Sunsynk Battery Not Charging — System Mode, SolarMan and BMS Faults
Hybrid inverter battery not charging from solar or grid despite settings appearing correct. Most common cause: System Mode set incorrectly in SolarMan portal. Very few of these are actual hardware faults — they're configuration issues resolvable remotely.
This diagnostic guide covers every setting that can prevent charging. Work through the steps in order — most cases are resolved by the third step.
Book Remote Diagnostic — from £75 → Sunsynk hubSafety: Isolate at the DC switch if you notice any error lights on the inverter or battery.
6-step diagnostic process
Work through these steps in order. Most battery charging issues are resolved by Step 2.
Check System Work Mode in SolarMan
Log into SolarMan, navigate to Device → Settings → System Work Mode. If the mode is set to Selling First, this is almost certainly your problem. Selling First mode maximises grid export and does not force charge from the grid or solar. Change to Time of Use (or the mode appropriate for your tariff) to enable charging windows.
Check SoC target and minimum SoC headroom
In SolarMan, check the State of Charge (SoC) settings: target SoC (maximum charge level) and minimum SoC (discharge floor). If the battery is already at or above the target SoC, the inverter will not charge further — there's no work to do. For example, if minimum SoC is 90% and target is 90%, charging will never happen. Set minimum SoC to ~10-20% and target to 90-100% to allow the system to charge from near-empty to full.
Check BMS communication status
In SolarMan, on the Device page, check the battery status. It should display a valid percentage (e.g., 45% SoC, 78% SoC). If the battery shows 0% or Unknown, the inverter has lost communication with the battery BMS. This prevents charging entirely. Check the CAN or RS485 communication cable between the inverter and battery — it may be loose, disconnected, or damaged.
Verify charge schedule times and timezone
In SolarMan, check the Time of Use charge window times. The window must exactly match your tariff's cheap-rate period. Common windows: Octopus Go 00:30–04:30 · Octopus Flux off-peak 02:00–05:00 · Economy 7 approximately 00:30–07:30. Also check that the inverter clock is set to current local time. During British Summer Time (BST), if the clock is still on GMT, the window will fire 1 hour late.
Confirm grid charge is enabled
In SolarMan, find the grid charge settings (usually under Device → Settings → Grid Charge). Verify that grid charge is set to ON for the time windows where you want the battery to charge from the grid. If grid charge is disabled, the battery will only charge from solar generation, not from cheap-rate grid electricity.
Check CT clamp orientation if applicable
If your Sunsynk has a CT clamp installed (for import/export monitoring), check that it's oriented correctly. In SolarMan, at night with no solar generation, the grid reading should show a positive value (import). If it shows negative (export) while you're clearly consuming from the grid, the CT clamp may be backwards. This can prevent charging from activating correctly.
About Sunsynk charging issues
Sunsynk hybrid inverters (3.6kW, 5kW, 8kW, 12kW models) are widely installed across the UK from 2020 onwards. They integrate directly with SolarMan monitoring (formerly PVMaster) and support native Octopus Agile, Intelligent, and Go tariff integration via Time of Use schedules. However, the SolarMan portal doesn't always clearly surface these settings, and many systems are delivered with factory defaults rather than customer-optimised configuration.
The result: nearly 30% of Sunsynk owners report battery not charging overnight despite having a charged battery and a cheap-rate tariff available. In almost all cases, it's a settings issue — System Mode set to Selling First, SoC headroom exhausted, or timezone mismatches — not a hardware fault.
Sunsynk battery charging questions
Check System Work Mode in SolarMan — if set to Selling First, the inverter will export all solar to the grid rather than charging the battery. Change to Time of Use mode. Also check that the battery SoC is below the charge target and that there is headroom to charge. If the battery is already at the target SoC, the inverter will not charge further.
A battery showing 0% or Unknown state indicates the inverter has lost communication with the battery BMS. This is a separate issue from charging configuration. Check the CAN or RS485 communication cable between the inverter and battery. The cable may be loose, disconnected, or damaged. Do not open the battery enclosure yourself — contact a qualified engineer.
Intermittent charging is often caused by: charge window times drifting due to BST/GMT clock mismatch (the window fires 1 hour late), SoC target set too high leaving no headroom to charge, or grid charge being toggled on/off manually. Check your SolarMan logs to see whether the charge window is firing at the expected time. If consistently 1 hour late, the clock/timezone is the issue.
Yes. A battery management system (BMS) fault can prevent charging entirely. The BMS protects the battery cells from overcharge and may trigger a shutdown if it detects abnormal conditions — temperature imbalance, cell voltage issues, or communication errors. If the battery shows as 0% or Unknown in SolarMan (indicating lost BMS communication), or if the battery refuses to charge despite correct settings, a BMS fault is likely. A diagnostic visit from £75 can identify BMS-related faults and determine if the battery requires service or replacement.
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