There are always 3 Limits:
- Power in Watt
- Voltage in Volts
- Current in Ampere
Normally we are talking from overpaneling if the maximum peak power of moduls is greater than the MPPT datasheet. As the sun is rarely that bright to reach max datasheet peak power, overpaneling is a common method to improve performance of the installation. Space restrictions on sail boats may set diffrent rules than overpaneling.
If the produced Watt exceeds the capacity of the MPPT, the only thing what happens is that excess power will be lost. As the sun is rarely that bright, …
If the produced Volt exceeds the capacity of the MPPT, the semiconductors and / or capacitors inside the MPPT will be destroyed. You might exceed the voltage without overpaneling using installations of modul Watt power much smaller than allowed MPPT power. Its simply a question of series or parallel wiring.
If the Ampere exceeds the capacity of MPPT datasheet, the things become more complicated to understand. Mainly currents are also a question of series or parallel wiring of the modules and it is also possible to destroy MPPT by current without any overpaneling.
3a)
One of the MPPT features is a reverse polarity protection. In the RS450 datasheet it is mentioned, that reverse polarity protection will be destroyed if maximum short circuit current is exceeded. With other words: Nothing will be damaged as long as you do not accidently connect your modules in reverse polarity. You might do overpaneling the usable Watts for your installation with the RS450 MPPTs.
3b)
The main task of the MPPT is to limit charge current from modules to feed the battery. This is normally done by using a inductor. Currents through inductors are late against the voltage. Therefore a Mosfet will simply switch on and off the module in the time what fits to the required current to charge battery. As high currents require big inductors and big inductors cause high design costs, all design engineers try to keep the inductor as small as possible to have a competitive MPPT to sell.
To reach this goals, the MPPTs are based on 2 diffrent tricks. Trick no. 1 is to limit the MPPT to the connection of PV modules as power source. PV modules have a high intrinsic resistance. The size depends from modul installation size and sun brightness. This intrinsic resistance can be combined with the MPPT inductor to limit the current to the battery. According the law of Ohm, a higher modul intrinsic resistance will result in a lower short circuit current. This is the reason, why todays MPPT normally have a maximum short circuit current in their data sheet.
With trick number 2, electronic designers have the possibility to decrease their inductor size by faster switching. As todays Mosfets and other electronic components might switch pretty fast, it could be possible the MPPT might limit a current what is greater than its datasheet very succesfull. 20 years ago, SMA had some galvanic isolated inverters what where explicitely spezified for use with low impedance sources. I assume, Victrons RS450 MPPT can do this too, although they are not specifed in the datasheet for that use. Be carefull with the older Victron PWM charge controllers. The function completely relies on intrinsic modul resistance and they will be distroyed by overpaneling.
As a rule of thumb, overpaneling the current in moderate size will be ok. Most datasheets show current and voltage what is over the maximum processed power. Good MPPT designs with fast enough circuits will be also possible to limit almost infinite currents from low impedance sources like batteries although their are not specified for doing this.
If you dont understand this all, simply trust the Victron design calculator without asking how the calculation works.
