PWM or MPPT: choosing a charge controller and the cold-morning voltage

A charge controller is chosen on two numbers and one date. The numbers are the highest voltage the array can ever put on its terminals and the highest current it can ever push through them; the date is the coldest clear dawn the site will see, because that is when the voltage peaks. Most controllers that fail in the field were rated for the panel wattage on a warm day. This guide covers the two kinds of controller, the voltage window that decides how many modules go in a string, the current factor, and how to read the datasheet so the numbers line up.

What a charge controller does

A PV array is a current source whose output changes with every cloud, and a battery is a chemical store that must be charged in stages and never past its limits. The controller sits between them. It takes whatever the array offers, delivers it to the battery as a bulk current until the absorption voltage is reached, holds that voltage while the current tapers, then drops to float; the charging guide explains why those stages exist and how long they take. It also compensates the setpoints for battery temperature, blocks the battery from discharging back into the array at night, and on small units switches a DC load off before the battery is over-discharged.

Without one, the panels would drive a 12 V battery to 18 V or more on a sunny afternoon and boil it; with the wrong one, the panels are connected to a device rated below their voltage, and it is the controller that fails. The two kinds differ in how they get the energy from the array's voltage to the battery's.

PWM: a switch between the panel and the battery

A PWM controller is a transistor switch in series with the panel. While the battery is below its setpoint the switch is closed and the panel is wired straight to the battery, so the panel runs at battery voltage, plus the small drop in the cable and the switch. As the battery approaches the setpoint the switch is pulsed at a few hundred hertz with a shrinking duty cycle, which is the pulse-width modulation of the name, and the average current falls until it just holds the voltage. It is cheap, simple, rugged and electrically quiet.

Its cost is in the physics. A silicon module gives nearly its full current at any voltage below its maximum-power point, so a 36-cell module with V_mp of 18 V pulled down to 14 V still delivers close to I_mp, and the power is 14 × I_mp instead of 18 × I_mp: about a fifth is never collected. That is acceptable, and it is why 36-cell modules exist: they are built for 12 V batteries on PWM. Put a 550 W module with V_mp of 41.5 V on the same 12 V battery and the module is pulled to 14 V while its current stays near 13 A, so about 190 W of the 550 is collected. PWM is fine for a small 12 V system with matched modules: a gate light, a telemetry post, a small water pump. It is the wrong choice for a modern high-voltage module, a long string, a 48 V bank or a cold site.

MPPT: a DC-DC converter that lets the array sit at its best voltage

A module's current-voltage curve is flat from short circuit up to a knee and then falls steeply to zero at open circuit; the power, current times voltage, peaks at the knee, which is the maximum power point. An MPPT controller is a switching DC-DC converter, usually a buck stage, with a control loop that searches for that point: it nudges the input voltage, watches the power, and keeps moving toward the peak. The array runs at V_mp, the converter steps the voltage down to the battery's, and the current is stepped up by the same ratio less the losses. For the example array, 83 V at 26.4 A in becomes 57.6 V at about 37 A out.

Good units track within 2–5 % of the true maximum and convert at 96–98 %, so an MPPT controller collects 95–98 % of what the array can make, against the 70–80 % of a matched PWM unit and far less of an unmatched one. The gain is largest when V_mp is furthest above battery voltage: on cold mornings, when module voltage is high, and on a bank whose modules were never designed for it. The price is two to four times a PWM unit of the same current, which is almost always less than the panels the PWM unit would waste. An MPPT controller also permits strings of several modules in series, which halves or quarters the cable current and lets the array sit further from the battery.

The voltage window: cold mornings and hot afternoons

The controller's PV input has a maximum voltage, and exceeding it is not a derating but a failure of the input stage. The array's highest voltage is its open-circuit voltage, and V_oc rises as the cells cool, by about 0.25–0.30 % per °C for crystalline silicon. Datasheet values are at 25 °C. On a clear winter dawn, before the sun has warmed the glass, a module at −5 °C is thirty degrees colder than that and its V_oc is about 8 % higher; at −15 °C in Quetta or Murree, 11 % higher. The correction is V_oc,cold = n_s × V_oc × (1 + β × (T_min − 25)), with β the coefficient as a fraction, and it is applied with the site's record low, not its average, because the controller has to survive the worst morning of its life, not the typical one. IEC 62548 requires this correction; NEC 690.7 is the North American equivalent. Note that the well-known 1.25 factor is for current, not voltage: the temperature correction is the voltage factor.

The other side of the window is the hot afternoon. V_mp falls as the cells heat, at roughly 0.35–0.40 % per °C (the P_max coefficient on the datasheet, or about 1.3 × the V_oc coefficient if it is not given). A module on a roof in Multan in June runs at 65 °C or more, so its V_mp is about 14 % below the datasheet. For the controller to charge, the array must sit above the battery's absorption voltage by the controller's margin, typically 5 V to start and 1 V to keep running, on the datasheet. A string that fits at 25 °C and fails at 65 °C stops charging at exactly the hour the batteries are needed. The two limits together set the number of modules per string: the most that keep V_oc,cold under the maximum, the fewest that keep V_mp,hot above the minimum, and on a 48 V bank with 550 W modules and a 150 V controller, both say two.

Current: the 1.25 factor and edge-of-cloud

Datasheet current is at 1 000 W/m². Real irradiance exceeds that at altitude, where the air is thinner, and for seconds to minutes whenever the sun comes out beside a bright cloud whose edge reflects extra light onto the array: the edge-of-cloud effect, which can push a module 20–30 % above its rated current. IEC 62548 and NEC 690.8 both take the array's short-circuit current times 1.25 as the design current for the controller input, the cables and the protection. The controller's PV input current rating must be at least that; its charge-current rating on the battery side is a separate number and, for MPPT, a larger one.

The same factor sizes the protection. String fuses or breakers are rated between 1.5 and 2.4 × I_sc under IEC 62548 (the NEC's 1.56 sits in that range), and they are needed when more than two strings are paralleled, because a fault in one string then sees the combined current of the others flowing backward into it, above the module's maximum series fuse rating. Cable from the array is sized to the design current and to the voltage drop; cable from the controller to the battery is sized to the charge current, which on an MPPT unit is higher than the array current.

Reading the datasheet

• Maximum PV open-circuit voltage. The number V_oc,cold must stay under. MPPT units come in 100, 150, 250 and 450 V classes; PWM units are commonly rated for 12/24 V systems with a limit of 50 V or so, and a few for 100 V.
• Maximum PV current, or maximum PV short-circuit current. The number 1.25 × array I_sc must stay under. Some units can limit an oversized array's power; none can limit its voltage.
• Rated charge current, and the voltage it is rated at. The battery-side output. Makers state "nominal PV power" per bank voltage; divide it by the rated current to see whether they used the nominal bank voltage or the absorption voltage, because the two differ by 20 %.
• MPPT voltage range or minimum start voltage. The lower edge of the window: typically battery plus 5 V to start charging and battery plus 1 V to continue.
• Efficiency. A peak figure and, on a good datasheet, a curve; the peak is at a fraction of full load, and the full-load figure is what matters for a well-sized unit.
• Battery presets. Flooded, AGM, gel and LiFePO4 profiles with adjustable absorption, float and equalisation voltages; a lithium bank needs the lithium profile and, ideally, a communication link to its BMS.
• Temperature sensor. A remote battery temperature probe input, so the setpoints compensate for the battery's temperature rather than the controller's.
• Load output and communications. A low-voltage-disconnect load terminal on small units; RS485, Modbus, CAN or Bluetooth on larger ones, which is what lets a controller be logged and alarmed from a monitoring system.

The example in numbers

Four 550 W modules (V_oc 49.5 V, V_mp 41.5 V, I_sc 13.9 A, I_mp 13.2 A, −0.27 % per °C) as two strings of two on a 48 V bank, at a site whose record low is −5 °C.

1. String V_oc = 99.0 V at 25 °C and 99.0 × 1.081 = 107.0 V at −5 °C: a 100 V controller fails, a 150 V class is needed.
2. Array I_sc = 27.8 A; design current 1.25 × 27.8 = 34.8 A for the PV input, the cables and the protection.
3. Array power 2 200 W; absorption 57.6 V; MPPT charge current 2 200 ÷ 57.6 = 38.2 A, so a 40 A unit at 95.5 % of its rating, or a 50 A unit with margin. Rated at the nominal 48 V the array could give 45.8 A, and a 40 A unit will limit to about 1 920 W when the bank is low.
4. String V_mp = 83.0 V at 25 °C and 83.0 × 0.8596 = 71.3 V at 65 °C, above the 62.6 V minimum: the array charges on a hot afternoon. Power delivered at STC about 2 134 W.
5. On PWM instead: the rating follows the current, 34.8 A, so also 40 A, but the array is pulled to 57.6 V and delivers about 57.6 × 26.4 = 1 521 W: 31 % thrown away.

The charge controller sizing calculator reproduces these figures and lets you change the site temperature, the bank voltage or the string layout and watch the window and the rating move; the battery charging time calculator takes the 38 A onward to the bank.

Common installation errors in Pakistan

• 72-cell or 550 W modules on a 12 V PWM controller. The most common one, in tube wells, shops and gate lights, because the modules are cheap per watt. The system collects a third of what was paid for and the owner concludes that solar is weak.
• No temperature correction on a 100 V controller in the north. Two modules in series read 99 V on a warm day and 107 V on a January dawn in Islamabad; the controller lasts until the first cold clear morning.
• Strings paralleled without fuses. Three or more strings on one pair of terminals, so a shorted module sees the other strings' current flowing back into it, above its series fuse rating.
• Controllers rated by panel wattage. "A 40 A controller is for 1 000 W" is only true at one bank voltage; the rating that matters is the charge current at your bank's absorption voltage, and for PWM the array current.
• No DC isolator, and the wrong order of connection. The array is connected live and the terminals arc; or the array is connected before the battery, and a controller that detects the bank voltage from the battery mis-detects or is damaged. Battery first, then PV, with an isolator on the PV side that makes the sequence safe every time.
• Undersized battery cable and no temperature sensor. The controller reads its own terminals, not the battery, so a long thin cable makes it think the battery is full early; and without the probe the setpoints are wrong in both seasons.

Ahmedonics designs solar-powered instrumentation, pumping control and telemetry where the controller is one component of a monitored system; the arithmetic above is what we apply to every one of them, and the calculator is that arithmetic made public.

References

• IEC 62548:2016, Photovoltaic (PV) arrays — Design requirements — temperature-corrected maximum voltage, the 1.25 irradiance factor, string fusing and isolation
• IEC 62109-1:2010, Safety of power converters for use in photovoltaic power systems — Part 1: General requirements
• IEC 61215-1:2021, Terrestrial photovoltaic (PV) modules — Design qualification and type approval — Part 1: Test requirements — STC measurement and temperature coefficients
• NFPA 70 (NEC) 2023, Article 690: Solar Photovoltaic (PV) Systems, 690.7 and 690.8 — maximum voltage by temperature correction and the 125 % current factor
• Module and charge controller manufacturers' datasheets — temperature coefficients, PV input limits, rated charge current and the voltage at which it is rated, minimum start voltage