30% being a reasonable floor absolutely is true. Far less stress to go 100 to 30 and back to 100, then 90 to 20 and back to 90, etc. Keeping 30 as an operational floor lets you use full capacity as needed and remain at full functional charge with maximum lifespan retention/recovery.
EV’s charge/operate that way to extend pack lifespan primarily. 4.2v float is fine if you aren’t routinely low end stressing it. Yes, the internal resistance bit is true, but that really does start to kick in around 25-30. A lot of datasheets I’ve worked with talk about 30% and stress zones. Far better to remain at 100% float for battery lifespan than to routinely dip below 30%. Calendar aging is not as cut and dried as it may seem. This is blatantly obvious with cellphones, of course, but holds true for lots of other implementations too. I expect 90% capacity on 10-year-old batteries stored properly almost always, usually – at a minimum. For capacity ratings, I’ve always done 100% DoD discharge tests to get around the manufacturer ratings for capacity. Cycle durability of course, you are generally right. But cycle durability is what truly matters in the long run for lifespan, not calendar aging, for batteries that often see use. And 30% is a sweet spot between usable capacity and lifespan extension to often double the manufacturer’s rated cycle count. Remember, cycle count means you’re actually using it – so I’m not charging or discharging any less, only doing so to specific levels. I’d also point out a lot of research is also indicative of low discharge levels being the leading factor to degradation, not capacity float charge status. Heat is the the number one factor (outside of or at high end of design spec heat, as often seen in consumer devices). Low discharge is factor #2. High / full charge stress comes in around #3. Better to engage #3 than #2 or #1 for lifespan retention. From: Mark Tinka <mark@tinka.africa> Sent: Friday, April 11, 2025 12:39 AM To: Gary Sparkes <g...@kisaracorporation.com>; Vasilenko Eduard <vasilenko.edu...@huawei.com>; North American Network Operators Group <nanog@lists.nanog.org> Subject: Re: [NANOG] Re: Small Capacity UPS On 4/10/25 22:44, Gary Sparkes wrote: This is… not quite correct. Age can be mostly mitigated by proper storage – at around 40% SoC for storage charge. I routinely pull out 15-20 (and older!) packs that were manufacturer properly storage charged and once jumped up a bit return with 80-90% capacity! I'm not talking about batteries that are in storage. Batteries in storage are not providing value to anyone :-). Yes, it is true that Li-Ion batteries store better than LA, because Li-Ion self-discharge can be 1.5% - 2% per month. LA batteries self-discharge at a rate of 4% - 6% per month. But that is not what we are talking about here. But 30% is a reasonable floor for normal usage, going below that IS a stress zone for the battery – you sharply reduce the number of possible full cycles if you delve into this range regularly. No, not true at all. While you do reduce SoH when you discharge below 20% or if you hold SoC above 80% for too long, how far you go in either direction and for what period does matter. The lower you discharge below 20%, the more internal resistance you introduce into the battery. The higher you charge above 80%, the faster you cause capacity fade due to a high battery voltage. Battery manufacturers all generally use a DoD (Depth of Discharge) of 80% to rate a battery, i.e., discharging from 100% down to 20%. EV's typically charge to 80% and discharged to 30% to increase mileage. But as the battery ages, this gap widens. This range does not really apply to stationery applications, because EV's put more stress on their batteries than a home backup solution, for example. Obviously, when you actually need to use the battery, use it up entirely, that’s what it’s there for. This guideline’s more in general in terms of if you can stop at that point, you really, really should do so. For what it’s worth, I was misremembering the floors slightly. It was 40% and 20% I remember, where stopping discharge at 40% gave you nearly *double* the number of full cycles compared to a 20% discharge. IE say you get 600 if you routinely go to 20%, routinely going to 40% could net you 1500 instead. The less you give the battery a full charge (so keeping it below 100%), the longer it will last. The less you discharge a battery, the longer it will last. But the largest capacity loss actually occurs when the battery remains at a higher voltage for too long. However, cells can only balance when they have a full charge, although once balanced, they will remain balanced even at lower voltages for several months before they begin to drift and need to be re-balanced. In practical terms, the battery becomes useless to you with a partial discharge because you are limiting the amount of capacity you get out of it. And while you will get more cycles from a partial discharge, you are still going to have to deal with calendar aging, which is inevitable. Current research and testing data indicates that the biggest reason for capacity loss is calendar aging and maintaining a high voltage for too long (100% SoC), rather than cycling. Mark. _______________________________________________ NANOG mailing list https://lists.nanog.org/archives/list/nanog@lists.nanog.org/message/3ZZ3RNKEOL5QRYKLPJBNGCAT6QS5TWXP/
