MPPT Solar Battery Charger

[imranahmed]

Please let me know about MPPT Solar Charger working principle and how does it works?

 

[BradtheRad]

In bright sun a 12V PV panel puts out voltage greater than battery voltage. So it doesn't absolutely need MPPT. Charging current is ample even as battery voltage rises above 14 or 15V.

In dim sun the panel may not produce greater than battery voltage. MPPT is a strategy which provides voltage boost while at a cost of reduced charge current. The aim is to find the right combination which pushes maximum power into the battery. It should do so at all stages of the charging cycle, whether battery voltage is 10.5V or 14.4V.

 

[imranahmed]

In bright sun a 12V PV panel puts out voltage greater than battery voltage. So it doesn't absolutely need MPPT. Charging current is ample even as battery voltage rises above 14 or 15V.

In dim sun the panel may not produce greater than battery voltage. MPPT is a strategy which provides voltage boost while at a cost of reduced charge current. The aim is to find the right combination which pushes maximum power into the battery. It should do so at all stages of the charging cycle, whether battery voltage is 10.5V or 14.4V.

I read many articles about MPPT, they says that we read V and I from PV panel and calculates power from P(nn)=V(nn)I(nn) and then calculate P(nn-1) and V(nn-1).
I am slightly confused about P(nn)-P(nn-1) formula.

 

[KlausST]

Hi

It just means. Measure current and voltage then calculate power.

Then modify the current in one direction.
Read the voltage and calculate the power.

Usually the power has changed. If it increased you should do the next step in the same direction.
If the power decreased, then do the next step into the opposite direction.

So you continuously look for the best power output.

Besides this there are other approaches.

Klaus

 

[BradtheRad]

It's easier simply to measure current into the battery. Battery voltage doesn't change much in the short term. Thus when you maximize charging current you have optimum utilization of PV power.

Electronically a common strategy of MPPT is referred to as 'perturb and observe'. Change switching frequency or duty cycle and watch to see if current increases. If not then see if it helps when you change things in the opposite direction. Etc.

 

[tonigau]

Please let me know about MPPT Solar Charger working principle and how does it works

Solar PV panel info: (This will help understand the How_It_Works part)
If you read the rear label of a solar PV panel it will show some values like Vmpp = 30.1V, Impp = 8.3A (this is a 250W panel)
Multiply these & its the panel rated Watts. This is the Maximum Power Point (MPP) of this panel at sun = 1kWm3 & 25deg.C. (STC)
Solar PV Panels are a Constant Current source, this means the volts can be dragged down by the load (Battery or heater elements etc.) but the current stays fairly constant. (relative to sun intensity).
._ Example... Say we have a 250W PV panel connected to a 2ohm resistor (heater element), at full sun the panel will output its 8.3A, [ 8.3A*2ohm = 16.6V] result 8.3A*16.6V = 137.78W, Way below the MPP of the panel. With a 3.5ohm resistor we get PV=29.05V, much closer to the MPP of the panel.
So by varying the load resistance on the panel we can vary the panel voltage. Power = Volts x Amps so it makes sense to maximize the Volt, but only up to the Panels Vmpp, above this the internal silicon starts to conduct. (a bit like a big zener diode).
When sun intensity is less the panel current is reduced, if the load resistance (or effective resistance) stays the same the panel volt will drop, so we need to increase the load resistance to bring the panel volt back up. Its all about Watts= Volt x Amp at the panel output for max power output.
In the case of a battery if we connect the 30.1Vmpp PV panel direct (or "PWM" type charger) to a 12V battery the Panel Volt will equal Battery Volt (13.6V*8.3=112.88W, this is fine if you have just a few LED lights.
For a 24V battery (26~ 28V) charging direct from 60 cell panel is quite acceptable, even with reduced sunlight as the battery Volt is still fairly close to MPP.

Working principal & How it works: MPPT Charger
An MPPT charger creates a volt (& current) difference between the PV Panel & the Load (battery). Decreasing battery charge current will result in an increase of PV panel Volts & vice-versa. (just like the varying load resistance)
This is typically achieved with a DC-DC buck converter running in mainly open loop. A micro-controller will "look" for the Maximum Power Point & adjust DC-DC Converter output current either up or down depending on the input & load conditions. A Common method is by calculating Watts from PV panel Volt x Current, make adjustment & observe if W is better or worse. When the Max Power Point is found it will hover around this until load or PV current changes then it will hunt for the new MPP.
So from the solar panels perspective the MPPT charger is adjusting the 'effective resistance' seen by the panel to enable its volts to increase while its current is constant (relative to solar intensity).

Another reason for choosing an MPPT charger is to enable higher Volt array 60V ~ 120V etc... to reduce input current in larger systems..
Something to consider is MPPT chargers no longer track the power point once battery charging reaches the CV mode, for a lead type battery this can be a long time at reducing charge current.
I just charge my 8cell LiFePo4 at full current to 3.55V per cell & stop, no HF switching, tiny 100A charger.

There is so much info around on Solar MPP Tracking chargers, just keep searching & re-read those articles ...

----------------------------------------------
STC= Standard Test Conditions, MPPT = Maximum Power Point Tracking, CV= Constant Voltage

 

[cupoftea]

A rough way to do MPPT is to note the open circuit voltage of the panel when under sunlight.
Then you make your charger regulate the current that it pulls from the panels, such that the panel voltage is 90% of its open circuit voltage.....if you do this, then you will tend to be getting the maximum power out of the panel.

So basically, close the charger feedback loop on its input voltage....and regulate it so that input voltage is regulated to 90% of panel open circuit voltage. I think the LT8705 can actually do this for you...so too can many other MPPT chips.