Van Power Troubleshooting: Why Your System Won’t Charge or Hold Power

Almost every van electrical fault is one of six things, and almost everyone diagnoses them in the wrong order — replacing a solar controller when the problem was a blown fuse, or buying a second battery when the inverter was quietly draining the first. This guide works through the four symptoms van owners actually report, in the sequence that finds the fault fastest: cheapest and most likely first, expensive and rare last.

Before Anything Else: Get a Baseline

You cannot diagnose what you cannot measure. Two tools do ninety per cent of van fault-finding:

A multimeter (£15–40) reads voltage at any point in the system. That alone identifies most faults, because electricity leaves evidence — a voltage that should be present and is not tells you exactly which side of a connection has failed.

A shunt battery monitor reads current in and out. It turns “my battery keeps going flat” into “I am drawing 1.8A with everything switched off”, which is a solvable problem rather than a mystery.

Know these four numbers for a healthy 12V LiFePO4 system before you need them:

• Resting voltage, fully charged: 13.3–13.6V
• Resting voltage, roughly half: 13.0–13.2V
• Resting voltage, nearly empty: 12.0–12.8V
• Charging voltage: 13.8–14.6V depending on stage

LiFePO4 has a famously flat discharge curve — it sits around 13.1V for most of its usable range — which is exactly why voltage alone is a poor fuel gauge and a shunt monitor is worth its price.

Symptom 1: The Battery Isn’t Charging

Work outward from the battery, not inward from the panels. Each step takes a minute and eliminates a whole branch.

Step 1 — measure at the battery terminals while charging should be happening. Engine running, or sun on the panels. Above 13.8V means charge is arriving and the problem is elsewhere (see Symptom 2). Sitting at resting voltage means nothing is reaching the battery.

Step 2 — check every fuse in the charging path. A blown fuse looks identical to a good one through smoked plastic. Test continuity with the multimeter rather than trusting your eyes. This single step resolves a large share of “dead system” reports.

Step 3 — measure at the charger output. At the MPPT or DC-to-DC terminals. Voltage present here but not at the battery means the fault is in the cable or a connection between the two. Nothing here means the charger itself is not producing.

Step 4 — check the BMS hasn’t disconnected. This is the one people miss. A LiFePO4 BMS will refuse charge if the cells are below roughly 0°C, and it will disconnect entirely after a deep over-discharge. Both look exactly like a dead battery. If the pack is cold, warm the van and try again; if it over-discharged, some BMSs need a mains charger to wake them and will ignore solar entirely.

Step 5 — check the alternator path specifically. If solar charges but driving does not, the fault is in the DC-to-DC charger or its ignition trigger wire. Many units need a switched live to tell them the engine is running — a broken trigger wire means the charger sits idle no matter how long you drive.

Symptom 2: Solar Produces Nothing (Or Almost Nothing)

Check the panel open-circuit voltage first. Disconnect the panel from the controller and measure across its leads in daylight. A 200W 12V panel should show 18–23V open circuit even under cloud. Near zero means a dead panel, a broken MC4 connection or a damaged cable. Correct voltage means the panels are fine and the fault is downstream.

Then check the controller is actually seeing them. Most MPPT controllers display input voltage. If the panel measures 20V at its leads but the controller reads 0V, the fault is the cable or the connectors between them — nearly always a badly crimped MC4 that has corroded.

Suspect shading before anything else in summer. A single roof vent shadow across one cell of a series string can cut output by 40–70%. If production collapses at a particular time of day, that is shading, not a fault. Watch the array across an afternoon before spending money.

Rule out “working correctly, in Britain.” A 400W array producing 250Wh on a December day is not broken — that is the correct output for the season. Our year-round van power guide has the month-by-month figures. Check them before assuming a fault, because a large share of winter “solar failures” are simply winter.

Symptom 3: The Inverter Cuts Out Under Load

Four causes, in order of likelihood.

The BMS discharge limit. A 2,000W inverter pulls roughly 185A at 12V. If the battery’s BMS is rated 100A continuous, it cuts out — every time, at the same load. Nothing is faulty; the components were never compatible. Check the BMS rating against the inverter’s draw.

Voltage drop in the DC cable. Inverters shut down below roughly 10.5–11V at their own terminals. Undersized or overlong cable can drop enough under load to trigger that while the battery itself is still at 12.8V. Measure at the inverter terminals under full load, not at the battery — if there is more than about 0.5V difference between the two, the cable is too small. The sizing tables are in the wiring and electrical safety guide.

Surge on startup. Microwaves, compressors and power tools draw two to three times their running wattage for a few seconds. An inverter with insufficient surge headroom trips on the inrush and runs the same appliance perfectly once started by other means.

Thermal shutdown. An inverter in a sealed locker with no airflow will cut out after ten or twenty minutes at high load, then work again once cool. If failures are time-dependent rather than load-dependent, it is heat.

Symptom 4: Power Drains Overnight With Everything Off

This is the most common complaint and the easiest to measure. Switch everything off, then read the shunt monitor. Anything above about 1.5A of standby draw needs finding.

Typical culprits, in order of size:

• Inverter left switched on: 8–30W, which is 190–720Wh a night. This is the cause more often than everything else combined.
• Power station in standby with outputs enabled: 5–15W
• Diesel heater controller: 1–3W
• Alarm, tracker or radio memory: 2–6W
• Solar controller and BMS: 1–3W combined, and entirely normal

To isolate it: with the monitor showing the standby draw, pull one fuse at a time and watch the number. The circuit that makes it drop is your answer. Five minutes, no guesswork.

If standby draw is genuinely low and the battery still empties, the problem is not a drain — it is that charging is not keeping up. That is a sizing question, not a fault, and the charging guide covers it.

The intermittent fault that is always a connection

If the inverter works some days and not others with no pattern in the load, stop suspecting the inverter. Intermittent faults are connections, essentially without exception — a lug that makes contact when cold and loses it when the van warms and the metal expands, or a terminal that reconnects when you drive over a bump.

Find it by measuring voltage across each connection rather than to ground: probe both sides of a single lug under load. A healthy high-current connection drops close to nothing across itself. Anything above roughly 0.2V across one joint is a bad connection generating heat, and it is both your fault and a fire risk. Remake it — clean the lug, re-crimp with the correct tool, torque to specification, heat-shrink.

Symptom 5: Capacity Has Quietly Collapsed

The battery charges and discharges normally but lasts half as long as it used to. Three likely causes.

Cold-weather charge damage. If the pack was charged below 0°C over a winter — no low-temperature cut-off, battery in an external locker — lithium plating has permanently removed capacity. Irreversible, and the reason that specification matters so much.

Cell imbalance. A pack that never reaches 100% drifts out of balance over months, and the BMS starts cutting out early to protect the highest cell. Fix it with a full mains charge held at absorption voltage for several hours, repeated monthly. Batteries with Bluetooth will show you the individual cell voltages — a spread above about 0.1V confirms it.

It was never the capacity claimed. Budget packs using B-grade cells frequently deliver 85–90% of rating from new. A shunt monitor counting actual amp-hours out of a full charge tells you what you really bought — worth doing inside the warranty period, as covered in the LiFePO4 buying guide.

The Ten-Minute Diagnostic Sequence

When something is wrong and you do not know where to start:

1. Battery resting voltage — is the pack actually flat, or does it just seem to be?
2. Battery voltage while charging — is anything arriving?
3. Continuity on every fuse in the affected path
4. Voltage at each charger’s output terminals
5. Standby current with all loads off
6. Voltage at the load terminals under full load

Those six readings identify the fault in the overwhelming majority of cases, and they cost nothing but ten minutes. Only after all six should you start suspecting a failed component — controllers and inverters fail far less often than the connections around them.

Faults That Need Immediate Attention

Stop using the system and investigate now if you find any of these:

• A cable, lug or fuse holder too hot to hold for two seconds after sustained load
• Any smell of hot plastic near the battery or inverter
• A battery case that has swollen or distorted
• A fuse that has blown twice — something is wrong upstream and the next one may not blow in time
• Corrosion or green deposit on a high-current terminal

All five are heat or fault-current problems, and all five get worse rather than better. A loose high-current connection is the classic van fire, and it announces itself exactly this way before anything visible happens.

When It Is Cheaper to Replace Than Diagnose

Fault-finding has diminishing returns. An MPPT controller costs £90–180; if you have confirmed correct panel voltage arriving at its input and nothing coming out, further investigation is not worth your evening. The same applies to a £120 DC-to-DC charger.

Batteries are the opposite — at £500–900, they justify real diagnostic effort, and a pack that appears dead is often a BMS that has latched off and needs a mains charger to reset. Try that before writing it off.

Verdict: Measure First, Buy Second

Most van power problems are a blown fuse, a corroded connection, an inverter left switched on, or a component pairing that never matched — a 100A BMS behind a 2,000W inverter. Almost none are the expensive part failing.

Fit a shunt monitor before you need it, keep a multimeter in the van, and work outward from the battery in the order above. If you would rather remove most of these failure modes entirely, an integrated unit like the Bluetti AC200L or Elite 300 puts the connections that fail inside a factory-sealed case — which is a large part of what you are paying for.