Solar School-Zone Beacon Fading on Overcast Afternoons
A solar-powered school zone beacon on a two-lane collector uses two 12-inch LED beacon heads, a 12 V flasher, a 100 W panel, a 100 Ah AGM battery, and a charge controller with low-voltage disconnect at 11.8 V. The school zone operates 7:15-8:15 a.m. and 2:30-3:30 p.m. on school days. The school resource officer reports that on overcast days the beacons are dim or off by 3:15 p.m., while on sunny days they work. The battery is five years old. At 10 a.m. after a sunny morning, a technician measured 12.6 V at the battery with the controller indicating float, and 12.1 V at the load terminals while the beacons were flashing. Each head draws about 0.6 A while flashing. A street tree planted last year now shades the panel from mid-afternoon. The site has no maintenance log beyond the original installation date.
1. Check the system energy budget on paper. Does the sizing explain the failure?
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Run the numbers before touching hardware:
- Load: 2 heads x 0.6 A = 1.2 A at 12 V, about 14.4 W. One hour in the morning and one in the afternoon is roughly 29 Wh per day.
- Supply: a 100 W panel with even 3 effective sun hours in winter gives about 300 Wh per day before controller losses.
- Storage: a 100 Ah AGM is about 1,200 Wh nominal; at 50% usable depth of discharge that is 600 Wh, far more than one day of load.
On paper the system has a large margin, so the failure is not a sizing problem. That changes the diagnosis toward the battery, the charging path, or a load larger than assumed. Verify the flasher current with a clamp meter, including any lamp-check feature, and check the controller's low-voltage disconnect setting. The measured 12.1 V while flashing indicates the battery is sagging under a small load, which is a sign of age or sulfation rather than an undersized array.
2. What tests separate a failed battery from a charging problem or a shading problem?
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Separate the three possibilities with measurements, in this order:
- Battery capacity. A 12.6 V open-circuit reading after a sunny morning only says the battery is charged, not that it can deliver. Load-test at a known current or do a timed discharge. A five-year-old AGM at a school-zone site is near the end of its expected life. If it falls to the disconnect point quickly under a 1-2 A load, replace it.
- Charging. Measure panel open-circuit voltage and short-circuit current at midday, then check charge current into the battery. If the new tree shades the panel through the afternoon, the battery may never recover from the morning load.
- Controller. Verify bulk, float, temperature compensation, and disconnect setpoints. A controller stuck in float after a deep discharge will leave the battery chronically low.
Only after these three checks can you say whether the tree, the battery, or the controller is the primary cause.
3. What do you recommend, and in what order?
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Order the work by cost and likelihood:
- Load-test and replace the battery first; it is the most likely failure and the cheapest fix.
- Photograph and measure the tree shadow at the worst-case time, low winter sun, and request pruning or relocation, or move the panel.
- Verify controller setpoints against the battery manufacturer's data and correct any that are wrong.
- Add the site to a seasonal inspection list with battery voltage and panel current recorded each fall.
Do not simply lower the disconnect voltage or add a second panel to compensate for a failing battery. That masks the problem and shortens battery life further. If the site is in a shaded corridor, a larger battery with a smaller depth of discharge is a better fix than a larger array.