How battery charger with MPPT work?

Thread Starter

tornedoar

Joined Oct 31, 2019
4
hallo, I have a question that I look up on the internet and nothing gives me direct answer.
anyway, when we have a battery charger from solar panels and work on MPPT, how does it exactly work? I saw datasheet for some of 12V system solar panels and it says most of the time Vmp is about 18-17 volt but to charge batteries I need 13-14 volt only. so how I can charge batteries and still do MPPT? I need two DC-DC converters? like one for MPPT and another one for charging a battery and include BMS "battery management system"? but honestly it does not make sense to me to have two DC-DC converters, so if anyone gives me to answer or give me any pdf or Matlab file to see how it could work?
 

crutschow

Joined Mar 14, 2008
23,502
Basically MPPT monitors the power from the solar panels and adjusts the battery charger voltage/current for optimum power output from the panels.
It does this by periodically increasing or decreasing the charging current in small increments so see where the maximum power point is as determined by the voltage times the current from the panel.
In effect it continually dithers around the maximum power point to keep the panels at that point as the solar power changes due to clouds or change in sun position during the day.

So you don't need two DC-DC converters, just one whose output charge current to the battery can be controlled by a command from the MPPT processor.
 

TeeKay6

Joined Apr 20, 2019
448
hallo, I have a question that I look up on the internet and nothing gives me direct answer.
anyway, when we have a battery charger from solar panels and work on MPPT, how does it exactly work? I saw datasheet for some of 12V system solar panels and it says most of the time Vmp is about 18-17 volt but to charge batteries I need 13-14 volt only. so how I can charge batteries and still do MPPT? I need two DC-DC converters? like one for MPPT and another one for charging a battery and include BMS "battery management system"? but honestly it does not make sense to me to have two DC-DC converters, so if anyone gives me to answer or give me any pdf or Matlab file to see how it could work?
@tornedoar
An MPPT charger will attempt to load the solar panel such that its output is at the MPPT voltage of that panel at its current insolation level, temperature, etc. An ideal charger will convert the voltage and current from the solar panel to a voltage and current suitable for whatever load is connected to the output of the charger. If your panel produces 100W at 18V when at MPPT, then 100W will be available at the charger output. An ideal charger would be able to provide that 100W as 100V@1A or 1V@100A or any intermediate combination. However, the charger cannot force a battery load to accept power beyond the battery's specification. If your battery can be charged at no more than 15V@3A (=45watts), then the charger essentially has two choices: 1. Operate at a non-MPPT output voltage from the panel or 2. Dump the excess energy into some dummy load. Since there is no advantage to burning unneeded power, the reasonable choice is to change the operating point of the charger such that it outputs the desired 15V@3A even if that means the solar panel will not then be operating at its MPPT. As another example, consider a battery can accept 15V@15A (=225watts) during charging from the same solar panel; then the charger will provide an output of whatever voltage is appropriate to force the battery to accept current at a level such that the power into the battery is 100W; that will mean the voltage will be somewhat less than 15V so that current into the battery is limited to the appropriate level to yield V*I=100W. What this means is that you cannot get optimum performance from the output of some random panel charging some battery of random V*I need. An efficient system will match the panel output to the battery's need. This is of course considerably complicated by the fact that the output of a panel depends on both its loading and its insolation (as well as temperature, age, etc) and the need of the battery depends on its depth of discharge, its charging characteristics, its temperature, its age, its type (lead acid, lithium, etc), etc.
 
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