Identify whether one charging source or all sources are affected, then check controller status, battery protection and net current before opening wiring.
First identify whether the problem affects one charging source or every source. Compare charger status with net battery current: a working charger can still leave the battery discharging when loads exceed its output.
Record the evidence
Write down battery and charger model numbers, temperature, error codes, charge stage and when the problem occurs. Use installed displays or apps first. Do not infer lithium state of charge from one voltage reading or bypass a BMS cutoff.
| Pattern | What to investigate |
|---|---|
| Solar only | Controller status, light, shade, array compatibility and its protected wiring |
| Driving only | Vehicle-approved connection, charger enable signal, input conditions and charger status |
| Shore power only | External supply, permitted load and converter/charger status |
| Every source | Battery protection, settings, shared connections and net running loads |
Charging while driving
A smart alternator changes its output under vehicle control. A compatible DC-DC charger may need a specified ignition or enable signal. Check the vehicle and charger manuals before changing it; a wire that happens to become live with the key is not automatically an approved connection.
For example, a charger delivering 30A while house loads use 5A puts a net 25A into the battery. An ideal two-hour period recovers 50Ah. If the loads instead use 35A, the bank loses 5A while the charger is operating normally. Thermal limits, supply variation and battery acceptance change the real result.
Use smart-alternator charging guidance and the DC-DC sizing comparison to review the design. Do not increase charger current beyond the vehicle, battery or wiring allowance.
Solar and shore sources
For solar, read the controller's status and compare panel voltage/current with the exact array design. Shade and a full battery can reduce output without a fault. Follow the specified isolation sequence; do not disconnect PV connectors under load.
For shore power, check the site's supply and the converter's normal indicators without opening live AC equipment. A plugged-in cord does not prove that the charger is enabled or receiving power. Repeated protective trips, heat damage or a mains fault require a qualified installer.
Battery and settings
Confirm every source uses the exact battery's permitted voltage, current and temperature rules. Not all lithium batteries use the same cold cutoff, heating behaviour or recovery procedure. A BMS alarm can explain refusal to charge; follow manufacturer support guidance rather than repeatedly disconnecting the battery.
Check shared wiring only after safe isolation and verification. A fuse that has opened needs its underlying cause investigated. Never upsize it merely to restore operation or put a multimeter's current range across the battery terminals.
The Victron Lithium Smart manual illustrates why protection and recovery are model-specific, including its external BMS requirements. It is not a substitute for your battery's manual.
Why does it charge on shore power but not while driving?
The driving source may have an input, enable, protection or compatibility issue. Record its status under the manufacturer's test conditions and inspect the approved connection design.
Is the charger broken if state of charge falls?
Not necessarily. Loads may exceed its output, or the monitor may be misconfigured. Compare net current and individual source data.
Should I reset a tripped BMS?
Use the manufacturer's diagnostic and recovery instructions. Do not bypass the protection or improvise a live disconnection sequence.
Read the charging systems guide, battery monitor guide, fuse guide, and use the planner to review demand and charging time.
Worked planning check
A 30A charger with 35A of running loads leaves the battery discharging at about 5A. The charger may be working correctly even while state of charge falls. Reduce nonessential loads, record each source's status and follow its manual's diagnostic sequence.
The figures or dimensions in this example are explicit planning assumptions, not a product test or an installation specification. Replace them with your equipment data and actual measurements.
Planning questions
Why is the battery discharging while the charger is on?
The loads may be using more than the charger supplies. Compare source output with net battery current before diagnosing a charger failure.
Can a full battery stop accepting solar charge?
Yes. Charge-stage and battery-protection behaviour can reduce current. Check the controller status and battery data; do not diagnose from a voltage reading alone.
Should I reset a battery BMS by disconnecting it?
Use the exact manufacturer’s diagnostic and recovery process. Do not bypass protection, improvise a reset sequence or disconnect live PV connectors under load.
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Jack Kennedy is a self-builder who has built two vans. He created Roam Wired to simplify conversion planning and share free tools, supported by affiliate links.
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