Solar Panels Not Charging Campervan Battery: Troubleshooting Guide

Your solar panels are fitted, the sun is out, and the battery is not charging. Before you pull the system apart, work through this checklist — most solar charging faults have a simple cause that takes minutes to diagnose.

This guide focuses specifically on solar charging faults. For a broader diagnosis including DC-DC charger and shore power faults, see our leisure battery not charging guide.

Before You Start: Check These First

Is the battery actually low? MPPT controllers reduce or stop charging as the battery approaches full. A LiFePO4 battery at 13.2-13.4V at rest is essentially full. Check the battery voltage first — if it is above 13V, the controller may be correctly tapering charge.

Is it genuinely sunny? UK solar output drops dramatically in overcast conditions. On a heavily overcast day, a 200W panel may produce only 10-20W — barely enough to register as "charging" on a monitor. Test on a clear day around midday before concluding something is wrong.

Did the system ever work? If this is a new installation that has never charged correctly, the problem is different to a previously-working system that has stopped. New installs often have wiring polarity issues; established systems more often have loose connections or component failures.

Check 1: MPPT Controller Status

The MPPT controller is the first place to look. Open its app (Victron Connect, Renogy, etc.) or check its status LEDs.

What the App Should Show on a Sunny Day

• PV input voltage: Should be above the battery voltage — typically 18-22V for a single 12V-nominal panel, higher for panels in series
• PV input current: Should be measurable (0.5A minimum on a partially cloudy day)
• Output charging current: Should be positive (charging the battery)
• Controller state: "Bulk", "Absorption", or "Float" — all indicate normal charging. "Fault" or "Off" indicates a problem.

MPPT in Float Mode

If the controller shows "Float" and minimal charging current, the battery is nearly full. This is correct behaviour, not a fault. Leave the system overnight, deplete some battery capacity, and recheck.

MPPT Showing No PV Input

If PV voltage reads zero or near zero on a sunny day:

• The panel cable is not connected to the controller
• A panel-side fuse has blown
• MC4 connectors are not fully engaged
• Panel cable polarity is reversed (see Check 3)

MPPT Showing PV Voltage But No Output

If PV voltage is present but charging current is zero:

• BMS has blocked charge input (see Check 5)
• Controller output fuse has blown
• Controller has tripped into protection mode — try power cycling (disconnect battery and panels for 30 seconds)

Check 2: Measure Panel Output Directly

Tools needed: Multimeter set to DC voltage

Disconnect the MC4 connectors from the MPPT controller. Measure voltage between the positive and negative panel leads in open circuit conditions (in sunlight):

• A single 100-200W 12V-nominal panel in full sun: 18-22V Voc
• Two identical panels in series: 36-44V Voc
• Two identical panels in parallel: 18-22V Voc

If voltage is zero: the panel or its wiring has a fault. If voltage is present but lower than expected (e.g. 10-12V instead of 18-22V): one or more cells may be damaged or shaded.

Reconnect the MC4 connectors after testing.

Check 3: Check Polarity

A reversed polarity connection — positive connected to negative — is the most common installation error on new solar systems. Most MPPT controllers have reverse polarity protection and simply will not charge. Some older or cheaper controllers can be damaged by reverse polarity.

Panels to MPPT: Check that the positive panel lead (marked + on the panel junction box) connects to the positive PV input terminal on the controller.

MPPT to battery: Check that the positive controller output connects to battery positive. A reversed battery connection will prevent charging and may damage the controller.

Check 4: Check for Shading

Shading is a common and often overlooked cause of reduced or absent charging. In a typical panel installation where cells are connected in series:

• Shading 10% of a panel's cells can reduce output by 30-50%
• A single shaded cell acts as a resistor against all the other cells in that string
• Partial shade from a roof vent, aerial, or satellite dish throughout the day can significantly reduce daily yield

Check: At the time of day you expect maximum output, look at what shadows fall on your panels. Even narrow shadows from roof rails, TV aerials, or ventilation pipes shade cells throughout the day.

Solutions:

• Reposition panels away from shadows if possible
• Fit bypass diodes (most modern panels have these built in) to mitigate series-cell shading effects
• Use a string inverter or individual MPPT per panel for complex shading situations (rare in campervan use)

See our solar panel shading guide for a full treatment.

Check 5: BMS Cold Blocking Charge (LiFePO4)

LiFePO4 batteries with a BMS will block charge input when cell temperature falls below 0°C. On cold mornings, the battery may not accept solar charge until it warms up.

Symptoms: Controller shows PV voltage and attempts to push current, but battery voltage barely rises and charge current reads near zero.

Check: Open the battery Bluetooth app and check cell temperature. If below 5°C, the BMS has blocked charging to protect cells.

Solution: The battery will begin accepting charge once it warms. In a heated van interior, this happens within an hour of morning sunlight warming the space. If the battery is in an unheated external location, warming may take longer. See our self-heating LiFePO4 guide for a long-term solution.

Check 6: Inspect MC4 Connectors and Panel Junction Box

MC4 connectors — the waterproof push-lock connectors used in all solar installations — occasionally fail or are not fully engaged.

Check:

• Are all MC4 connectors fully clicked together? Push firmly until you hear/feel the click
• Any damaged, cracked, or melted connector housings? Replace immediately
• Any corrosion at the connector contacts? Clean with contact cleaner or replace
• Is the panel junction box (on the back of the panel) sealed? Any sign of moisture ingress?

A partially engaged MC4 connector causes high resistance — the system may appear to work but generates less than expected output and the connector may overheat. This is a safety issue as well as a performance one.

Check 7: Check Cable Runs for Damage

Cable run from roof to MPPT controller may have been damaged by:

• Chafing against a sharp edge or panel mount bracket
• Pinching at a cable gland or through a hole
• UV degradation on exposed cable sections
• Physical damage (van body damage, something dropped on the cable)

Check: Visually inspect the entire cable run from panels to controller. Look for flattened sections, abraded insulation, or signs of heat damage. Use a multimeter in continuity mode to check cable resistance — very high resistance on a short run indicates damage.

See our solar cable gland guide and cable routing guide for correct installation that prevents this type of damage.

Check 8: Panel Damage

Physical panel damage — cracked glass, delamination, or internal cell failure — causes permanent output loss.

Inspect: Look at each panel from different angles for:

• Hairline cracks in the glass (these can be subtle)
• Dark patches or discolouration on individual cells (hot spot damage)
• Bubbling or delamination of the encapsulant (more common in flexible panels)
• Discolouration of cells visible from the back (yellow/brown patches indicate overheating)

A single cracked cell reduces the output of its entire series string. Widespread cracking can reduce panel output to near zero. Panel damage from impact or thermal stress requires panel replacement.

Systematic Testing Approach

If you cannot identify the fault from visual inspection:

1. Disconnect panels from MPPT controller
2. Measure panel open-circuit voltage in direct sunlight — should be Voc per spec sheet
3. Reconnect panels to MPPT, disconnect battery
4. Measure MPPT output voltage — should be configurable charge voltage
5. Reconnect battery and measure charge current in the app

Working through the system section by section tells you where the fault lies.

FAQ

My system worked fine all summer but stopped in autumn — why?

Reduced charging is normal in autumn — lower sun angle, shorter days, and more overcast weather. If output has dropped to near zero rather than just reduced, check for a new fault (loose connection from vibration, a connection that has corroded over summer). But some reduction is entirely expected — see our winter solar guide.

The MPPT controller shows charging current but the battery voltage doesn't rise — what is wrong?

There may be a significant load consuming the solar output. Check what appliances are running — a fridge, diesel heater fan, or other appliance may be consuming all available solar. Also check for a fault causing high current draw (a shorted circuit, a stuck relay).

Do MC4 connectors need regular maintenance?

Not typically — they are weatherproof and designed for outdoor use. After 3-5 years, it is worth checking for any discolouration or brittleness on the outer housing. Replace any that show degradation. Keep a few spare MC4 connectors and a crimp tool in the van for field repairs.