After Dark | By Piotr Mikus
After Dark (Series): Browse the series: https://solarlightingnightshift.com/category/after-dark/ | Previous: The Approved List Is Not a Sizing Methodology
(What to demand in a solar street lighting specification for cold climate installations):
- Battery chemistry named explicitly, with the manufacturer published charging temperature floor documented, not the operating temperature range
- If the charging floor is at or above the minimum ambient temperature the project location sees, a battery heating system is required, and its power consumption is included in the load calculation
- Cold weather battery capacity derating applied to the sizing, not the rated capacity at 77 degrees Fahrenheit
- HOMER Pro sequential simulation running the actual project location’s weather, with heater load included and state of charge tracked hourly
- Pole, foundation, and wind load calculations per AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, sized against the actual battery, panel, and enclosure EPA, not a marketing brochure image
A battery brochure has never shoveled a driveway. Winter has.
Every technical decision in a solar street lighting system has a downstream cost. Change the battery chemistry, and the enclosure grows. Grow the enclosure, and the wind load changes. Change the wind load, and the pole rating changes. Change the pole, and the foundation depth changes. Change the foundation, and the installation crew, equipment, and permit package change with it.
Most of these consequences are hidden in the summer, when the sun is generous and the battery does not care whether it is 60 degrees or 85. In winter the consequences show up all at once, in the same week, and usually in the middle of a design review that was supposed to be already closed.
This is what happens when one battery decision travels through the whole system.
Winter Doesn’t Care About Marketing
The brochure says all weather. The datasheet says something quieter.
Manufacturer marketing lives in language like all weather, all climate, and four season performance. Manufacturer datasheets live in language like charging temperature range, discharging temperature range, and derated capacity at low temperature.
The two documents describe the same product. They almost never agree with each other. Marketing is written to sell a system that works in Miami. Datasheets are written to protect a warranty against a claim from Winnipeg. If the specifier only reads the marketing, the design lands in Miami. The installation lands wherever the DOT signed the contract.
Winter reads the datasheet.
The Battery That Changed the Laws of Physics
Physics does not need a marketing department. It has been consistent for a while.
Some products claim charging down to minus 20 degrees Celsius. That is a bold claim. Lithium chemistry does not agree.
Standard LiFePO4 cells accept charge cleanly between roughly 0 and 45 degrees Celsius. Below zero, lithium plating begins on the anode. Plated lithium does not participate in the cycling reaction, permanently reduces capacity, and creates the kind of internal short-circuit conditions that make battery engineers nervous. NMC chemistry has a similar floor, sometimes slightly lower. NiMH is more forgiving at the low end but pays for it with lower energy density and a much shorter cycle life. All three chemistries operate cleanly above their charging floors. All three degrade or refuse to accept charge below them.
A product that claims to charge at minus 20 without a self heating system or an external heater is either using a specialty low temperature cell variant with published test data, or it is claiming performance the underlying chemistry cannot deliver. The specifier’s job is to ask which one. If the answer is a self heating pack, the next question writes itself.
The Watt Nobody Budgets
A battery heater is an appliance. It has never worked for free.
The mitigation for low temperature charging is a heating blanket, pad, or self heating cell circuit. Whatever the form factor, it draws power. That power comes from the battery. The battery is the same one running the luminaire.
In a grid tied system this is uninteresting because the grid is infinite. In an off grid solar system, every watt the heater draws is a watt the light does not deliver. The heater has priority in the control logic, because if the battery freezes to death the light dies with it. The luminaire, which the DOT specification identifies as the critical component of the installation, becomes the secondary load behind battery thermal management.
Nobody includes the heater load in the sizing calculation unless the specification demands it. The submittal shows the luminaire wattage and the panel wattage and the battery capacity, and pretends the heater does not exist. Winter arrives and the heater shows up anyway. The energy budget was written for a system that does not exist in the climate it was installed in.
Bigger Batteries Have Bigger Friends
Weight rarely travels alone.
Cold weather sizing pushes the battery capacity up. A depleted, derated, partially heated battery pack in January needs more nameplate capacity than a summer optimal pack to deliver the same usable energy. More capacity means more cells, more mass, and a larger enclosure to house them.
A larger enclosure means a larger effective projected area for wind load calculations. Wind load on the top of a solar pole is the design driver for pole and foundation selection under AASHTO’s Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. When the EPA goes up, the pole class goes up. When the pole class goes up, the anchor bolt pattern changes, the base plate thickness changes, and the foundation depth and diameter go up with them.
A cold climate solar pole is a different animal than its warm climate cousin. Different pole rating, different foundation, delivered by a different truck, installed with a different crew. The battery decision does not sit in isolation. It rewrites every downstream decision that follows it.
The Light Is Still On…
Which is wonderful, unless you are the one trying to see where you are going.
Assume every previous problem was solved. The battery charges in the cold. The heater runs on borrowed sunlight. The pole holds against a February wind. The light is on when the driver approaches the intersection at 2 a.m. in January.
Is it delivering the criteria the roadway classification requires?
ANSI/IES RP-8 does not measure success in lumens produced. It measures success in maintained illuminance and maintained luminance on the road surface, at the uniformity ratios and glare limits stated for the roadway classification. A luminaire dimmed to 30 percent output to conserve battery capacity is not delivering the criteria the standard requires, no matter how proudly it is still illuminated.
People do not drive on lumens. They drive on maintained lighting performance. The cold physics chain is complete only when the last link is verified: the light on the road, at the required level, all night, every night, through the coldest week of the year.
Closing Thought
Every engineering decision creates another engineering decision. Change the battery chemistry and the pole gets heavier. Add a heater and the sizing gets tighter. Ignore either and the light goes out. The cold physics chain does not care whether the specifier followed it. It just delivers the bill.
Sources and Where to Verify
- Battery manufacturer datasheets (LiFePO4, NMC, NiMH): charging temperature range, discharging temperature range, and low temperature capacity derating tables, publicly available on manufacturer websites
- AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (LTS-6, current edition), for pole and foundation sizing against EPA and wind load
- ANSI/IES RP-8-25 (2025), Part 1, Section 6.10.2, adaptive lighting minimum criteria floors by roadway classification
- IEEE 1562, Guide for Array and Battery Sizing in Stand-Alone Photovoltaic Systems
- HOMER Pro sequential simulation methodology, 8,760-hour state of charge tracking with parasitic load modeling
Piotr Mikus is a roadway lighting designer and specifier focused on solar powered street lighting and controls.
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