Introduction
The battery bank can be a major cost and maintenance item in a solar traffic signal. Size it for the required autonomy, actual load, battery chemistry and documented operating limits. Correct sizing alone does not guarantee a 15+ year system life. This guide explains the calculations.
Autonomy Days
Autonomy days are the number of days the system can run with no solar input. For traffic signals:
- Critical intersection: agree the required backup duration and failure response with the authority
- Intersection autonomy: define and verify the project-specific requirement.
- Remote intersection: set autonomy from access constraints, local weather, load and the approved outage-response plan.
Specify the required solar autonomy for the site and operating schedule, then confirm the battery and charging design against that requirement. Do not assume a standard duration from the urban or remote location alone.
Depth of Discharge
Battery life depends on how deeply you discharge it. For deep-cycle batteries:
- AGM lead-acid: verify permitted discharge depth and service-life data for the selected model
- For a gel lead-acid battery, use the selected model’s documented depth-of-discharge limit and life data under the stated temperature and duty cycle; do not assume 60% DoD and 5-7 years for every model.
- Lithium iron phosphate (LiFePO4): confirm allowable depth of discharge and life criteria for the selected battery, temperature and duty cycle.
As an arithmetic example, a 100 Ah battery operated at 50% depth of discharge has 50 Ah nominal usable capacity; at 80%, it has 80 Ah. The permitted depth depends on the selected battery and operating conditions. Compare AGM and LiFePO4 using model-specific usable capacity, life data and quotations, not a fixed price multiplier.
Sizing Calculation
Battery capacity calculations should include the daily energy demand, required autonomy, permitted depth of discharge, system losses, temperature, ageing and design reserve. Confirm the sizing method and usable capacity with the selected battery and charging-system documentation.
Idealized arithmetic example: assume a 200 Wh/day load, 3-day autonomy and 50% depth of discharge. The following subtotal assumes 100% discharge-path efficiency and excludes temperature, ageing and design reserve; it is not a final battery selection.
Battery capacity = 200 × 3 / 0.5 = 1200 Wh
At 12V system voltage: 1200 Wh / 12 V = 100 Ah
Confirm the available AGM or LiFePO4 battery model and capacity against the project’s load and autonomy calculation.
Worked example: illustrative assumptions
Assume that an approved phase schedule gives the signal indications a combined average input power of 30 W, and that the controller and communications draw a combined 5 W continuously. These are hypothetical calculation inputs, not measurements or specifications of a GAOQIAO product. Each load must be assessed separately: do not apply the LED duty cycle to continuously operating equipment, and do not apply it again to a power figure that is already an average.
The average total load is 30 W + 5 W = 35 W. For 24 hours of operation, daily load energy is 35 W × 24 h = 840 Wh/day. More generally, daily load energy is the sum of each load’s input power multiplied by its operating time, with any applicable duty cycle included once.
For an illustrative 3-day period without charging, the loads need 840 Wh/day × 3 days = 2520 Wh. Assume a nominal 12 V battery bank, an allowable depth of discharge of 80%, and a discharge-path efficiency of 90%. The minimum nominal stored energy under these assumptions is 2520 Wh ÷ (0.80 × 0.90) = 3500 Wh; nominal capacity is 3500 Wh ÷ 12 V ≈ 291.7 Ah. The efficiency here covers the path from battery to loads; it is not the solar charging efficiency.
This arithmetic is not a final battery selection. Confirm the permitted depth of discharge, discharge limits, usable capacity at the design temperature, ageing allowance and manufacturer requirements. Check that the chosen bank can supply peak current and that its voltage range, protection, battery management and charger are compatible. Actual usable capacity may require a larger nominal bank.
Size the solar array separately using the design-season solar resource, shading and the charging-path losses. It must meet daily demand and the specified recovery requirement after a low-sun period. Do not assume that a battery capacity calculation alone demonstrates year-round energy balance or a guaranteed service life.
Featured Products
GAOQIAO supplies solar traffic signals and solar wireless controllers with battery and panel sizing worksheets. See the solar site survey guide.
Conclusion
Size the battery using the required autonomy, actual load profile, usable capacity, battery chemistry and site conditions. Send GAOQIAO the location and operating requirements to confirm the proposed battery configuration and the scope of any sizing support before ordering.
Get a Quote
Email: sky-sale@njgq.net
WhatsApp/Phone: +86-18805173545
Tel: +86-25-84471080
Frequently Asked Questions
AGM or lithium for a traffic signal?
Compare AGM and lithium using the selected models’ documented cycle life, permitted depth of discharge, temperature limits, replacement cost and service access; do not assume a universal 3-5 year AGM replacement cycle or 10+ year lithium life at 2-3x the cost.
How long does the battery last?
Battery service life and replacement criteria depend on the selected model, operating conditions and manufacturer guidance. Use inspection and capacity-test results together with the project’s required autonomy to decide when replacement is needed.
Can I add more batteries later?
Adding batteries in parallel requires an approved battery and charging-system configuration. Confirm manufacturer requirements for battery compatibility, protection and any battery-management system, and have a qualified person assess the installation before expansion.
Relevant GAOQIAO products
These public GAOQIAO catalogue pages are related to the guide. Open them for the representative description, then confirm the latest drawing and configuration before ordering.
- Traffic signal controllersWired and solar controllers with 6, 22, 44 and 56 output options.
- LED traffic signal heads200mm, 300mm and 400mm signal heads; confirm the selected model and applicable project requirements.
- Signal polesVertical, signal arm and double-arm poles; confirm suitability for the approved site design.
- Spare parts and accessoriesSpare LED modules, lenses, visors, surge arrestors and anchor bolts.