Help me to understand the usefulness of a MPPT inverter on a yacht

Discussion in 'OnBoard Electronics & Controls' started by BertKu, Apr 5, 2015.

  1. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    That circuit works on that principle. It just doesn't track the maximum power point. It must be set by choosing the resistors. MPP doesn't change that much for a given solar panel, thus it works OK for that. It can't handle 15 A, but you can get say 5 A at 17 V transformed to 6.4 A at 13 V.

    When you are charging with a solar panel, you rarely get 14.4 V at the battery, because the charging current is low compared to battery size. That would happen only when the battery is almost full and there is no consumption.

    You should be able to order LT3763 directly from Linear or perhaps from Digikey.

    There is no midnight sun in southern Finland, but the night is very short and quite bright in June and July. You won't get any solar power during night.
     
  2. BertKu
    Joined: May 2009
    Posts: 2,498
    Likes: 43, Points: 58, Legacy Rep: 223
    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi Joakim, I have my panels in serial and thus I work with 28.8 Volt. My panels provide me with 11.8 Ampere, short circuit current 12.03 Ampere. They are 235 watt panels 1 meter 95 by 95 cm and when folded open 1.95 meter by 1.90 meter. Thus you feel, I am wasting my time by making a doubler should the voltage fall below 30 Volt. My thought was to double the voltage in the second large capacitor and then discharge, when a certain voltage is reached. Either by a diac and a thyrister or via an A/D input of a PIC18F2431. I should get some extra current in when the elco is discharged. I haven't calculated what it would be over a period of 1 hour. I symphatise with you when you are working with SMD components and struggle to solder them. What is your thought, am I wasting my time or I could get 5% or more extra energy in the batteries. My current use on my motor is from 30 Ampere to 50 Ampere.
    Bert
     
  3. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    You can check what kind of power you should expect at your location from e.g. http://pvwatts.nrel.gov/pvwatts.php
    Then check your solar panel datasheet or measure your self the MPP at different powers. If you are using standard 12 V panels to charge 24 V lead acid batteries, I wouldn't care about the moments the panel voltage is lower than charging voltage. From those you will likely get less than 1% more in one day. Using MPPT will give you 10-40% more depending on the voltage at the batteries.

    If you would like to use my circuit, you need to change quite a few components, but using LT3763 is still OK. You need to change the R5/R6 and R8/R9 to cope with higher voltage. Also Q1 and Q2 need to be changed, since the ones named in the text can't tolarate more than 30 V, which is easily exceeded with 24 solar panels. You also need to change the inductor for bigger current and resistors R2 and R7 to lower value (e.g. 2.5 mohm for 20 A maximum current).
     
  4. BertKu
    Joined: May 2009
    Posts: 2,498
    Likes: 43, Points: 58, Legacy Rep: 223
    Location: South Africa Little Brak River

    BertKu Senior Member

    Thank you Joakim, I may consider to use your circuit and make a unit. At least, you are able to answer my questions I have for myself, thank you for that. At present I have a constant voltage circuit made, which kicks in at 27.2 Volt. This after a relay contact opens when reaching 27.2 Volt and the solar panels gets disconnected between batteries and Solar panel and is via the Constant Voltage circuit connected to the batteries. As soon the Voltage drops to 25.6 Volt, the Relay contact short circuit the constant voltage circuit and charges the batteries directly from solar panel. Thus a window of 1.6 Volt, 0.8 Volt per solar panel. (Which I can vary via the microprocessors program.)

    I understood, rightly or wrongly, if the 2 solar panels charges the batteries directly, i.e. 12 Ampere charging, because I don't have a MPPT circuit build in, that is all what I am getting.

    While I could have pumped 15 Ampere or even higher charging currents into the batteries, when using a quality MPPT charger or SEPIC circuit.

    Thus, my conclusion is that it is worthwhile for a yacht to use the best available MPPT charger, or to make a circuit like yours.

    Look in my case, if I run low on energy, I just wait a few more minutes before sailing back and get some more current into the battery bank, or I use my reserve battery-bank energy. At my age, speed or time is not an issue.
    Bert
     

  5. BertKu
    Joined: May 2009
    Posts: 2,498
    Likes: 43, Points: 58, Legacy Rep: 223
    Location: South Africa Little Brak River

    BertKu Senior Member

    You know Joakim, I am in a two mind. I have the gut feeling, that if I run my brushless motor directly on the solar panels, I will have automatically have Maximum power point. The reason why I say this. I am controlling the speed with a PWM circuit (Pulse With Modulation) and the solar panel will only give me 12 Ampere Maximum at 42.2 Volt, but because the solar panel will charge the 68.000uF /100 Volt Capacitor to the maximum Voltage, as soon I start revving up the motor, automatically I utilize the full power coming from the solar panels. This in contrast to charging my battery which drops to 11.5 Volt when being charged by the solar panels direct. In view that I have time to charge my batteries over a period of time, I don't think it is worth for me to go to the trouble and expense to make a MPPT circuit. However I have now full and better understanding of the working of a MPPT charger at each load and it is thus easier to make a decision. I will see whether I can measure the voltage at the motor side, this will not be easy. This differ for yachties who have just a battery to charge. Thanks for your inputs.
    Truly appreciated.
    Bert
     
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