Battery-Based Charge Controller Technology Explained

The charge controller in a battery-based PV system has but one main function: to keep the batteries from being over-charged. Most charge controllers can now be configured for battery voltage (12, 24, 48 V nominal), battery type (flooded-cell, AGM, gel), and they offer multi-stage charging algorithms (bulk, absorption, float, equalization), resulting in dependable performance and enhanced battery life.

Some controllers offer temperature compensation (increase target voltages when the batteries are cold and reduce voltages when they’re warm), can be fine-tuned to a particular application, offer voltage-controlled switches, and provide status indicators or displays. At least one controller can now talk to the operator!

PWM vs. MPPT Charge Controllers

One key remaining difference in battery-based charge controller technology and pricing is pulse width modulation (PWM) vs. maximum power point tracking (MPPT) architecture. PWM charge controllers are among the first “smart” controllers, and they may include many of the features mentioned above. These controllers use PWM technology to accurately maintain target voltages for the absorption, float, and equalization stages by varying and limiting charge current.

However, a characteristic of the PWM controller is that the PV module operating voltage is essentially set by the battery charging voltage when the controller is operating in its bulk stage. For example, although a module’s maximum power point voltage might be 17V,  it will operate at 13V if the battery’s instantaneous bulk charging voltage is 13V. The 4V loss means a 24% power reduction under these conditions, which translates into a longer bulk charge period.

An application complication when using a PWM charge controller is that the module- and array specs must be carefully matched to the operational environment and the particular battery target voltages. Module voltages that are too high will not deliver full power, resulting in less than expected system performance.

This can be especially disheartening in the winter when module voltages are typically higher than in summer. And, large low-voltage arrays require (expensive) large gauge wiring to minimize power losses in the wiring home run between the array and the controller.

Charge Controller Breakthrough Technology

MPPT charge controllers are a true technology breakthrough. They effectively operate as smart DC-DC converters and provide many benefits to PV system design and operation. At the most basic level, an MPPT controller can track a module’s optimal voltage and current points independently of the battery voltage to deliver maximum power (maximum voltage x maximum current = maximum power).

Borrowing from the example above, a module operating at 17 V and 4 A in bulk mode via an MPPT controller can deliver 13 V and almost 5A.

By converting “excess” module voltage into additional charge current, the bulk charging state is shortened compared to a PWM unit. This feature is especially valuable during short winter days when a cold module’s even greater “excess” voltage is converted into even more charge current.

High battery-current MPPT charge controllers can be even more sophisticated. For example, these can take arrays in the 18 V to 60 V (nominal) range and convert the “high” array voltage down to a lower voltage battery bank, all while proportionally increasing the available charge current.

This feature allows for much broader and more competitive selection of PV module specifications, all while providing for an optimized PV system which can more easily meet operation expectations. And, by using a “high” voltage array to charge a “low” voltage battery bank, you can reduce your home run cabling costs.

MPPT controllers invariably cost more than PWM models. However, everything else being equal, MPPT controller-based systems require smaller – as less costly – PV arrays. Including reduced balance of system costs, an MPPT controller-based PV system can be quite competitive from a financial perspective.

Give Inverter Service Center a call at 800-621-1271, and we’ll be happy to discuss your system configuration options.

About the Author – Jim Goodnight – also known as “crewzer” — is a retired solar industry application engineer, product manager, and forum moderator and has previously written for Home Power and Solar Professional magazines.

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