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LiFePO4 vs Lead Acid a cost analysis for energy storage.

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  • Originally posted by Naptown View Post
    It proves that you don't really understand the workings or advantages of MPPT.
    Try reading the stickies here about them. In order to get the greatest harvest from them the Vmp of the panels has to be about 50% higher than the battery voltage.
    And your cost per watt is way off for the PV.
    Here is an example
    100 Watt 36 cell panel VMP of 17.
    Imp of 5.9

    Charging a 12V battery you are only getting 70 watts into the battery. A loss of 30%
    Go to Mppt and you get upwards of 95 watts into the battery. A loss of 5%
    OH and you can't series the panels to reduce wire size or compensate for long runs from panels to controller with PWM or the losses get worse.
    On a 100 watt system it probably doesn't make much difference but get over 200 watts and it makes a big difference.
    I will try to answerer all your questions if I can.

    I promise I understand how an MPPT works in the most intimate detail. I used to design DC-DC converter. And I also understand the algorithm part of an MPPT.
    Small offgrid panels use 36 cells for 12V and 72cells for 24V battery that is to be able to compensate for high temperatures in hot climates.
    Mppt will be most effective in cold and sunny days and not as effective in hot summer days. With a 36 cell panel and 12V Lead Acid battery the average gain over a year compared with an PWM controller is not higher than 15% will wary a bit with location (climate).
    Those 15% can be also obtain using a simpler and less expensive PWM controller and a solar PV panel that is 15% larger.
    Notice I use % so the size of the system is irrelevant. A larger MPPT controller will also be more expensive.
    How is 1$/Watt way off for solar PV panels ? Maybe very small solar panels cost a bit more but I think 1$/Watt is a good average price for PV panels.

    As for your example 100W panel example

    A 12V battery dose not usually have 12V during charging it will be quite a bit more (13 to 14.4V) depending on the charge rate and SOC of the battery.
    Also that 17V at max power point is calculated for cells with 25C temperature it will be quite a bit lower in the summer.

    For small offgrid systems you usually have the panels quite close to the controller and batteries (usually mounted on the roof or somewhere quite close) so there are almost no savings on copper. Plus at high voltage difference between PV and battery the MPPT (DC-DC converter) will get quite inefficient dissipating even more heat and reducing the life of the MPPT controller even more (mostly the electrolysis will fail).
    An PWM dose not get that hot and it dose not need electrolytic so it can last as much as the solar panels over 25 years where MPPT will be changed quite a few times adding to the cost of operation.

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    • Originally posted by Sunking View Post
      I am done with you. Not worth my time. You are selling a product, I am not.

      I do not sell a product there is no profit from my Open Source solar BMS. But I'm also busy with the Kickstarter campaign so I will be glad to take a break from this discussion.

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      • Originally posted by electrodacus View Post
        I do not sell a product there is no profit from my Open Source solar BMS. But I'm also busy with the Kickstarter campaign so I will be glad to take a break from this discussion.
        Good -
        [SIGPIC][/SIGPIC]

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