Most often the controller never started: the panel voltage sits below the threshold it needs, or above the maximum it accepts, or the battery is outside the range where the controller will charge at all. The sequence of measurements narrows it quickly.
A charging system that does nothing is easier to diagnose than one that underperforms, because each stage can be measured independently.
Panel first
Open-circuit voltage at the panel leads, in sunlight, disconnected. A figure close to the datasheet means the panel and its cable are fine. A much lower figure points at shading, a damaged bypass diode or a connection; nothing at all points at the cable, a connector or the panel itself.
Controller next
The controller has a starting condition — typically a panel voltage above the battery voltage by a margin — and a maximum input it must not see. Both are published. A string that is inside neither will sit there doing nothing without reporting a fault.
Battery last
A battery outside the controller’s acceptable range, or a lithium pack whose management system is refusing charge because of temperature or a protection event, will show as a controller that is connected and idle.
The connections in between
Polarity at the controller, a blown inline fuse, a mismatched or partly seated connector pair, a crimp that separated inside its insulation. Each of those produces exactly the same symptom as the electronic causes, and each is faster to check.
What people ask before they order
Where do I measure first?
Open-circuit voltage at the panel leads, in sun, with nothing connected. If that is absent or low, the fault is upstream of the controller and nothing downstream matters yet.
Can a cold battery stop charging?
On lithium packs, yes — many management systems block charging below a threshold by design, and the controller then appears to be doing nothing. The pack’s documentation states the threshold.
Last reviewed 16 September 2026