Originally Posted by DarinB
Booster... Thank you for the insight into the specs. I have a question about the discharge comparison to regular AGMs at 50%. If you can take these down to 80-100% so without damage and then bring them back to full charge, why would they not be more cost effective than regular AGMs that should not be discharged to less than 50% if they truly have 3X the cycles? I am not a real "battery person" so I would like your perspective.
The 50% rule has been a topic of discussion here several times over the last couple of years, and IMO the rule makes claims that are not backed up by the manufacturers actual specifications. There are also several variants of the rule of differing harshness.
What you will most often here is that never going below 50% SOC on a lead acid battery will "double its life".
Some people have morphed that into saying that if you ever go below 50% you will cut your life in half.
Some even have claimed going below 50% even a couple of times will destroy the batteries.
The other discussions get much further into the data and calculations and may be worth a read if you are interested in that kind of detail. They are very good discussions.
Bottom line conclusions for me are that, yes, your batteries will last for twice as many recharges at 50% compared to 80%, but when you look at the total energy in and out the difference is closer to 15% different, not double, and energy is what counts. So if you go to 80% down, you get 60% more capacity from the same batteries than at 50% so you recharge less often and use less of the recharge cycles .
Another point is that having some cycles below 50% even down to 80% don't make the entire life of the battery go the 80% point of the charts as the cycles. The cycles essentially average, so if you had 1/2 your cycles going to 80% down and 1/2 the cycles going to 20% down, you would get essentially the same number of cycles as you would if they were all at 50%.
This all said, I would not want to have a system that was always taken to 80% down mainly because at that point you have no reserve left if something doesn't work out right someday. Similarly, I would not want to have a system that was designed so big that it would never, ever, go below 50% as it would big, expensive, and very heavy.
Our setup was designed to be in the 50% discharge or less most of the time, but if we need the extra capacity because of wanting to stay offgrid longer or poor solar conditions we have it available and are certainly not afraid to use it.
If you read the past discussions, you will find some people that agree with this and some that don't, but IMO, the data very plainly shows that the 50% rule is severely overhyped by vendors trying to sell lots of batteries our use the rule to convince you AGMs are much more expensive per AH of capacity than they really are.
I will point out that from the testing I have done and from battery and charger manufacturer's specs, you are much more likely to shorten your battery life significantly due to poor charging of them. Nearly all the chargers out there, shore, alternator, and solar will nearly always leave lead acid batteries either over or under charged.