There’s an old rule of thumb in battery engineering: if you want the battery to last longer, don’t drain it as deeply each cycle. It is correct, but only half the picture. Where the battery sits on the charge scale - not just how far it swings - matters just as much for lifetime. On a grid-scale asset, the right question isn’t “how many cycles can this battery take?” but “which operating pattern earns the most money after paying for the wear it causes?”
What the traditional curve actually says
The textbook plot shows cycle counts climbing from a few thousand at 100% depth-of-discharge (DoD) to tens of thousands at 20%. For a modern LFP cell tested at 0.5C and 25°C, manufacturer curves typically show something like 6,000 equivalent full cycles to 80% state-of-health at 100% DoD, 15,000+ at 50%, and 30,000+ at 20%. Academic studies on large-format prismatic LFP cells [2] confirm the shape. Shallow cycles really do extend life - the curve is real.
Depth of discharge and SoC window - not the same thing
DoD is how much energy is drawn per cycle. The SoC window is where along the charge scale that cycle sits. A cell that cycles between 10% and 90% charge and a cell that cycles between 0% and 80% charge both have 80% DoD, but the first averages 50% state-of-charge and the second averages 40%. That difference matters.
The Dahn Lab study [1] on LFP/graphite pouch cells measured it directly with ultra-high-precision coulometry: a 0–25% SoC window beats a 75–100% window on lifetime, even at similar DoD. Lithium inventory loss and iron dissolution accelerate when the cell sits full. The familiar advice “keep DoD low to extend life” is incomplete. The better version is: keep average SoC moderate, and use only the width of the window you actually need.
Why the right window depends on the revenue stack
A battery that earns almost all its revenue from one deep evening-peak spread per day - the pattern across most continental European markets today - only needs about 80% DoD once a day. Cycling 10–90% captures close to all the available spread and significantly reduces degradation versus 0–100%. The opportunity cost of the extra 10% on each end is tiny spreads the optimiser almost never captures profitably after degradation cost.
A battery earning from ancillary services such as secondary reserve needs headroom in both directions - it can neither sit at 100% nor at 0% if it is contracted to respond symmetrically. Typical operating windows are 20–80%, often tighter. A battery doing dual-use (arbitrage plus frequency response) might operate 15–85% on arbitrage windows and tighten to 30–70% during ancillary windows.
A battery running in a high-spread, volatile market where the optimiser repeatedly sees multiple high-value windows per day - GB’s Balancing Mechanism is the standard example - may rationally run 0–100% on the days it pays, and lean on the faster degradation as a cost of doing business, because the uplift from capturing every spread swamps the extra wear.
Cost the window, don’t just choose it
The disciplined way to pick a window is to attach a cost to degradation, in €/MWh, and let the operating optimiser route around it. Ask: how much extra capacity loss does the next 5% of DoD buy, or the next 10°C of cell temperature? That cost curve turns the SoC window from an engineering choice made once into a revenue-optimisation output that shifts day by day. DNV’s 2024 Battery Scorecard is the cleanest public benchmark: operating fleets span a wide range of observed degradation, with assets that run tighter windows and better thermal management clustering at the low end and assets that don’t drifting measurably above it.
One final nuance
Manufacturer warranties typically specify a DoD band and an average temperature. Cycling outside the band does not just hurt lifetime - it voids the warranty. Before any window decision, the warranty text is the binding constraint.