The irrigation controller that stops in June
A community garden installed a solar-powered drip irrigation system. A 12 V pump draws 4.5 A and runs for 25 minutes twice a day under timer control. The 60 W panel faces north but is shaded by a macrocarpa from about 1:30 pm, and a June measurement at midday showed 18 W output. The battery is a 12 V 40 Ah lead-acid, and the installer advised not to discharge below 50% depth of discharge. The pump is 6 m of cable run from the controller, wired in 1.5 mm² copper. In June the system failed: after two days the battery voltage fell to 11.2 V, the pump ran slowly then stopped, and the relay was heard chattering before it failed. On inspection the relay contacts were pitted and blackened. The garden's summer performance had been fine.
1. Use the numbers given to show why the system fails in June, and explain how the chattering relay and pitted contacts follow from that.
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The system has almost no winter margin, and the failure chain runs from the energy budget through to the relay.
Daily load Pump power = 12 V × 4.5 A = 54 W. Runtime = 25 min twice a day = 50 min = 0.83 h. Energy = 54 × 0.83 ≈ 45 Wh per day, plus roughly 10% for controller losses, so about 50 Wh.
Winter generation 18 W at midday is the peak, not the average. Even assuming 3 effective sun hours, output is 54 Wh before charging losses; at 0.85 charging efficiency that is about 46 Wh delivered. With the afternoon shading, the effective hours are likely fewer, so generation is at or below consumption in June. Every cloudy day is a deficit.
Usable storage 12 V × 40 Ah = 480 Wh nominal; 50% depth of discharge gives 240 Wh usable. That is five days of load if the battery were new and healthy - but a lead-acid battery at low state of charge, in cold weather with reduced capacity, and with rising internal resistance, delivers less. Two days to 11.2 V means generation is near zero on those days and the battery is weaker than nominal.
Why the relay failed At 11.2 V the battery is under load, so the pump sees less still. Voltage drop along 6 m of 1.5 mm² at 4.5 A is about 2 × 6 × 0.0175 × 4.5 / 1.5 ≈ 0.63 V, plus connection losses - the 0.8 V observed. The pump terminal voltage is therefore around 10.4 V, near or below the controller's low-voltage cut-off. The controller repeatedly tries to start, hits cut-off, and drops out: the relay chatters. Every make and break under load arcs, and repeated arcing pits and carbonises the contacts. The relay is a symptom, not the cause; replacing it alone will reproduce the failure.
2. Recommend changes to the system, ranking them by effect and justifying each with reference to the energy budget.
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The priority is to fix the deficit first and the voltage drop second, because no amount of storage fixes a system that generates less than it uses.
1. Remove or work around the shading (largest effect). Moving the panel clear of the macrocarpa, or raising it, restores the array to something closer to its 60 W rating. A north-facing unshaded 60 W panel in June should deliver roughly 100–130 Wh a day at 3–4 effective hours. That alone moves the budget from deficit to surplus and is cheaper than more battery.
2. Increase panel capacity to about 120–150 W if the shade cannot be removed, to cover the partial-shading losses. Trade-off: cost and mounting.
3. Reduce cable voltage drop by replacing the 6 m run with 4 mm² copper, which cuts the drop to roughly 0.24 V, or by relocating the controller closer to the pump. This ensures the pump actually sees the battery voltage and stops false low-voltage cut-off.
4. Reduce runtime or split the schedule to run before 1:30 pm, so the pump draws on the panel while it is producing, rather than draining the battery. Cuts load or shifts it, and costs nothing.
5. Replace the battery only after the above, and specify a deep-cycle type with a suitable capacity - for example 60–80 Ah - if two overcast days of autonomy are required. Trade-off: $300–$500 and disposal of the old battery.
6. Fit an automotive-style relay or a solid-state switch rated for inductive loads, and add a snubber, so the switching itself is not the weak point.
3. Describe a monitoring and testing plan that would confirm the fixes work and give early warning of future failure.
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The plan must measure the same quantities the diagnosis used, over a period that includes bad weather - one sunny week proves nothing.
Baseline before changes Record panel open-circuit and operating voltage and current at 9 am, midday and 3 pm on three days, battery voltage at the same times, and pump current while running. Confirm the 0.8 V drop by measuring at both ends of the cable under load.
After the changes
- Energy audit: a small data logger (or daily manual readings) recording panel output, battery voltage and pump run time for four weeks in June–July. Success: battery stays above 12.0 V at the end of each night and returns to float voltage on most days.
- Voltage drop check: measure at the pump terminals under load; success is under 0.3 V drop and pump terminal voltage above the controller's cut-off with margin.
- Overcast-day stress test: deliberately let one overcast day pass without intervention and confirm the system recovers within two days.
- Relay inspection: inspect contacts after one month and again after three. Clean, unpitted contacts indicate the chattering has stopped; pitting means the cause is unresolved.
- Battery health: record specific gravity or resting voltage monthly, and note the date the battery was installed.
Set a threshold for action - for example, if battery voltage falls below 11.9 V on two consecutive mornings, the garden checks panel shading and connections before adding load. Log everything in a simple table the garden coordinator can maintain.