What you need
- A charge controller sized for your panels and battery voltage. MPPT for anything bigger than a trickle panel (MPPT vs PWM).
- A fuse or breaker on the battery cable, close to the battery.
- A way to switch the panels off: a PV breaker or disconnect between panels and controller. Victron's manual asks for one.
- MC4 solar cables from the panels, and flexible, fine-stranded copper cable for the screw terminals.
The order
From Victron's MPPT manual; most controllers work the same way.
- Check polarity on every cable before connecting anything.
- Battery to controller first, fuse out. Positive to positive, negative to negative, then insert the fuse. Wait about 10 seconds while the controller detects 12, 24 or 48 V, and confirm it picked the right one.
- Panels to controller, with the PV disconnect off, then switch it on.
- Set the battery type (lithium or AGM) in the app or on the display. How to charge a LiFePO4 battery has the settings.
To shut down, do it backwards: panels off, then battery. Victron warns that the wrong order can disable or damage the controller.
Series or parallel
| Series | Parallel | |
|---|---|---|
| How | + of one panel to − of the next | all + together, all − together |
| Voltage | adds up | stays the same as one panel |
| Current | stays the same as one panel | adds up |
| Good for | MPPT controllers, long cable runs, thinner wire | partial shade, low-voltage controllers |
Victron's minimum: the panels' working voltage should be at least 5 V above the battery's, so a 24V battery needs at least two 12V panels in series.
The cold-morning check
A panel's voltage rises as it gets colder, and it's highest at sunrise on the coldest day. Add up the open-circuit voltage (Voc) of the panels in series and correct for cold. With Victron's 150W 12V panel (22.3 V Voc, -0.35% per °C):
| Coldest morning | One panel | 2 in series | 3 in series | 4 in series |
|---|---|---|---|---|
| 77°F (25°C) | 22.3 V | 44.6 V | 66.9 V | 89.2 V |
| 32°F (0°C) | 24.3 V | 48.5 V | 72.8 V | 97.0 V |
| -4°F (-20°C) | 25.8 V | 51.6 V | 77.4 V | 103.2 V |
A 100 V controller must never see more than 100 V, so four in series is over the limit at -4°F. Three is safe almost anywhere. Use your own panel's Voc and temperature coefficient from its label or datasheet. For more panels, use a series-parallel layout (two strings of three, say) or a higher-voltage controller.
Then the rest
- Inverter: straight to the battery, on its own fuse, with heavy cable. What size inverter.
- Battery voltage: 12V vs 24V vs 48V.
- If it isn't charging: solar not charging your battery? Check these 7 things.
- Every part, including fuses and the disconnect: the complete parts list, and the cabin guide sizes the panels, controller and battery.
Questions people ask
Can I connect a solar panel directly to a battery?
Not safely, except for a tiny trickle panel made for it. Without a charge controller nothing stops the panel overcharging the battery, and a lithium battery needs a set charge voltage. Use a controller sized for the panels.
Do I connect the battery or the solar panel to the charge controller first?
The battery. Victron's MPPT manual says to connect the battery first so the controller can detect the system voltage, then the panels. To shut down, reverse it: panels off first, then the battery.
Should I wire my panels in series or parallel?
Series raises the voltage and keeps the current (and wire size) down, and an MPPT controller makes good use of it; parallel keeps the voltage low and copes better with partial shade. The limit is the controller's maximum PV voltage, counted on the coldest morning.
How many 12V panels can I put in series on a 100V controller?
Usually three. A Victron 150W 12V panel is 22.3 V open-circuit at 25°C; four make 89 V on paper, but at -4°F (-20°C) its voltage rises to about 25.8 V, and four reach 103 V, over the limit. Four only work where it never gets colder than about 15°F.
What fuse do I need between the charge controller and battery?
The size your controller's manual gives. For Victron's SmartSolar MPPT 100/30 it's 35–40 A, and for the 100/50, 55–70 A. Put it close to the battery positive terminal.
Sources
- SmartSolar MPPT 100/30 & 100/50 manual (connection order, battery fuse, 100 V limit, Voc and temperature, PV disconnect) Victron Energy
- BlueSolar monocrystalline panels datasheet (150W-12V: Voc 22.3 V, Isc 8.69 A; Voc coefficient -0.35%/°C) Victron Energy
Every number on this page comes from these sources. Spotted something out of date? Tell me.
Size it for your place
A few taps and the guide works it out for your situation and your winter sun.