A shunt regulator protects a battery by diverting the panel’s current when the battery voltage reaches its ceiling, with a zener diode sensing the voltage and a transistor or relay doing the switching. The shunt must sit across the panel on the panel side of the blocking diode, never across the battery, and because a panel is current-limited a low-resistance shunt of 1 ohm wastes only about 20 W from a 4.5 A panel.
Part of Solar Panel Construction
A hand-built shunt charge regulator for a solar array: where the parts go, how the trip point is set, and what the shunt dissipates.
The Shunt Goes On The Panel Side Of The Diode
The carrier describes a shunt regulator that, when battery voltage reaches the float level (13.8 V), uses a relay or power transistor to short the panel output through a resistor, with a zener set to trigger at 14.0–14.4 V, a blocking diode and a heat sink. The blocking diode’s position decides whether this works. The panel’s positive goes first to a node where the shunt connects; from that node the blocking diode leads to the battery. When the shunt turns on, it pulls the node down, the blocking diode becomes reverse-biased and the battery is isolated. If the shunt were instead across the battery, it would short the battery itself.
A solar panel is a current-limited source: shorted, it delivers about its short-circuit current at close to zero voltage (the corpus says panels can sit at open circuit; that they tolerate a short is general knowledge, not from the other articles). So the shunt dissipates only I²R with a small R, not the panel’s whole power. That differs from wind, where the Charge Controller article requires a dump load that always loads the turbine; for a panel nothing spins up.
What You Need
- A zener diode, an NPN transistor, resistors and a 10 kΩ potentiometer — the parts list in the Charge Controller article
- A relay or power MOSFET with current rating above the panel current with a margin (Load Calculation: 1.25)
- A blocking diode rated for the panel current — Bypass Diodes explains which diodes to use
- A flyback diode across any relay coil, and a small capacitor to stop chatter (Charge Controller)
- A variable supply and a voltmeter for calibration — Metering
- A heat sink if a resistor or transistor takes sustained power
Method: Build And Calibrate
Step 1: Set the layout. Panel + → node → blocking diode → battery +; the shunt element from the node to panel −; the sensing circuit across the battery.
Step 2: Build the sensing stage. A divider scales the battery voltage down to a zener; when the zener conducts it turns on a transistor, which operates the relay or MOSFET. The Charge Controller article uses a pot in series with a fixed resistor for the divider.
Step 3: Calibrate on the bench. With a variable supply across the battery terminals, raise the voltage until the shunt operates at your chosen ceiling (the 14.4 V of that article for flooded cells), then lower it and check that it releases 0.5–1.0 V lower (hysteresis); lock the pot.
Step 4: Choose the shunt element’s size with the arithmetic below.
Step 5: Fit the regulator between panel and battery with a fuse on the battery lead.
Step 6: Season adjustment. The Charge Controller article suggests raising the setpoint 0.5 V in winter and lowering it 0.3 V in summer if there is no temperature compensation.
Worked Example: Trip Point And Shunt Power
Trip point. The Charge Controller article’s parts list asks for a 14.0 or 14.4 V zener and also a divider; the two do not fit together as written, because a divider is needed only to bring the voltage down to a lower zener. Choose the zener first. Assume a 5.1 V zener (a common value; the choice is mine) and a transistor base-emitter drop of 0.6 V (silicon, from Junction Diode). The divider must give 5.1 + 0.6 = 5.7 V when the battery is at 14.4 V, a ratio of 5.7 ÷ 14.4 = 0.396. With a 10 kΩ bottom resistor the top is 10 ÷ 0.396 − 10 = 15.3 kΩ. A real zener has a tolerance (general knowledge), so the pot, not the arithmetic, sets the final trip.
Shunt power. Take the 59 W panel from the earlier examples at 14.4 V (4.1 A), and assume a short-circuit current of 4.5 A (an assumed figure; measure yours). A 1 Ω shunt pulls the node to 4.5 × 1 = 4.5 V, well below the battery, so the diode isolates the battery, and the shunt dissipates 4.5² × 1 = 20 W. A MOSFET with an assumed on-resistance of 0.05 Ω wastes 4.5² × 0.05 = 1.0 W. By comparison, a shunt resistor sized to take the full 4.1 A at 14.4 V (3.5 Ω) would absorb 14.4 × 4.1 = 59 W, which is the heat-sink case the carrier mentions; the low-resistance shunt is far cooler.
Diode loss. When charging, the blocking diode drops about 0.6 V at 4.1 A: 0.6 × 4.1 = 2.5 W.
Common Mistakes
- Shunt across the battery — it shorts the bank; the shunt belongs on the panel side
- Skipping hysteresis — the relay chatters and burns its contacts
- A relay rated for the load, not the panel — contacts weld
- Adjusting the setpoint on the live array — use a bench supply
- Forgetting the blocking diode — the battery discharges through the shunt
Safety
The panel stays live while the shunt works, and a mis-wired shunt shorts the battery
The array is producing voltage whenever there is light, and when the shunt is on the panel is shorted, so it can burn a careless hand through a hot shunt or a welded relay. If the shunt is wired across the battery instead of the panel, it short-circuits the bank, which can deliver hundreds of amps and melt wiring; fuse the battery lead. A series string reaches a dangerous voltage (three 18 V panels make 54 V, six make 108 V); cover the array or disconnect it at the array and treat it as live regardless (Lethal Current).
What’s Next
- Learn next: PWM Controller Basics — smoother control with a switching transistor
- Combine with: Float Charging — choosing the voltage the regulator holds