Four 150 Wp Modules, One 30 A PWM Controller: A 600 Wp School System That Never Reaches Full Charge
Trainees at a government higher secondary school in Uttar Pradesh inspect a 600 Wp stand-alone system that runs six 9 W LED tubes and one 12 V fan, about 1.1 kWh/day. The array is four 150 Wp polycrystalline modules (Vmp 18.2 V, Imp 8.24 A, Voc 22.5 V) wired all in parallel with 4 mm2 cable into a 30 A PWM charge controller. The battery is a 12 V, 150 Ah flooded tubular, two years old. The complaint is that lights are dim by 22:00 even after a clear day.
At 12:30 on a clear March day the trainee records: array-side current 32.4 A, battery-side current 30.1 A, battery terminal voltage 13.6 V while the controller display shows 14.4 V in absorb, controller case temperature 46 C, module backsheet 52 C, and 0.8 V dropped between the controller's battery terminals and the battery posts. Dawn rest voltage is 11.95 V and all six cells read specific gravity 1.16.
1. Account for the gap between the array's 600 Wp nameplate and the power actually reaching the battery, and list the separate faults these readings expose.
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Start by establishing what the array could produce, then what the topology allows it to deliver.
Available at MPP: backsheet 52 C means cells near 55 C, so thermal derating of about 0.4%/K gives 600 x (1 - 0.004 x 30) = 528 W.
Delivered: a PWM controller does not track; it switches the array directly onto the battery, so the array is forced to operate at about 13.6 V. Power = 32.4 x 13.6 = 441 W. That is roughly 87 W (about 16%) thrown away purely by clamping the array below Vmp. An MPPT controller would recover most of it.
Three further faults are visible in the same data:
- Controller overload. Four modules give 4 x 8.24 = 33 A at Imp, above the 30 A rating. The 30.1 A battery-side reading and 46 C case confirm the controller is limiting and running at its ceiling; in June this becomes a failure.
- Cable/terminal drop. 0.8 V at 30 A is about 0.027 ohm in the battery leads and terminals. The controller regulates its own terminals to 14.4 V, so the battery only sees 13.6 V during absorb. It therefore never reaches gassing voltage, the absorption timer expires early, and the battery sits chronically undercharged, which matches the uniform 1.16 SG and 11.95 V rest voltage.
- Cable sizing. 4 mm2 carrying 33 A in a warm enclosure is at or beyond its derated ampacity.
Uniformly low SG points to undercharge, not a failed cell.
2. Is the array adequately sized for the load in December? Support your answer with an energy balance and comment on autonomy.
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Do the balance month by month, because the system is marginal at the low-sun end of the year.
December generation: about 3.9 peak sun hours in this region. 600 Wp x 3.9 h = 2.34 kWh at STC. Apply 0.92 for dust, wiring and mismatch = 2.15 kWh; apply the PWM clamping loss of about 16% = 1.81 kWh into the battery; apply 0.85 for battery round-trip and self-discharge = about 1.54 kWh delivered to the loads, against a 1.1 kWh demand. Margin is only about 36%, and it disappears if there is one cloudy day.
Autonomy is the real failure. The bank is 150 Ah x 12 V = 1.8 kWh nominal, of which about 0.9 kWh is usable at 50% depth of discharge. That is less than a single day's load (1.1 kWh). The battery can never ride through a cloudy day, and it will be cycled deeply every night, which explains the two-year life.
Sizing check: two days of autonomy at 50% DoD and 0.85 efficiency needs 1.1 x 2 / (0.5 x 0.85) = 5.2 kWh, i.e. roughly 430 Ah at 12 V, or a 24 V 220 Ah bank. So both the array and the battery are undersized at end of life; fixing only the controller will not stop the nightly deep discharge. Recommend the controller/cable repair immediately, and include a second pair of modules and an enlarged bank in the upgrade note.