Answers to Frequently Asked Questions for the SCC3 12 Volt, 20 Amp Solar Charge Controller

(c) 2010-2020, Document revision: July 15, 2020

---------- Battery Sizing and Compatibility Questions ----------

Q: How do I select the size of battery for my solar power system?
There are a number of factors to consider when selecting the battery's amp-hour rating. Generally, the best starting place is to calculate the amp-hours consumed by the load you will be powering. As an example, a 1 amp light bulb that needs to be powered for 5 hours will take 5 amp-hours of battery capacity, that is the absolute minimum battery size you will need. Battery efficiencies tend to be around 80%, so you will need to put in more amp-hours during charging than you consume at night.

Batteries last much longer if they are not fully discharged every day, so extra battery capacity is a good idea. PV panels only produce a small fraction of their normal output in cloudy weather. If you need power in cloudy weather, increasing the capacity of the battery and PV array is recommended. People tend to expand their solar power systems over time, so it is usually a good strategy to choose larger batteries and PV panels than you initially require.

Keep Peukert's Law in mind, it explains how a lead acid battery's amp-hour rating becomes lower as the rate of discharge increases.

Q: What kinds of battery can be used with the SCC3?
The most common rechargeable battery types used in solar power systems are of the lead acid variety. Both wet cell and sealed gell cell lead acid batteries will work. Wet cell batteries are typically used in larger systems and gell cells are used in smaller or portable systems. Wet cell Nickel Iron (NiFe) batteries will work with the SCC3. The SCC3 will also work with Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) cells, these types of batteries typically have a much lower amp-hour capacity. For systems with NiCd and NiMH batteries, the SCC3 thermistor should be in thermal contact with the battery pack.

Q: Can I use my SCC3 with any type of Lithium battery?
Lithium (LiFePo) cell batteries should only be charged by the SCC3 if they are the type with internal protective circuitry. Lithium cells without protective circuitry can burst into flames if they are overcharged and become hot. The Tenergy part number 30006 cells are one example of a protected cell. Be sure that the maximum current from your PV panel is less than the maximum charging current that the LiFePo cells are rated to handle. A set of four 3.7V LiFePo batteries will have a charged voltage of 14.8V, which is near the high end of the normal SCC3 charging range. The SCC3 charging range can be raised by changing R1 from 270K to 300K.

Q: Can I use an automotive or motorcycle starter battery with the SCC3?
Yes, but it is not recommended, especially if the battery has been previously used. Automotive batteries are optimized for delivering hundreds of amps of current for a few seconds, followed by immediate recharging. Solar power systems typically deeply discharge the battery at night, this is bad for starter batteries and they will have a short lifespan. Start with a new deep-cycle battery, it will prevent many headaches.

Q: Can I run more than one battery in parallel with the SCC3?
Yes, but the batteries should all be the same brand and age to prevent the stronger batteries from losing their charge into the weaker batteries at night. The PV array should be suitably sized to charge the total capacity of your battery array.

---------- Photovoltaic Panel Sizing and Compatibility Questions ----------

Q: How do I select the size of the PV panel that is right for my battery?
A good rule of thumb for amps of charging current is based on the battery amp-hours divided by 20 (C/20). Here are some examples: a 10 amp-hour battery, or a common 7-AH UPS battery can be charged with a 6W panel that puts out around .5A. A 20 amp-hour battery can be charged with a 12W panel that puts out about 1A. A series-pair of Trojan T105 6V/225 amp-hour batteries can be charged with a 120W panel that puts out about 10 amps. These are only approximations, quite a bit of variation is acceptable. Too much charging current may damage a battery and too little current will never be able to fully charge the battery. If the battery is powering a constant load, you will need enough PV current to run the load *and* charge the battery at the C/20 rate.

Q: Will the SCC3 work with my (brand X) PV panel?
PV panels are characterized by their open-circuit voltage in full sun and their short-circuit current in full sun. As long as those ratings are within the SCC3 specifications and the charging current is matched to the battery's needs, the panel should work. Beware that amorphous panels may drop their output voltage if they get hot, resulting in much-reduced battery charging.

Q: Can I use different types of PV panels in parallel?
Usually, it depends on the type of panels you have. If your panels are all mono/polycrystaline and they have the same number of cells (typically 36), they should work in parallel. The same is true if all of your panels are amorphous types and have similar open circuit voltage ratings.

If you are unsure whether your panels can work together, connect the panels in parallel (+ to + and - to -) and see if there is any leakage current by placing an ammeter between the + and + leads when the panels are in full sunlight. Also try this with one of the panels in the sun and the other in the shade. If the leakage current is more than a few tens of milliamps, you may be able to isolate it by putting a suitably rated schottky diode between each PV panel's + lead and the SCC3 PV + input. If you do this, be sure to solder a bridge wire across the two pins of D1 (20L15T) on the SCC3 board so that there is only diode in series with each panel. Be sure to use Schottky diodes since they have a lower forward voltage drop than regular silicon diodes and will waste much less power.

---------- Alignment Questions ----------

Q: What float voltage should the SCC3 be set to?
That depends on the battery type. The best place to find the right float voltage setting is from the battery manufacturer's specications. If that is not available, a good rule of thumb is 13.8V for 12V gell cell batteries and 14.5 V for 12V wet cell batteries. For systems that don't get discharged on a daily basis, lower float voltage settings may make the battery last longer. Banks of NiMH cells should be set to about 1.25V per cell, so a 10 cell NiMH battery should be set to 12.5V. Again, consult the battery manufacturer's data for the best setting. Users of NiMH and NiCd batteries should put the SCC3 thermistor in thermal contact with the battery pack.

Q: The measured float voltage from my SCC3 has gone up since it was first installed, what's wrong?
If the thermistor is mounted near the battery and the thermistor wiring breaks or corrodes, the float voltage will rise by several volts. Replace the wiring and possibly the thermistor. Dirt or flux residue on the SCC3 board can also cause the float voltage to change. Disconnect and clean the solder side of the SCC3 board with alcohol and a toothbrush.

Q: Can the SCC3 be aligned when the battery is disconnected?
No, the battery must be connected for the SCC3 to operate correctly.

---------- Configuration Questions ----------

Q: The older SCC2 kit worked at 6V, can the SCC3 be modified for 6V operation?
No, the circuit activation and voltage regulator sections of the SCC3-e1 won't work reliably below around 8V and those circuits are powered from the battery.

Q: Can I use the SCC3 kit for solar charging a 10V battery?
Yes, 10V is about the lowest battery voltage that the SCC3 can charge, see the above question. You will need to reduce the value of R1 to 220K to lower the minimum float voltage setting. Note that 10V battery systems are very uncommon and a 12V battery with one dead cell will not have correct discharge properties due to the high internal resistance of the dead cell.

Q: Can I use the SCC3 kit for solar charging a 24V or higher voltage battery?
The SCC3 can be used to charge a 24V battery, but the kit needs to be modified with a few additional user-supplied parts. While the SCC3 can be modified to work with a 24V battery system, the circuit would require a complete re-design to work with systems above 24V. With the stock 12V SCC3, the open-circuit voltage of a PV panel that is used to charge a 24V battery can be more than 36V, and that is above the ratings for some of the components in the unmodified SCC3. See the SCC3 circuit page for the 24V modification details.

Q: Can I use two SCC3 kits in series to charge a 24V battery?
No, the SCC3 circuit is not designed to operate in that manner.

Q: Can I use multiple SCC3 kits in parallel for a higher maximum charging current?
Yes and No. If each SCC3 board has its own PV panel or panels, the battery side of the boards can be connected in parallel to charge one battery or bank of batteries. The float voltage adjustments should be set so that the LEDs on both boards blink when the desired float voltage has been reached. The thermistors on the SCC3 boards should be located close together to insure good thermal tracking between the boards. Multiple SCC3 board cannot be wired with parallel PV inputs because the boards will not synchronize with each other, causing one board to overload. See the section below on SCC3 Circuit Extensions for info on increasing the maximum current of an SCC3 board to 60 Amps.

---------- Operation Questions ----------

Q: Can I connect a load to the battery when the PV is charging?
Yes. If the load current is higher than the PV charging current, the battery voltage will drop and the battery will discharge. If the load current is lower than the PV current, the battery's charge will increase, but more slowly than if no load were connected.

Q: My battery's voltage drops after the sun goes down, is there a problem with my SCC3?
No, this is normal. When the sun is shining on the PV, the SCC3 pulls the battery up to the float voltage setting. When there is no PV current, the battery voltage will drop. When a heavy load is connected to the battery, its voltage will drop. A typical unloaded 12V lead acid battery will drop to around 12.8V at night with no load. If the battery voltage drops below 10V, the battery is either fully discharged or the load is taking more current than the battery can deliver. If the battery drops below 10V immediately after being fully charged, one or more of the cells is defective and the battery should be replaced.

Q: My battery eventually discharges when the PV panel is left disconnected or stored in the dark, is there a problem with my SCC3?
No, all batteries will eventually self-discharge. The SCC3 also consumes a small amount of current (around 0.002 Amps) from the battery at night. If you have a portable system, putting the PV in a window that receives some direct sunlight each day is usually sufficient to keep the battery topped up. It is always best to locate the PV panel outside and aim it at the sun's noontime position in the sky. If you need to store your system in the dark for extended periods, it is best to disconnect one terminal of the battery. In such a system, the PV should be placed in direct sunlight for a few hours before powering a load from the battery.

Q: If I connect my PV panel to the SCC3 input, why is there no voltage on the battery terminals?
The SCC3 will not function if there is no battery connected to the battery terminals, that is where it gets the power to operate. Further, the battery should always be kept above 8V for the circuit to operate properly. Typically, lead acid batteries should always be kept above 10V or they will suffer internal damage and have a very short life. If you start with a new battery that is completely discharged, you may need to pre-charge the battery by connecting the PV panel directly to the battery until it reaches 12V.

Q: The heatsinks on my SCC3 get really hot, is this a problem?
For PV panels that are rated more than about 5 Amps, the SCC3 should be located in a metal box that has exposure to outside air. Above 10 amps, this becomes very important. Use the "rule of thumb" test, if the heat sinks are too hot to keep your thumb on, you need to increase the ventilation. Generally, a fan is not required for the SCC3, but it can be a good addition if your system runs above 15 Amps and the ambient air temperature is above 70 degrees F (about 25 degrees C). D1, the 20L15T Schottky diode is the hottest running component on the SCC3, it should be be well ventilated.

Q: Are there any hazards that I should be aware of?
Fires and burns caused by short circuits across the battery terminals are a serious concern with systems that use large batteries. All connections to the battery should be insulated and fused and batteries with exposed metal terminals should always be enclosed in a non-conductive enclosure. It is a good idea to install a Class-T fuse or DC-rated circuit breaker in series with the battery's + terminal, especially when using large batteries. Flooded (wet) cell lead acid batteries emit caustic sulpheric acid fumes and explosive hydrogen gas, especially when they are charged or discharged at a heavy rate. Large battery banks should be mounted in an enclosure that is ventilated to the outdoors.

Q: Can the SCC3 run a water pump or other DC load directly?
No, the SCC3 is generally only useful for charging rechargeable batteries. What you probably need is a buck-mode switching regulator circuit, also known as a Linear Current Booster (LCB). Here is a small Buck Mode Switching Regulator circuit, larger capacity LCB circuits can be purchased commercially. The buck mode regulator can also be used to efficiently power lower voltage devices from a 12V battery.

---------- Other Power Source Questions ----------

Q: Can the SCC3 be used to regulate charging from an AC powered DC power supply?
Yes, but the power supply needs to be current limited. This is simply a matter of putting a low-ohm high-watt resistor between the DC supply and the SCC3's PV input. The DC supply should output 1-4 Volts above the desired battery float voltage setting. Note that this is not going to make a very efficient charger since the resistor will waste a lot of energy. If you are trying to regulate a small "wall wart" power supply, beware that the voltage specs are often misleading, measure the voltage before connecting.

Q: I would like to run a line-powered battery charger, can I leave the SCC3 connected to the battery while the charger is in use?
Yes, the SCC3 has a built-in diode that prevents it from reverse current flow. Just connect the charger directly to the battery terminals as usual. Keep in mind that an unregulated AC charger will probably overcharge the battery if it is left connected for too long, this can shorten the life of the battery. A better way to control a backup charger is to use the CirKits LVD1 kit with a simple modification that allows it to control a backup battery charger.

Q: Can I use the SCC3 to regulate power from a wind generator?
No, the SCC3 is not designed for use as a wind generator charge controller. A wind generator can produce very high voltages and currents when exposed to gusts of wind, that would burn out the protection circuitry and possibly other parts of the SCC3. When the battery becomes full, the SCC3 opens up the circuit. A wind generator with no electrical load will spin out of control and can fly apart. Wind generators need a shunt-mode regulator that diverts excess power to a dump load. The WGR1 project is an example of a wind generator regulator circuit that has been used in conjunction with the SCC3.

Q: I have a self-regulated wind generator charging my battery, can I use the SCC3 and a PV panel to add supplemental power?
Yes, this will work. You should set the wind generator's regulator to approximately the same voltage as the SCC3's float voltage. Wind and solar tend to complement each other, it's often sunny and calm or windy and cloudy.

Q: Can I use the SCC3 to regulate power from a hydroelectric generator?
Not recommended, the problem is similar to regulating wind generator power (see above). The SCC3 will produce a variable mechanical load to the generator and this will likely cause the generator's bearings to wear out prematurely.

Q: Can I use the SCC3 to regulate power from a human-powered DC generator?
Sometimes. The SCC3 has been used to regulate pedal-powered DC generators, but the generator's output should always be lower than the maximum PV input voltage for the SCC3 (around 25V) or the transzorb will burn out. The SCC3 can be built without the transzorb if it is to be used in this manner. When the battery reaches the float voltage setting, pedaling becomes easier.

Q: Can I use the SCC3 to regulate power from a car/boat alternator?
No, alternators require a control circuit for the field coil winding, a special purpose regulator is required.

Q: Can I use the SCC3 to power an automotive 12V battery system?
Sometimes, some automotive electrical systems can produce high voltage negative spikes when the starter motor is shut off. This can damage diode D1 on the SCC3 board. Before connecting the SCC3 to a car's 12V system, it is advisable to look at the car battery voltage with an oscilloscope while starting the engine to make sure there are no large negative voltage swings.

---------- SCC3 Circuit Extension Questions ----------

Q: Can I use external circuit breakers with the SCC3?
Yes, and this is required for NEC code compliance in systems where that applies. DC-rated 20 amp circuit breakers can be located in series with the + leads for the battery and the PV panel. Breakers allow the SCC3 to be electrically isolated for safe installation and removal. eBay is a good place to find DC-raded circuit breakers.

Q: Will the SCC3 work with an AC power inverter?
Yes, just be sure that the inverter's current requirements are within range of your battery's capabilities and the inverter's supply wiring is connected directly to the battery through a fused disconnect. A 2KW inverter will consume around 200 amps when it is fully loaded, be sure to use the correct wire gauge for the inverter's DC cabling. Also, keep in mind that older square wave and modified square wave (mod sine) inverters will only power a limited number of devices. Many AC-operated devices will only run correctly with true sine wave inverters and may overheat when run on square-wave power.

Q: Can I modify the SCC3 to handle more than 20 amps of solar charging current?
The main power connector, circuit board traces and power diode on the SCC3 are all limited to 20 amps max, running more current through these components will burn them out and can possibly cause a fire. The 60 Amp Modification can allow the SCC3 to regulate higher levels of current.

Q: Can I use more than one SCC3 to charge my battery bank?
Yes, as long as each SCC3 and associated PV array are running at 20 amps or less. This method of wiring may have better reliability compared to a single high-amperage charge controller. In the event of a failure of one charge controller or PV array, the system will still receive a charge from the other charge controller(s).

Q: Can you recommend any voltage meters for my battery? sells a nice two wire LCD meter (part #DMS-20LCD-0-DCM-C) that can be directly connected to the battery circuit, it consumes very little power. Builders may be interested in looking at the BVM1 12 Volt Battery Voltage Monitor kit and the Expanded Scale Battery Volt Meter project.

Q: Can you recommend a current meter for my PV and battery circuits?
This Solar Panel Current Meter project is one solution. A bi-directional automotive amp meter can also be installed between the battery + terminal and the rest of the circuitry to monitor charging and discharging current.

Q: Can I tap into the excess solar power after the battery becomes charged?
Yes, details on a dump load circuit can be found here. There's no circuit board or kit available for the dump load circuit, it's not too hard to hand wire on a perforated board. Be sure to use adequate heat sinks for the FET and power diode. In general, it is best to prioritize on getting your PV system fully functioning before worrying about the power that is wasted after the battery is charged. Dump load power is "low quality" power, it comes in the form of irregular pulses and is generally best used for generating heat.

---------- SCC3 Construction/Debugging Questions ----------

Q: I built my SCC3 kit and it doesn't work, what's wrong?
Read the SCC3 Troubleshooting document for tips on repairing common assembly problems and performing circuit diagnostics.

Q: Can I use IC sockets for the chips?
- Yes, sockets can make repairs and troubleshooting easier if the SCC3 develops a problem. Be sure to use high-quality sockets, the type with "machined pins" are best. Sockets may cause reliability problems in dusty or high-vibration environments.

---------- Miscellaneous SCC3 Questions ----------

Q: Does the SCC3 use Maximum Power Point Tracking (MPPT)?
No, the SCC3 uses an efficient on/off switching mode of operation. MPPT charge controllers can be more efficient, but they are more complicated and may consume a fair amount of power to run their circuitry. They also tend to generate more RF noise because of the switching circuitry on both the input and output sides. MPPT controllers are typically best suited for larger systems with many PV panels or grid-tied systems.

Q: I saw someone selling what looked like the SCC3 kit, but his version claimed to have an RS-232 data output, please explain?
An unscrupulous person in Oregon had a business where he would buy kits from various companies and resell them for double the price, sometimes making outrageous claims about the kit's capabilities. Buyer beware. The SCC3 circuit was intentionally designed without a microprocessor or any associated data ports.

Q: How does the SCC3 compare to some of the inexpensive Chinese-made charge controllers?
CirKits has had reports from one person who tried using some of these charge controllers, the controller only allowed their 5 amp PV panel to charge the battery at 1 amp. Switching to the SCC3 increased the charge current to the panel's the full 5 Amps. The PV panel is one of the most expensive parts of a solar power system, don't waste precious PV power by settling for a low quality charge controller.

Go Back to the SCC3 kit page.