Choosing the Right Solar Charge Controller/Regulator
A solar charge controller (frequently called a regulator) is similar to a regular battery charger, i.e. it regulates the current flowing from the solar panel into the battery bank to avoid overcharging the batteries. (If you don’t need to understand the why’s, scroll to the end for a simple flow chart). As with a regular quality battery charger, various battery types are accommodated, the absorption voltage, float voltage can be selectable, and sometimes the time periods and/or the tail current are also selectable. They are especially suited for lithium-iron-phosphate batteries as once fully charged the controller then stays at the set float or holding voltage of around 13.6V (3.4V per cell) for the remainder of the day.
The most common charge profile is the same basic sequence used on a quality mains charger, i.e. bulk mode absorption mode float mode. Entry into bulk charge mode occurs at:
- sunrise in the morning
- if the battery voltage drops below a defined voltage for more than a set time period, e.g. 5 seconds (re-entry)
This re-entry into bulk mode works well with lead-acid batteries as the voltage drop and droop is worse than it is for lithium-based batteries which maintain a higher more stable voltage throughout the majority of the discharge cycle.
Lithium batteries (LiFePO4) do not benefit from re-entry into a bulk mode during the day as the internal impedance of the lithium batteries increases at high (and low) states of charge as indicated by the orange vertical lines in the chart below and it is only necessary to occasionally balance the cells which can only be done around the absorption voltage. A related reason is to avoid the Rapid and large variation in voltage that will occur in these regions as large loads are switched on and off.
Lithium batteries do not have a defined “float voltage”, and therefore the “float voltage” of the controller should be set to be at or just below the “charge knee voltage” (as indicated in the chart below) of the LiFePO4 charge profile, i.e. 3.4V per cell or 13.6V for a 12V battery. The controller should hold this voltage for the remainder of the day after bulk charging the battery.
The Difference Between PWM and MPPT Solar Charge Controllers
The crux of the difference is:
- With a PWM controller, the current is drawn out of the panel at just above the battery voltage, whereas
- With an MPPT solar charge controller the current is drawn out of the panel at the panel “maximum power voltage” (think of an MPPT controller as being a “Smart DC-DC converter”)
You often see slogans such as “you will get 20% or more energy harvesting from an MPPT controller”. This extra actually varies significantly and the following is a comparison assuming the panel is in full sun and the controller is in bulk charge mode. Ignoring voltage drops and using a simple panel and simple math as an example:
Battery voltage = 13V (battery voltage can vary between say 10.8V fully discharged and 14.4V during absorption charge mode). At 13V the panel amps will be slightly higher than the maximum power amps, say 5.2A
With a PWM controller, the power drawn from the panel is 5.2A 13V = 67.6 watts. This amount of power will be drawn regardless of the temperature of the panel, provided that the panel voltage remains above the battery voltage.
With an MPPT controller the power from the panel is 5.0A 18V = 90 watts, i.e. 25% higher. However this is overly optimistic as the voltage drops as temperature increases; so assuming the panel temperature rises to say 30°C above the standard test conditions (STC) temperature of 25°C and the voltage drops by 4% for every 10°C, i.e. total of 12% then the power drawn by the MPPT will be 5A 15.84V = 79.2W i.e. 17.2% more power than the PWM controller.
In summary, there is an increase in energy harvesting with the MPPT controllers, but the percentage increase in harvesting varies significantly over the course of a day.
A PWM (pulse width modulation) controller can be thought of as an (electronic) switch between the solar panels and the battery:
- The switch is ON when the charger mode is in bulk charge mode
- The switch is “flicked” ON and OFF as needed (pulse width modulated) to hold the battery voltage at the absorption voltage
- The switch is OFF at the end of absorption while the battery voltage drops to the float voltage
- The switch is once again “flicked” ON and OFF as needed (pulse width modulated) to hold the battery voltage at the float voltage
Note that when the switch is OFF the panel voltage will be at the open-circuit voltage (Voc) and when the switch is ON the panel voltage will be at the battery voltage voltage drops between the panel and the controller.
The best panel match for a PWM controller:
The best panel match for a PWM controller is a panel with a voltage that is just sufficiently above that required for charging the battery and taking temperature into account, typically, a panel with a Vmp (maximum power voltage) of around 18V to charge a 12V battery. These are frequently referred to as a 12V panel even though they have a Vmp of around 18V.
The MPPT controller could be considered to be a “Smart DC-DC converter”, i.e. it drops the panel voltage (hence “house panels” could be used) down to the voltage required to charge the battery. The current is increased in the same ratio as the voltage is dropped (ignoring heating losses in the electronics), just like a conventional step-down DC-DC converter.
The “Smart” element in the DC-DC converter is the monitoring of the maximum power point of the panel which will vary during the day with the sun strength and angle, panel temperature, shading, and panel(s) health. The “smarts” then adjust the input voltage of the DC-DC converter – in “engineering speak” it provides a matched load to the panel.
The best panel match for an MPPT controller:
- The panel open circuit voltage (Voc) must be under the permitted voltage.
- The VOC must be above the “start voltage” for the controller to “kick in”
- The maximum panel short circuit current (Isc) must be within the range specified
- The maximum array wattage. some controllers allow this to be “over-sized”, e.g the Redarc Manager 30 is permitted to have up to 520W attached
Choosing the Right Solar Controller/Regulator
The PWM is a Good Low-Cost Option:
f or solar panels with a maximum power voltage (Vmp) of up to 18V for charging a 12V battery (36V for 24V battery, etc).
When the solar array voltage is substantially higher than the battery voltage e.g. using house panels, for charging 12V batteries
An MPPT controller will yield higher returns compared with a PWM controller as the panel voltage increases. I.e. a 160W panel using 36 conventional monocrystalline cells with a maximum power amp of 8.4A will provide around 8.6A at 12V; while the 180W panel having 4 more cells will provide the same amperage but 4 additional cells increases the panel voltage by 2V. A PWM controller will not harvest any additional energy, but an MPPT controller will harvest an additional 11.1% (4 / 36) from the 180W panel.
For the same principle, all panels using SunPower cells with more than 32 cells require an MPPT charge controller otherwise a PWM controller will harvest the same energy from 36, 40, 44 cell panels as it does from a 32 cell panel.
Solar Charge Controller Features and Options
Boost MPPT Controllers
“Boost” MPPT charge controllers allow batteries to be charged that has a higher voltage than the panel.
Combined MPPT and DC-DC Chargers
The MPPT function is a natural adjunct to the DC-DC charger function and there are several quality brands that provide this with more under development. A single unit can be used by itself, as it automatically switches between alternator charging and solar charging. For larger systems, our favoured arrangement is to use a separate MPPT controller for the fixed roof-mounted panels and use the combined MPPT/DC-DC with portable panels. In this case, an Anderson connector is placed on the exterior of an RV which is then wired to the solar input of the MPPT/DC-DC unit.
Note that the battery capacity must be sufficient so that the combined charging current from simultaneous charging from the alternator and the roof solar panels does not exceed the manufacturer’s recommended maximum charging current.
Cheap controllers may be marked as an MPPT but testing has shown that some are in fact PWM controllers. Cheap controllers may not have the over-voltage battery protection which could result in the battery being overcharged with potential damage to the battery; caution is recommended. Normally, due to the increased circuitry, MPPT solar charge controllers will be physically larger than PWM solar charge controllers.
Multiple Solar Chargers
Properly wired, it is possible to add multiple solar chargers (any combination of type and rating) to charge a battery. Proper wiring means that each solar charger is wired separately and directly to the battery terminals. This ideal case means that each controller will “see” the battery voltage and is unaffected by the current flow coming from other charge controllers. This situation is no different from charging a battery from the grid/generator at the same time as charging from solar. With modern controllers, the current will not flow backwards from the battery to the controller (excepting a very small quiescent current).
Costa Water Resistant Solar Charger
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Costa Water Resistant Solar Charger is designed for charging iPhones, iPads, Androids, GPS devices and cameras.
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Right-to-Charge Laws Bring EV Promise to Apartments, Condos and Rentals
than 3.6 million electric cars are driving around the U.S., but if you live in an apartment, finding an available charger isn’t always easy. Grocery stores and shopping centers might have a few, but charging takes time and the spaces may be taken or inconvenient.
Several states and cities, aiming to expand EV use, are now trying to lift that barrier to ownership with “right to charge” laws.
Illinois’ governor signed the latest right-to-charge law in June 2023, requiring that all parking spots at new homes and multiunit dwellings be wired so they’re ready for EV chargers to be installed. Colorado, Florida, New York and other states have passed similar laws in recent years. [The Florida law, HB 841, enacted in 2018, applies to condominium associations, cooperatives, and homeowner’s associations. See details here.]
But having wiring in place for charging is only the first step to expanding EV use. Apartment building managers, condo associations and residents are now trying to figure out how to make charging efficient, affordable and available to everyone who needs it when they need it.
Electric cars can benefit urban dwellers
As a civil engineer who focuses on transportation, I study ways to make the shift to electric vehicles equitable, and I believe that planning for multiunit dwelling charging and accessibility is Smart policy for cities.
Transitioning away from fossil-fueled vehicles to electric vehicles has benefits for the environment and the health of urban residents. It reduces tailpipe emissions, which can cause respiratory problems and warm the climate; it mitigates noise; and it improves urban air quality and quality of life.
Surveys show most EV drivers charge at home, where electricity rates are lower than at public chargers and there is less competition for charging spots. In California, the leading state for EVs, 88% of early adopters of battery electric cars said they were able to charge at home, and workplace and public charging represented just 24% and 17% of their charging sessions, respectively. Nationwide, about 50% to 80% of all battery electric car charging sessions take place at home.
Yet almost a quarter of all U.S. housing structures have more than one dwelling unit, according to the 2019 American Housing Survey. In California, 32.5% of urban dwellings have multiple units, and only a third of those units include access to a personal garage where a charger could be installed.
Even if installing a personal charger is an option, it can be expensive in a multiunit dwelling if wiring isn’t already in place. And it often comes with other obstacles, including the potential need for electrical upgrades or challenges from homeowner association rules and restrictions. Installing chargers can involve numerous stakeholders who can impede the process – lot owners, tenants, homeowners associations, property managers, electric utilities and local governments.
However, if a 240-volt outlet is already available, basic charger installation drops to a few hundred dollars.
Right-to-charge laws aims for ubiquitous home charging
Right-to-charge laws aim to streamline home charging access as new buildings go up.
Illinois’ new Electric Vehicle Charging Act requires that 100% of parking spaces at new homes and multiunit dwellings be ready for electric car charging, with a conduit and reserved capacity to easily install charging infrastructure. The new law also gives renters and condominium owners in new buildings a right to install chargers without unreasonable restriction from landlords and homeowner associations.
California, Colorado, Florida, Hawaii, Maryland, New Jersey, New York, Oregon and Virginia also have right-to-charge laws designed to make residential community charging deployment easier, as do several U.S. cities including Seattle and Washington, D.C. Most apply only to owner-occupied buildings, but a few, including California’s and Colorado’s, also apply to rental buildings.
Chicago officials have considered an ordinance that would include existing buildings, too.
Sharing chargers can reduce the cost
There are several steps communities can take to increase access to chargers and reduce the cost to residents.
In a new study, colleagues and I looked at how to design shared charging for an apartment building with scheduling that works for everyone. By sharing chargers, residential communities can reduce the costs associated with charger installation and use.
The biggest challenge to shared charging is often scheduling. We found that a centralized charging management system that suggests charging times for each electric car owner that aligns with the owner’s travel schedule and the amount of charge needed can work – with enough chargers.
In a typical multiunit dwelling in Chicago – with an average of 14 cars in the parking lot – a small community charging hub with two level 2 chargers, the type common in homes and office buildings, can cover daily residential recharging demand at a cost of about 15 cents per kilowatt-hour. But having only two chargers means residents are waiting on average 2.2 hours to charge.
A larger charging hub with eight level 2 chargers in the same city avoids the delay but increases the cost of charging to 21 cents per kWh because of upfront cost of purchasing and installing the chargers. To put that into context, the average electricity cost for Chicago residents is 16 cents per kWh.
The future of charging management at multiunit dwellings will be automated for efficiency, with a computer or artificial intelligence determining the most efficient schedule for charging. Optimized scheduling can be responsive to the times renewable electricity generation sources are producing the most power – midday for solar energy, for example – and to dynamic electricity pricing. Automation can also eliminate delays for drivers while saving money and reducing the burden on the electric grid.
The current limited access to home charging in many cities constrains electric vehicle adoption, slows down the decarbonization of U.S. transportation and exacerbates inequities in electric vehicle ownership. I believe efforts to expand charging in multidwelling buildings can help lift some of the biggest barriers and help reduce noise and pollution in urban cores at the same time.
Eleftheria Kontou is Assistant Professor of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign. Kontou receives funding from the Department of Energy Vehicle Technologies Office, the National Science Foundation, the Illinois-Indiana Sea Grant, and the Office of Naval Research.