Choosing a charge controller
The controller sits between the panels and the batteries. Chosen badly, it wastes your production or fries on the first cold morning. Here's how to choose one, and a tool to check your wiring before you buy.
Tool: Check your wiringThe short answer
MPPT: almost always
- it gets 20 to 30% more out of the same panels, especially in the cold;
- it accepts panels wired in series, at higher voltage: less loss in the wires;
- it works with big 400 to 550 W panels, the ones made for houses.
PWM: only for a small stopgap
- one or two small “12 V” panels (about 18 V when working) on a 12 V battery;
- a trailer or shed with 100 to 200 W in total;
- it can't handle a big high-voltage panel: it wastes a good part of it.
Reading an MPPT controller's name
Many MPPT controllers carry two numbers in their name. They're the two limits to respect.
100/30
- 100
- maximum volts at the input, panel side. Never exceed it, even in deep cold.
- 30
- maximum amps at the output, battery side. It decides how many watts of panels it can take.
Check your wiring
The panel's numbers are on the label on the back, or on its spec sheet. In the cold, a panel's voltage rises: that's what fries controllers.
Example: a common 400 W panel. Replace with your own panel's numbers.
85.2 V
in deep cold, open circuit
52.1 V
working, on a hot day
1,600 W
of panels in total
67 A
of charging current needed
85.2 V in deep cold: that's under the controller's 100 V, with margin.
At most 2 panels in series on a 100 V controller.
In summer, the voltage stays high enough above the batteries': the controller can charge.
You need an MPPT controller of at least 100 V input and 70 A output.
We count Voc at the coldest temperature (with a 5% safety margin) and Vmp with cells at 65 °C. Amps: panel watts ÷ battery voltage, rounded up.
Words to know
- Voc
- The panel's voltage with nothing connected. The highest it can give. It rises in the cold.
- Vmp
- The panel's voltage when it works at its best. It drops when it's hot.
- Temperature coefficient
- How much the voltage changes per degree. -0.28%/°C: at -30 °C, Voc is about 15% higher than at 25 °C.
- In series
- One panel's + to the next one's −: voltages add up, current stays the same.
- In parallel
- All the + together, all the − together: currents add up, voltage stays the same.
- Battery setting
- The controller must know whether your batteries are lead-acid, AGM or lithium: they don't charge at the same voltages.
Classic traps
- Working out the series with the label's Voc, measured at 25 °C. In a cold winter, count 10 to 20% more.
- Buying a controller just big enough in amps, then adding panels: it tops out and wastes the extra.
- Forgetting to pick the right battery setting: a lithium battery charged like lead-acid fills poorly or gets damaged.
- Connecting the panels before the battery: most manufacturers say to connect the battery first and disconnect it last.
- Putting three or more strings in parallel without a fuse on each: if one string shorts, the others discharge into it.
To calculate
Next step
Choosing an inverter
From the small modified sine wave inverter to the 120/240 V all-in-one, and a tool to find the size you need.