Battery management with MPPT and PWM solar charge controllers

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MPPT charge controllers are usually more expensive to purchase than PWM charge controllers, but under certain conditions they operate with almost no energy loss. Find out more about when it is worth buying and what you need to consider with regard to the location of the solar system here!

PWM vs. MPPT Charge Controller: Sensible Battery Management for Off-Grid Systems

As a link between the solar system and the electricity storage system, a solar charge controller ensures that the battery is not damaged due to voltage fluctuations or overcharging. PWM and MPPT charge controllers are often used. An overview of the advantages and disadvantages.

When are PWM or MPPT charge controllers used in battery management?

Battery management is used to protect accumulators (rechargeable batteries), i.e. battery storage systems of solar systems, for example, from deep discharge or overcharging. A charge controller is the simplest form of battery management. In photovoltaic systems, such as those typically used in residential buildings, such a charge controller is already integrated into the system's inverter and does not have to be purchased separately.

In the case of off-grid systems, such as those used for motorhomes, it makes sense to insert an additional charge controller between the modules and the battery. The inexpensive PWM solar charge controllers are often used here, but under certain circumstances, the MPPT charge controllers, which are usually more expensive, can be advantageous.

What is the difference between PWM and MPPT charge controllers?

A solar charge controller fulfills two important tasks: It protects the battery of a solar system from damage by cutting off the power supply when the battery is at maximum charge. At the same time, it exploits the potential of the solar system in the best possible way by optimising the charging capacity. Thus, charge controllers are an indispensable part of the battery management system and an important safety device of the PV system.

The input voltage of the solar system almost always differs from the permissible charging voltage of the battery. In motorhomes, the modules deliver around one hundred volts, while the battery has a charging voltage of 12 or 24 V. The charge controller has to compensate for this difference.

The so-called pulse-width-modulation position controller (PWM charge controller) is a simple form of power electronic control in which only the charging current is regulated. When the battery is charged, the charge controller interrupts the connection to the modules.

The MPPT charge controller (short for Maximum Power Point Tracking, german is characterized by the fact that it measures the power curve of the module and can adjust the voltage so that the maximum charging power is reached. If the voltage of the battery changes during the charging process, the MPPT charge controller adjusts the current and voltage values so that the optimum charging performance is achieved again. To do this, the regulator gradually increases the voltage to the maximum power. This process is repeated periodically, so that even with fluctuations due to changing irradiation conditions, almost the maximum output is always achieved.

How the MPPT charge controller works

While the PWM charge controller reduces the input voltage of the solar system overall, the MPPT charge controller converts the excess part of this voltage into charging current. This means that the possible output of the solar system is not throttled, but the energy generated is retained with almost no loss. The efficiency is around 98%.

This difference in the functioning of the charge controllers is particularly noticeable when the nominal voltage of the solar modules is high: the higher the nominal voltage, the greater the energy loss when it is throttled to the voltage level of the battery. The MPPT charge controller keeps losses low. Even though it is usually more expensive to buy, it pays for itself over time. Such a solar controller can be particularly useful for motorhomes due to the changing locations. However, there are some special features to consider.

Pay attention to the nominal voltage when buying a solar charge controller

The nominal voltage of a solar system is determined by the selection of modules. The more modules, the higher the nominal voltage and the more profitable it is to use an MPPT charge controller. In addition, various (environmental) influences have an impact on the voltage of the system. Since the use of an MPPT solar charge controller makes more sense the more the system voltage and battery voltage differ from each other, these effects should be taken into account when buying. In particular, the following must be taken into account:

Shading: Shadows and other poor lighting conditions reduce the voltage in the solar panel.
Temperature: Very high temperatures of the solar cells reduce the voltage of the solar system.
Wind and insulation: If the air cannot circulate freely around the installed solar modules, the temperature increases and thus the voltage drops.

It should also be borne in mind that especially at very high temperatures, the voltage of a PV system can fall below the battery voltage, which prevents the battery from charging. Although the average temperature in Germany is only between about 9 and 10 degrees, with average values in 2021 of 19 degrees in June and 2.6 degrees in December. However, the solar cells themselves can become significantly warmer: 50, 60 or even 70 degrees are not uncommon in direct sunlight. The nominal power is measured at an ambient temperature of 25 degrees, deviations from this are accompanied by a (slightly) reduced power.

When is it worth buying an MPPT charge controller?

The higher purchase price of an MPPT charge controller compared to the PWM charge controller is justified if the additional solar power generated generates more income (or saves costs) over time than money was invested in the controller. Tests of MPPT charge controllers have shown that this is especially the case when

the temperature of the solar cells is often below 45 °C or above 75 °C,
if the place of use is in Central Europe (with comparatively cool temperatures and low solar radiation) (also relevant for solar charge controllers in motorhomes),
if the solar system operates at a nominal voltage that is far above the battery voltage, and
if the system generates a relatively large amount of electricity, as even a slightly higher efficiency thanks to an MPPT charge controller has an even stronger effect.

In tropical or subtropical climates, however, the use of an MPPT charge controller seems to be less worthwhile.

To ensure that all factors can really be taken into account, it is advisable to seek detailed advice before buying a charge controller.