A charge controller sits between a solar array and a battery, stops the array from pushing the battery past its charging ceiling, and prevents the battery from draining back through the panel at night. Salvage one from a solar installation or vehicle if you can, set it to the ceiling that suits your battery type, and choose one rated for at least 1.25 times the array’s current. Only a very small panel on a very large bank can get by without one, at the cost of regular water top-ups.
Part of Solar Panel Construction
Why a solar array needs regulating before it reaches a battery, the kinds of controller, and how to choose and set one.
A Panel Keeps Pushing Current Until Told To Stop
A panel delivers current whenever there is light, whether or not the battery needs it. On a full lead-acid battery that current goes into splitting water, boiling off electrolyte and corroding the positive plates (Charging Circuits). The charge-controller article calls overcharging the main way lead-acid batteries are destroyed. At night the opposite problem arises: the battery is the higher voltage and can feed current backwards through the panel, so a blocking diode or switch is required (Energy Storage & Batteries).
Solar panels differ from wind turbines in one useful way: they can safely sit at open circuit (Power Electronics), so a controller may disconnect the panel when the battery is full. The Charge Controller article says series controllers that disconnect are for solar panels only, whereas a wind turbine must always keep a load.
Controller Types, From Salvaged To Built
- A salvaged commercial controller — found on solar installations, in RVs and boats, and in battery maintainers and trickle chargers (energy-storage article). Usually the best choice.
- Blocking diode plus a car voltage regulator — the energy-storage emergency option, a regulator that limits to about 14.2 V; crude, built for an alternator, not a panel.
- A shunt regulator — diverts excess current away from the battery (Shunt Regulator Design).
- A series or PWM controller — switches the panel on and off to hold the voltage (PWM Controller Basics).
Settings follow the battery. The charge-controller article lists the ceiling for a 12 V flooded battery as 14.4–14.8 V, for sealed AGM 14.1–14.4 V and for gel 13.8–14.1 V, and advises the gel values when unsure. The sources for lead-acid voltages disagree, so see Charging Circuits for how to find the right value for your own cells. The temperature correction in the same article is −5 mV per °C per cell.
What You Need
- A controller — salvaged, with its rated current known
- A blocking diode if the controller has none
- A fuse between battery and controller — Fuse Design
- A voltmeter and ammeter
- Wire sized for the current — Wire Selection
Method: Connect A Controller
Step 1: Read the controller’s rating and settings, and set the charging ceiling for your battery type.
Step 2: Connect the battery first (general knowledge: many controllers read the battery voltage to set their system voltage), through a fuse close to the battery.
Step 3: Cover the panel with an opaque sheet, then connect it to the controller, then uncover it.
Step 4: Check polarity at every connection before powering (Polarity Check).
Step 5: Watch the first day: battery voltage should rise to the ceiling and hold there while the current falls.
Worked Example: Sizing A Controller, And When To Skip One
Sizing. A 59 W panel into a 12 V battery at the 14.4 V ceiling carries 59 ÷ 14.4 = 4.1 A. With the 1.25 margin from Load Calculation, 4.1 × 1.25 = 5.1 A, so a controller rated above that, say the next common size up, is the choice. This is the current into the battery; the corpus gives no panel short-circuit current for this example.
Skipping one. The carrier’s 9 W panel gives 0.5 A. On a 100 Ah flooded battery that is C/200. Once the battery is full, a battery needs only about 4 to 7 mA to cover self-discharge (the float arithmetic: 3–5% a month), so nearly all of the 0.5 A would go to gassing. At 6 hours of sun, 0.5 × 6 = 3 Ah a day, and at 0.336 g of water per ampere-hour (Water Level Maintenance) that is 3 × 0.336 = 1.0 g a day per cell, about 30 mL a month. A top-up chore, not a disaster, but it shortens plate life; do not extend it to larger panels.
Common Mistakes
- No controller on a large panel — the battery boils dry
- A wind-turbine setting or controller on a panel, or the reverse — a series controller on a turbine lets it overspeed
- Connecting the panel before the battery — some controllers misread the system voltage
- Setting the ceiling from a different battery type — gel and flooded differ
- Undersized wire — voltage drop starves the charge (Voltage Drop)
Safety
The array stays live whatever the controller is doing
A controller regulates the battery side only; the panel side is still producing voltage whenever there is light, rain included, and strings of panels add up (three 18 V panels make 54 V, six make 108 V). Disconnect or cover the array before working on the wiring, and treat it as live regardless. The battery side can deliver hundreds of amps into a short and makes hydrogen when charging, so fuse the bank and ventilate the top of the space (Hydrogen Gas Hazard).
What’s Next
- Learn next: Shunt Regulator Design — the simplest built controller
- Combine with: Series vs Parallel Configuration — matching array and bank