How Does Humidity-Freeze Cycling Increase Cell-to-Cell Voltage Spread

Humidity-freeze cycling widens cell-to-cell voltage spread because not every cell and connection ages at the same rate. HF10 uses 10 cycles with a hot-humid stage around 85°C/85% RH and a low temperature of −40°C, giving a temperature span of about 125°C.^[1][2] If one contact corrodes, one crack grows, or one solder joint becomes resistive while nearby cells stay stable, their electrical behavior starts to separate. At 10 A, only 5 mΩ adds 50 mV of resistive drop. That is already large enough to create a clear voltage outlier in cell-level testing.

Read the Voltage Direction Correctly

A higher voltage is not always worse, and a lower voltage is not always worse. The same defect can move voltage in different directions depending on how the module is being tested.

Take a cell that has developed extra series resistance. Under fixed forward current, more voltage is needed to push the same current through that path, so its measured terminal voltage can rise. Put the same damaged cell into normal power-producing operation and the result can look different. If it has lost active area and can no longer supply enough current, its operating voltage may fall instead.

So a note such as “Cell 18 changed by 30 mV” does not tell the full story. The test current, temperature, illumination and probe positions need to be known as well.

The same applies to a PV module I-V test. Voc, Vmp, current and fill factor are useful only when the test conditions are controlled and recorded.

Use Current to Expose Resistance

Resistance problems become easier to spot when current rises. The basic relationship is:

V_R = I × R

For the same 5 mΩ defect:

These numbers are the voltage drop across the added resistance itself. They are not automatically the exact change in measured cell voltage, because the probe may also include other parts of the circuit.

In practice, the trend matters more than one reading. A cell that stays close to the group at low current but moves farther away as current increases is a strong candidate for a resistance-related fault.

This hypothetical pattern strongly supports added series resistance. It still does not tell you where the resistance sits, so the physical current path needs to be checked next.

Check Corrosion Along the Current Path

Corrosion does not have to spread across the whole module to matter. One damaged contact can be enough to create a visible voltage outlier.

The main places to inspect are:

• fine metal fingers on the cell;
• busbars;
• soldered contacts;
• cell-to-cell ribbons or wires;
• local contact points around the interconnection.

If corrosion adds only 3 mΩ to one cell position:

That is enough to explain why one joint can become abnormal while the rest of the module still looks stable.

Moisture exposure is also uneven in real modules. Edge regions, weak interfaces and contaminated contacts can see more moisture or react faster than other areas. Once corrosion starts locally, voltage spread grows because only part of the cell population is changing.

EVA degradation can generate acetic acid under heat and moisture. Controlled corrosion research has found that acid exposure accelerates PV degradation and that solder-based interconnections can be particularly susceptible.^[3]

That does not mean every EVA module will corrode. The actual result depends on encapsulant chemistry, moisture transport, metallization, solder, contaminants, temperature and module construction. The behavior of module encapsulation materials therefore needs to be judged together with the rest of the module design.

Measure Crack Isolation, Not Crack Length

A long crack can be electrically mild, while a shorter crack can remove a large useful area from the circuit. Crack length by itself is not enough.

What matters is whether the current-collection paths still cross the crack.

Suppose a crack eventually disconnects 20% of a cell. Only 80% of the original area remains electrically connected.

For the same total current, the average current carried by the remaining connected area increases by:

100 ÷ 80 = 1.25

That is about a 25% increase in average current density across the remaining connected area.

This is a simplified area calculation; current is not perfectly uniform across a real cell. It does show why a cell can become electrically stressed even though the string current has not changed.

The damage can also appear suddenly. A crack may still have several metal paths bridging it, so the cell continues to work. As those paths disappear, the remaining current gets concentrated into fewer connections.

A hypothetical voltage trend could therefore move from 5 mV to 6 mV to 8 mV and then jump to 35 mV.

This is not an industry threshold. It simply shows how an apparently stable crack can become a much larger electrical problem once the final useful conductive path is lost.

Extended experimental work has shown this type of crack progression. In one study, modules containing mechanically introduced cracks were exposed to 200 humidity-freeze cycles. Crack growth increased electrically disconnected cell area, and power loss reached up to about 10% in the tested modules.^[4]

The 200-cycle result is an extended research test, not the normal 10-cycle HF10 qualification sequence.

Check Solder and Ribbon Damage Separately

Solder and ribbon faults often look similar in voltage data, but they are not the same failure.

A solder joint can still look attached while its effective electrical contact becomes much smaller.

If only half of the original contact area remains effective, local current density can roughly double if current sharing is otherwise similar.

That does not mean contact resistance must exactly double. Real contact resistance also depends on pressure, solder condition, conductor geometry and the remaining contact points.

From a production or supplier-quality point of view, this distinction matters. A recurring solder-interface problem calls for a different process review from a recurring ribbon fracture or cell-grid defect.

Rule Out Temperature Before Calling It Damage

A few degrees of temperature difference can create the same size of voltage shift that might otherwise be blamed on degradation.

For crystalline-silicon cells, Voc has a negative temperature coefficient. Around normal operating temperatures, about −2 mV/°C per cell is a useful order-of-magnitude example, although the exact value changes with cell design and irradiance.^[5]

Using −2 mV/°C only as a simple Voc example:

If the spread being investigated is only 8–10 mV, a few degrees of temperature difference can completely change the diagnosis.

Use the same temperature sensor position and wait until the module is stable before comparing pre-test and post-test readings. The same issue affects flash testing; module power measurements can drift with temperature and test conditions even when permanent degradation has not occurred.

Make Sure the Change Is Larger Than Test Noise

Small millivolt changes are easy to overinterpret.

A hypothetical healthy cell measured five times might give:

• 602 mV
• 604 mV
• 601 mV
• 603 mV
• 602 mV

The observed range is already 3 mV.

If the later result is 603 mV, there is no strong evidence that the cell has changed. Repeated post-test readings around 630 mV tell a very different story. A roughly 27–29 mV shift is far outside the original variation.

Before treating a small voltage change as real, check:

• probe position;
• probe contact;
• current-source stability;
• cell temperature;
• illumination uniformity;
• instrument accuracy;
• measurement delay and stabilization.

Probe placement deserves special attention. If the probe includes part of the ribbon or solder connection, the number contains both cell-junction voltage and interconnection loss. That can be useful when looking for a weak cell position, but it is not a pure junction-voltage reading.

Compare Each Cell With Its Own Baseline

The highest-minus-lowest value is easy to calculate, but it can hide the actual pattern.

The stronger comparison is:

ΔV_cell = V_after − V_before

Cells do not have to move in the same direction.

That is also why total module voltage can hide local damage. If one cell moves by +25 mV while another moves by −24 mV, their combined change is only about +1 mV. The string total looks almost unchanged even though both cells have moved well away from their original condition.

Do Not Use One Universal mV Limit

A 20 mV or 40 mV spread is not automatically good or bad.

The useful comparison is against:

• the pre-test distribution;
• measurement repeatability;
• the number of cells that moved;
• supporting EL, I-V or thermal evidence.

Consider two hypothetical 60-cell modules.

Both modules can end with a large maximum-to-minimum spread, but the corrective action would not be the same. Module A points toward a local failure. Module B calls for a wider review of materials, process conditions or the test setup itself.

Use Cell Location to Narrow the Cause

The position of the abnormal cells can save a lot of unnecessary failure analysis.

For supplier-quality or production analysis, a physical cell map is far more useful than a spreadsheet that lists Cell 1, Cell 2 and Cell 3 without showing where those cells sit in the module.

Confirm the Fault With EL, I-V and Infrared

Voltage spread narrows the search, but it should not be the final diagnosis.

EL testing is useful because cracks and electrically inactive areas can be invisible during normal visual inspection. IEA PVPS also identifies EL and infrared imaging as useful non-destructive methods for finding cracks, hotspots, mismatch and other PV module faults.^[6]

A dark EL area is not proof of a crack on its own. Series resistance, current injection, shunting, contact quality and cell temperature can also change EL brightness.

Infrared adds another useful check:

P = I²R

For a 5 mΩ local resistance:

Doubling current from 10 A to 20 A increases local resistive heating four times. A small contact defect can therefore be difficult to see at low current but obvious in a controlled thermal test.

Check Reverse Bias Before a Weak Cell Becomes a Hotspot

When a damaged cell cannot supply the current demanded by the rest of its series path, its voltage can fall below zero.

At that point, the cell is absorbing power rather than producing it.

For example, a cell at −5 V while 5 A passes through the path is dissipating:

P = 5 V × 5 A = 25 W

This is only a simple electrical example; the actual reverse voltage and current depend on the cell, module circuit and operating condition.

Twenty-five watts concentrated in a small cell area can create serious local heating.

Bypass diodes reduce stress across groups of cells, but they do not clamp every weak cell individually. A diode normally protects a substring, and a weak cell can experience reverse bias before the substring reaches the point where the diode conducts.

NREL work on cracked cells describes the link between electrical disconnection, current mismatch, reverse bias, hotspot heating and bypass-diode operation.^[7]

Read the Actual Module Circuit

“All cells carry the same current” is only correct within one series path.

Modern PV module designs can include:

• half-cut cells;
• parallel branches;
• multiple bypass-diode sections;
• multi-busbar layouts;
• wire-based connections;
• shingled circuits.

Parallel branches can carry different currents. Two cells that sit next to each other on the laminate may not belong to the same electrical path.

Before comparing their voltages, check the actual module circuit. Otherwise, a normal branch-to-branch difference can be mistaken for humidity-freeze damage.

Use This 60-Cell Example as a Diagnostic Model

The following numbers are hypothetical. They show how to read cell-level data; they are not an industry pass limit.

A conventional 60-cell module is measured before and after environmental stress using:

• dark forward bias;
• 25°C module temperature;
• 8 A forward current;
• the same probe positions;
• the same measurement equipment.

The average changes by only 1 mV:

0.001 ÷ 0.604 × 100 ≈ 0.17%

The voltage spread changes from 9 mV to 42 mV:

42 ÷ 9 ≈ 4.7 times

Looking only at the average would make the module appear almost unchanged. The cell-level distribution tells a different story.

Cell 18: close to normal at low current but +32 mV above the group at 8 A. Infrared also shows heat near the ribbon. Check the contact and series path first.

Cell 43: a pre-existing EL crack develops a much larger dark area after cycling. Check whether part of the cell has become electrically isolated.

Cell 57: abnormal even at low current, with little increase as current rises. Check leakage, shunt behavior and measurement repeatability before blaming series resistance.

Know What HF10 Can Prove

HF10 is useful for finding weak cells, interfaces, materials and electrical connections under a defined accelerated stress. It is not a direct 25- or 30-year lifetime simulation.

Outdoor modules also face UV radiation, daily temperature movement, system voltage, wind, snow, salt, pollution, hail and installation stress. Accelerated tests should not be converted directly into a fixed number of field years.

NREL reliability work has long emphasized that qualification and accelerated stress tests are useful for identifying weaknesses and comparing designs, but they should not be treated as simple numerical lifetime clocks.^[8]

For B2B qualification, supplier comparison or process improvement, a PV testing and failure-analysis program is most useful when chamber testing is combined with cell-level electrical data, EL, I-V, thermal inspection and material analysis.

Match the Fix to the Failure Pattern

For manufacturing and supplier-quality teams, the useful outcome is a clear link between the electrical pattern and the process step that needs attention. A resistance-driven outlier calls for work on contacts and interconnections. A growing crack-related EL area points toward mechanical handling and process stress. Edge-heavy failures shift attention toward moisture paths and laminate quality.

Finally

Start with the size and shape of the change, not the final voltage number alone. HF10 exposes the module to 10 cycles across roughly 125°C. At 10 A, 5 mΩ creates 50 mV across that resistance; a 5°C temperature difference can account for roughly 10 mV of crystalline-silicon Voc variation; and losing 20% of connected cell area raises average current density in the remaining area by about 25% at the same total current. Compare each cell with its own baseline, repeat the test at controlled temperature, change forward current, then use EL, I-V or infrared to confirm the physical cause before deciding on corrective action.