A lithium iron phosphate cell rated for 6,000 cycles at 25 °C is describing a laboratory.
The moment that same cell sits in a rooftop enclosure in Lagos, Kano or Port of Spain, three
things change at once: ambient temperature rises, depth-of-discharge discipline changes
because the grid keeps failing, and the cell spends more time at high state of charge.
None of this is exotic. It is the normal operating condition of these markets. And the gap
between the datasheet number and the field number is where most storage projects quietly lose
their second year of economics.
Ambient temperature is the obvious one — and the most often skipped
Cycle-life figures are conventionally quoted at 25 °C. LiFePO4 tolerates heat far better than
NMC — that is its main argument — but "tolerates" is not "does not degrade". Capacity fade
accelerates with temperature, and the relationship is not linear.
The consequence for procurement is simple: ambient temperature at the installation site,
not the annual regional average, should be an input to the cycle-life claim. A system
specified against a 25 °C figure in a market with a 38 °C design summer temperature has been
specified against a number that will never be observed.
Ask for the degradation curve against ambient temperature, not a single cycle number. And ask
what the cabinet's thermal management is doing — the cell inside a cabinet in full sun can sit
10 °C above site ambient.
Procurement clause: cycle life shall be stated at the design ambient temperature of the
installation site, not at 25 °C, and the supplier shall state the measured cell temperature
inside the enclosure at rated load.
Grid availability changes the duty cycle, not just the backup time
The datasheet assumes a daily cycle at a defined depth of discharge. With unreliable supply, the
duty cycle is set by the grid rather than the designer.
This matters more than it first appears. When the grid fails repeatedly through the working
day, a backup system sized for a shallow daily cycle spends its life at deep discharge — the
condition that does the most damage per cycle. The same nominal 96 kWh cabinet can see a very
different annual throughput in Lagos than in a European installation with stable supply.
The common design error is sizing autonomy from the nominal outage rather than the distribution
of outages. The long tail is what consumes battery life: how often the outage lasts eight hours,
how often it lasts twenty.
Procurement clause: autonomy shall be sized from the measured outage duration
distribution for the site, not from a nominal or average figure, and the assumed annual cycle
count shall be stated explicitly.
Time at high state of charge is the quiet one
Less discussed, and relevant wherever solar oversupply is common. A system running from a large
PV array on a sunny grid day can sit near full charge for long periods. Calendar ageing at high
state of charge accumulates whether or not the battery is being cycled.
This is one argument for a mixed strategy — some storage for daily time-shifting, some reserved
for grid events. The reserve portion spends its life at moderate charge, and the frequently
cycled portion carries a shallower depth of discharge.
Putting it together
Take a 250 kW commercial load with a 400 kWh bank on a coastal West African site. Sized from a
datasheet cycle-life figure at one full cycle per day, the arithmetic looks comfortable. Adjust
for the three factors and it looks different:
| Consideration | Effect on annual throughput |
|---|---|
| Design ambient well above the 25 °C reference | Reduce stated cycle life by the supplier's own temperature derating — in writing |
| Outage-driven deep discharge instead of a shallow daily cycle | Increases depth-of-discharge wear per cycle |
| Long periods at high state of charge | Calendar ageing in addition to cycle ageing |
| Combined | Annual throughput materially below the datasheet-derived figure |
The conclusion is not that the system will underperform. It is that the procurement has to be
written so the supplier's stated numbers apply at real operating conditions. A supplier who
won't provide a temperature-derated cycle life hasn't understood the market they're quoting into.
Six clauses that close most of the gap
None of these cost anything to include in a tender document:
- Cycle life stated at the installation site's design ambient temperature, not at 25 °C
- Depth of discharge specified as a maximum, not a typical value
- Autonomy sized from the measured outage duration distribution, not an average
- Annual cycle count stated as an explicit design assumption in the contract
- Enclosure thermal management described, including measured internal cell temperature
- Warranty written against the specified duty cycle, with a defined remedy if actual operating conditions exceed the stated assumption
Why it matters commercially
Storage economics in these markets are usually modelled on datasheet cycle life. When field
performance falls short of the model, the project looks like a bad investment — when in fact the
model was built on a number that never described the site.
Requiring suppliers to state performance at real operating conditions isn't adversarial. It's
the only way both sides have the same expectation.
I'm a solar engineer at Force Solar, where we build commercial and industrial solar systems from
10 kW to 1 MW and LiFePO4 storage cabinets from 96 kWh to 5 MWh. Technical questions welcome:
https://forcesolarpower.com/contact/
Full version with the site-specific worked example: https://forcesolarpower.com/blog/battery-degradation-tropical-ci/
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