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.