How to Select a Battery for a Solar Traffic Sign
solar traffic sign battery, AGM vs lithium battery, traffic sign battery
SOLAR POWER & BATTERIES
9/6/20265 min read
How to Select a Battery for a Solar Traffic Sign
The battery is one of the most important components in a solar-powered traffic sign. It stores energy collected during the day and keeps the sign operating at night, during cloudy weather, and through periods of limited sunlight.
Selecting a solar traffic sign battery requires more than comparing amp-hour ratings. Climate, daily power consumption, required backup time, battery weight, charging characteristics, and expected service life should all be considered.
This guide compares AGM, GEL, and lithium iron phosphate batteries and explains how to choose a suitable traffic sign battery for a specific application.
Start With the Sign’s Power Requirements
Before selecting a battery, determine how much energy the complete system uses each day. This may include:
LED light bars or internally illuminated sign faces
Radar sensors
Pedestrian push buttons
Wireless communication equipment
Controllers and monitoring devices
Standby power consumption
A continuously illuminated sign may use much more energy than a pedestrian-activated RRFB system. Radar speed feedback signs can also have different power requirements depending on traffic volume, display brightness, and operating mode.
A simplified sizing calculation is:
Required battery energy = Daily energy use × required backup days ÷ usable battery capacity
Additional capacity should be included for cold weather, battery aging, charging losses, and extended periods of limited sunlight.
AGM vs. GEL vs. LiFePO₄ Batteries
Battery typeInitial costWeightCold-weather useExpected lifeImportant considerationAGM lead-acidLow to moderateHeavyGenerally strong cold-temperature discharge performanceModerateRequires correct charging and adequate capacityGEL lead-acidModerateHeavyCan operate in cold conditions, but available capacity decreasesModerate to goodSensitive to excessive charging voltageLiFePO₄ lithiumHighestLightGood discharge performance, but charging below freezing requires protectionLongRequires a compatible controller and BMS
Actual performance and service life vary considerably by battery model, temperature, depth of discharge, charging method, and system design.
AGM Batteries
AGM, or Absorbent Glass Mat, is a sealed lead-acid battery technology commonly used in solar traffic equipment.
AGM batteries are widely available, relatively affordable, maintenance-free, and compatible with many existing solar charge controllers. They also provide good cold-temperature discharge performance.
Some specialized AGM batteries have published operating ranges as low as -40°F (-40°C). However, this does not mean the battery will provide its full rated capacity at that temperature. Cold temperatures reduce available energy and can cause voltage to fall more quickly under load. The solar system may therefore require a larger battery bank in northern climates.
The battery’s state of charge is also important. A fully charged lead-acid battery has much better freeze resistance than a deeply discharged battery. Allowing an AGM battery to remain discharged during winter can damage it and significantly shorten its life.
AGM is often a practical choice when:
Initial cost is important
The installation experiences very cold winters
Battery weight is not a major concern
The system already uses a lead-acid charge controller
Replacement batteries need to be locally available
High-quality deep-cycle or pure-lead AGM batteries may offer improved charge acceptance, cycle life, and extreme-temperature performance, but they generally cost more than standard AGM batteries.
GEL Batteries
GEL batteries are another type of sealed lead-acid battery. Instead of holding the electrolyte in fiberglass separators, the electrolyte is suspended in a gel.
A deep-cycle GEL battery can perform well in applications with steady loads and repeated cycling. It is sealed, maintenance-free, and resistant to electrolyte leakage.
However, GEL batteries usually accept charge more slowly than AGM batteries and are particularly sensitive to excessive charging voltage. A solar charge controller must have a charging profile specifically approved for the selected GEL battery.
GEL batteries can operate in cold environments, but their available capacity still declines as temperature falls. AGM batteries may provide better power performance than GEL batteries at temperatures below freezing, depending on the product design.
GEL may be appropriate when:
The application uses relatively low and steady current
Deep-cycle performance is more important than rapid charging
The controller provides the correct GEL charging profile
The battery manufacturer has documented performance for the project temperature range
GEL is not automatically better than AGM simply because the installation is outdoors. The complete battery specification and charging system must be evaluated.
LiFePO₄ Batteries
Lithium iron phosphate, or LiFePO₄, batteries offer several advantages for solar traffic signs. They are much lighter than lead-acid batteries, can provide more usable energy from a similar rated capacity, and may deliver a longer cycle life when correctly managed.
Their lower weight can simplify installation, especially when the battery must be mounted in a cabinet or transported to a remote location.
The main cold-weather concern is charging. Many standard LiFePO₄ batteries should not be charged below approximately 32°F (0°C). Manufacturer limits vary—for example, some systems specify a minimum charging temperature of 41°F (5°C). Charging outside the approved range can damage the cells or reduce battery life.
A LiFePO₄ battery used in a cold-climate solar sign should therefore include one or more of the following:
Low-temperature charging cutoff
Internal battery heating
An insulated battery enclosure
Temperature monitoring
A charge controller coordinated with the battery management system
LiFePO₄ is often a strong choice when:
Lower weight is important
Long cycle life is a priority
The battery is cycled frequently
The system can support a compatible BMS and charge controller
Low-temperature charging is controlled or heating is provided
Lithium batteries should not be selected based only on their rated capacity. The BMS, solar controller, enclosure temperature, and winter charging conditions are equally important.
How Temperature Affects Battery Life
Cold weather temporarily reduces available battery capacity, while excessive heat accelerates battery aging.
This applies to both lead-acid and lithium batteries. A battery cabinet exposed to direct summer sunlight may become much hotter than the surrounding air. Whenever possible, the enclosure should be shaded, protected from solar-panel heat, and designed according to the battery manufacturer’s ventilation and temperature requirements.
Lead-acid batteries also require temperature-compensated charging. Charging voltage that is suitable at 77°F (25°C) may not be suitable during a very cold winter or inside a hot roadside cabinet.
Which Battery Is Best?
There is no single battery chemistry that is best for every solar traffic sign.
For a very cold location, a properly sized deep-cycle AGM battery is often a practical and reliable option. A published operating rating of -40°F may be available on selected AGM models, but the system must still account for reduced winter capacity and the battery manufacturer’s charging limits.
GEL can be suitable for deep-cycle, low-current applications when the controller has an exact GEL charging profile.
LiFePO₄ is attractive when low weight, greater usable capacity, and longer cycle life justify the higher initial cost. In freezing climates, it should include low-temperature charging protection or an appropriate heating system.
The final decision should consider:
Daily system energy consumption
Required days of backup operation
Lowest and highest battery temperatures
Available solar energy during winter
Permitted depth of discharge
Charge-controller compatibility
Battery weight and enclosure size
Replacement cost and expected service life
Zoodico can help customers evaluate battery capacity, solar-panel size, charging controls, enclosure requirements, and operating conditions for solar-powered RRFB systems, LED-enhanced traffic signs, radar signs, and other electronic traffic safety equipment.
Need help selecting a battery and solar-power configuration? Contact Zoodico Traffic Solutions to discuss your sign type, location, operating mode, and project requirements.
Battery temperature ratings, charging limits, cycle life, and usable capacity vary by manufacturer and model. Always follow the battery and charge-controller manufacturers’ specifications. Some AGM products publish operating ranges down to -40°F, while representative LiFePO₄ manuals restrict charging to temperatures above freezing.
Sources: Trojan AES AGM specifications, Discover Battery operating manual, and Victron Lithium Smart Battery manual.
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