system is doing a decent job or not in a very short period of time. New RVs probably don't include them because they use less than stellar charging equipment and don't want anyone to figure that out.
My hope, for anyone that has been following this excellent discussion would be:
* Don't sweat the deeper discharges so much as they are relatively minor battery killer.
* Get a good battery monitor system and be certain to get it set properly to determine full charge and SOC with your batteries bank size and style. The setup numbers are very, very, important if you want good data.
* Based on what you see on the battery monitor data, determine if you need any charging improvements, and if you do, is it worth the upgrading costs in comparison to the cost of replacing batteries more often. The cost calculation is pretty important to do, as if you are shortening the life of a single $80 wet cell you probably would have a hard time justifying charger improvements. If you are shortening the life of $2500 of AGMs, you have a lot more incentive to get better charging.
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