Solar Charger 200ds232 rev0

An unconventional, scalable high efficiency 12V solar power system and battery charge controller with low voltage cutout to protect the battery.  (Ideal for systems of 50W or less)

This design is based on an earlier version that can be found here.  The only change made regards the the low voltage LED. It's behavior is inverted and this removes the need for the jumper.


Low idling current
This circuit was designed for small to medium lead acid systems and feature a lowish idle current ( 5mA ) which increases battery life on small capacity systems.

Easily obtainable parts
All the parts in this design are through hole parts and can be found from a number of sources. None of the parts need programing and only a voltmeter and an adjustable supply is needed to calibrate the board. This makes it easy and cheap to build and maintain.

Late generation
This is the 4rd iteration of the design, with improvements and at every step.

Some of the older, development revisions :

The solar controller uses shunt solar regulation, when battery voltage exceeds a set voltage, typically set to 13,8V.

Low voltage battery protection
The circuit disconnects the battery if the if the battery voltage drops below an adjustable point, typically 10.5V.

Why not MPPT?

MPPT benefits
Maximum power point tracking controllers are basically microprocessor controlled switch mode converters that vary the duty cycle up and down, hunting for the panel voltage that relates to the maximum power output of the Photo Voltaic cells. These converters currently provide the highest conversion efficiency. They dynamically adjust to allow for changes in environmental conditions. An example would be if your battery is at 11V and the solar panel currently provides the optimum power at 15V, the MPPT would take in 15V at say 4A (60w) and provide the battery with 11V at 4,9A (53.9W) this includes a loss of 10% in the conversion. The same panel when connected directly may only put out 11V at 4,2A (46.2W).

MPPT drawbacks
The 2 drawbacks are complexity and monetary cost.
A microprocessor is inevitably needed and the circuit is generally more complex, this makes for a circuit that is less repairable and maintainable by its users that now require additional skills.
Also, MPPT converter cost more than simple controllers.

When to use MPPT?

If you accept the additional complexity, or was never going to builds, repair or maintain your system yourself, the only reason to choose or not to choose MPPT is monetary cost.

Currently (2011) on systems over 60W, it is almost always cheaper to use a MPPT converter.
that is because what you save on your PV panel, is more than the addition cost of the controller.

An example
Assuming you are paying 3$ / Watt for your panel. If your MPPT converter is going to cost $35 more than a simple converter, the MPPT has to save you $35 or more in panel costs to make it the better option,  that is to say 35/3 = 11,7W.
Assuming the manufacturer's claim of a 20% increase in performance is accurate, MPPT converters are then more cost efficient at power levels over 11,7W * 100 / 20 = 58.8W.

So for a 60W or less panel, a simple converter is cheaper, simply because it would be cheaper to spend the extra cash on a larger PV panel.

Common circuit problems

The most common solar charger consists of a Schottky diode to prevent the battery from draining into the PV panel and a shunt regulator that effectively short circuits the panel once the battery is fully charged.
One problem with this approach is diode losses and the resulting heat. If a 50W 12V panel supplies 4A to the battery,  the Schottky diode will drop about 0,4V across it dissipating about 1,6W of heat. This requires a  heat sink and loses power to heat. The problem is that there is no way of reducing the volt drop, paralleling diodes may share current, but the 0,4V will still be there. The circuit below uses a MOSFET in stead of the usual diode and the primary power loss is resistive.
For comparison, a 40W PV system using the circuit below with IRFZ48 mosfets has a loss of about 1/4W on Q2. This means less heat and more power for your battery.  More importantly though, MOSFETS have a positive temperature coefficient and can be paralleled to reduce the ON resistance. Unlike the diode system, the total power loss can be reduced.

Circuit diagram.

circuit diagram

Download a higher quality image.

Unconventional design
This circuit has a couple of unconventional solutions to the normal solar charger problem.

There is no diode between the solar panel and the load. This function is performed by Q2, a mosfet, used in reverse. The diode in the mosfet ensures that current will always flow from the PV panel to the load. If a significant voltage is present over Q2, Q3 turns on, charging C4, this allows U2c and U3b to turn mosfet Q2 on. Now the volt drop across the mosfet is determined by I*R, much lower than with a diode. C4 periodically discharges through R7, then Q2 is turned off. If current was flowing from the Photo Voltaic panel, the self induced EMF across inductor L1 ensures that Q3 is turned on promptly. This happens many times a second.  In the case where current was flowing to the PV panel at the time Q2 turned off Q3 will not be turning on again and D2 limits L1's self induced EMF. D2 may just be a 1A diode, but thats 1A continuously, and as the test is only performed periodically, D2 can handle currents much higher than 1A.

VR1 sets the maximum voltage. U2d's output gos high when the system voltage is above 13.8V, this turns Q1 on, shorting the solar panel out. Above 13,8V U2d also ensures U3b's output is low, effectively disconnecting the solar panel from the system voltage. This is needed because otherwise Q1 will short out not only the solar panel, but the system too, the battery is sure to provide enough current to destroy something if it is shorted out. L1 prevents a momentary short circuit on the load while Q1 and Q2 are controlled simultaneously.

High drive N channels
Q2 and Q4 are N channel mosfets and needs to be turned on with a voltage higher than the system voltage. For this, the circuit U2a and the associated diodes and capacitors create a high voltage VH. This voltage is used as the supply to U3, allowing the outputs to rise above the system voltage. U2b and D10 provides some regulation of the High drive voltage to about 24V. At this voltage, the Mosfets will see at least 10V across the Gate and Source, ensuring a low on resistance and therefor a low heat dissipation.

N channel mosfets typically have a significantly lower on resistance than P channel mosfets, this is the reason they were chosen for the design.

Low voltage cutout
Q4 and U3a and associated resistors and capacitor create a low voltage cutout circuit. Once again a MOSFET, Q4 is used in an unconventional way, the diode in the mosfet ensures current can always flow into the battery. If the voltage is above the set point,the mosfet is turned on, allowing a lower volt drop during charging, but more importantly allows current to flow from the battery to power the system when the solar panels are not providing the system with power. A fuse protects against damage in case of a short circuit on the load side. D11 will light up once the battery has been isolated. In real life that means D12 will be of no use, but it is used during calibration.

Kit form

A kit including all the parts to be inserted, the inductor core and the PCB, can be bought for $23 US and $7US for international registered airmail.

Bare printed circuit boards

A bare Printed Circuit Board can be bought for US $10 and US $7 for registered airmail.

Printed Circuit Boards

Here are pictures of the different layers of the double sided sided through hole plated board.




Here are PDFs for all of the layers.
Top layer
Bottom layer

Part list

R5,R18,R8,R13,R15,R14,    100K
R16,R1,R9,R11,R19, 10K


C1,C3, 1000UF 25V
C7,C6,C5,C4,C2,    100nF CAP
C9, 100UF 35V
C8,C12,C10,10uF 25V

D7,D3,D8,D5,D10,D6,D12,    1N4148
D4,D2,    1N5819
D1,    1.5KE16,    unidirectional tranzorb

D9,green led
D11,red led

Q3,BC327 pnp transistor
Q1,Q2,Q4,IRFZ44 to220-gds n channel mosfet

L1,19 turns 1mm wire on micrometals T68-52A

F1, 5A 20MM FUSE

U1,LM336-2.5 voltage reference

ST1,ST2,ST3, 2 pin screw terminal


To calibrate the unit, connect a variable power supply to the LOAD terminals. (ST2)

First we will set the Low voltage cutoff point.
Set your power supply to 10,5V.
Turn trim pot VR2 anticlockwise until LED D11 turns off.
Slowly turn VR2 clockwise until it turns on.

Next we will set the maximum system voltage
Set your power supply to 13,8V.
Turn trim pot VR1 clockwise until LED D9 turns off.
Slowly turn VR1 anticlockwise until it turns on.

The End.


The MOSFETS can be exchanged for IRFZ34 (cheaper) for smaller systems, or increased to IRFZ48 on systems that have higher current requirements.


A number of people have built this circuit and have used a number of different enclosures. Send me a pic of yours if you want to share it with the community.

Here is a one by Thomas Gfüllner of Germany.

Design files (DXF) available on thingiverse. Simple, clean and it you have access to a laser cutter or cnc mill, quick to make.

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