Size the MPPT charge controller
The charge controller sits between the array and the battery, stops overcharging and, with MPPT (maximum power point tracking), converts high array voltage efficiently into battery charging current.
How
- Charge current is roughly array watts divided by battery voltage. Pick a controller rated above that current, or split the array across several controllers.
- Check the controller's maximum PV input voltage against the array's open circuit voltage (Voc) on the coldest expected morning. Panel voltage rises as temperature falls. Example (US, NEC 690.7): agree a low design temperature with the inspector (often from ASHRAE data), then correct each panel's Voc with the datasheet temperature coefficient, or use the multiplier table in 690.7 (for example 1.14 at -6 to -10 °C). An IAEI worked example shows 16 panels of 39.1 V reaching about 690 V at -10 °C, over the 600 V dwelling limit. The maker's string sizing tool does the same sum.
- Use the PV string sizing calculator on this page for the cold Voc limit. Also check the hot string Vmp stays above the controller's minimum MPPT voltage, using the Vmp temperature coefficient on your panel datasheet.
- Set the controller's charge profile to match your battery chemistry and, for lead acid, use temperature compensation with a battery temperature sensor.
- For LFP, let the BMS communicate with the controller where possible, so charging stops at low temperatures.
- Wind and hydro need their own controllers designed for that turbine, usually with a diversion (dump) load. Do not use a PV controller for a turbine.
Safety: Panels produce voltage whenever light falls on them. You cannot switch them off at the panel.
Sources
- Balance-of-System Equipment Required for Renewable Energy Systems. US Department of Energy, Energy Saver, accessed 2026-11
- Battery temperature compensation (blog). Victron Energy, accessed 2026-11
- A Primer on Small Wind Systems. Bergey Windpower, accessed 2026-11
- Questions from the Field (NEC 690.7 maximum PV voltage). IAEI Magazine (International Association of Electrical Inspectors), accessed 2026-11