Some questions about MPPT and Batteries charger

Discussion in 'General Electronics Chat' started by Hakimos, Feb 17, 2015.

  1. Hakimos

    Thread Starter New Member

    Oct 14, 2014
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    Hello,

    I have a small project wich consist of creating a solar MPPT charger and I have some questions.

    1 - What does the MPPT do ? I know it's an algorithm implemented in microcontroler in order to control a DC-DC (Buck or Boost) converter but I don't really now how it works, does it change output voltage (battery) or input voltage (Solar panel) in this case ?

    2 - Is the output voltage of the DC-DC converted imposed by the battery or by the MPPT (With the variation of duty cycle) ?

    3 - Is C/10 the maximum current to charge a lead acid battery ? Can we charge with higher current ?

    4 - How to measure battery temperature ? Using LM35 sensor ?

    5 - What does kill the battery, the higher current or the higher temperature ?

    6 - I have a 10 Watts (MPP @ 17v 0.6A) solar panel and I choosed a 12V 7Ah lead acid battery, is it a good choice ?

    Sorry for my english and thank you for you help.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    1. Google is your friend to learn how MPPT works.
    2. The voltage is to support the battery you are charging.
    3. C/10 is recommended for many batteries. See the spec sheet of the specific battery you intend to use and charge at that rate.
    4. Any sensor you can place in contact with the battery will do.
    5. Both. Current makes the temperature rise (as does a hot day).
    6. As the rated panel max current is below C/10 of the battery it is a good match. However, MPPT on a 12V system will not bring any significant improvement over just connecting the panel thru a simple diode. There is some risk in overcharging with such a simple circuit, so a constant current limited with voltage monitoring would be better.
     
  3. Hakimos

    Thread Starter New Member

    Oct 14, 2014
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    1 - I've learned some articles, I just want to understand more, it's my first time. Just tell me if i'm right or wrong, the mppt change the output voltage (wich must be constant to charge battery) so the duty cycle will depend on input voltage ?
    3 - I'll see for the battery.
    4 - Thx, i'll get more information on google.
    5 - Ok, will see spec sheet of the battery
    6 - It's just for learning purpose.

    Thank you for your answers.
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,991
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    The MPPT circuit measures the panel output voltage and current to determine if it's operating at the maximum power point. In not, then it changes the output voltage (by adjusting the PWM duty-cycle) which changes the charging current (and thus also the input operating point) until the panel output is at the MPP.
     
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  5. Hakimos

    Thread Starter New Member

    Oct 14, 2014
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    Thank you for your answers guys, rly thank you.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What kills the lead-acid chemistry battery:

    1. Chronic under charge.
    2. Discharging too much.
    3. Letting the battery sit around uncharged for long periods.
    4. Overheating the battery by charging at too high a rate.
    5. Maintaining the battery (after recharging) at too high a float voltage.
    6. Not replacing charge lost to self-discharge during long-term storage.
    6. Stratification of the electrolyte in stationary flooded-cell batteries. Battery needs to have its electrolyte mixed by mechanical agitation or using a brief overcharge to cause bubbles to rise in the electrolyte (called "equalization").
     
  7. ak52

    Member

    Oct 15, 2014
    145
    4
    MPPT as the name suggests(Maximum power point tracking),is an algorithm used to track the point at which you get maximum power out of your solar panels.So in order to calculate power you obviously need to know the solar voltage form your PV panels,and the charging current.
    In the micro-controller , you constantly monitor/compare the power output.
    If the power of your current cycle is greater than the power calculated in the previous cycle,then you increase the duty-cycle,and vice-versa.
    Basically,
    if(current_power > old_power)
    {
    ++duty cycle;
    }
    else
    {
    --duty cycle;
    }
    The above algorithm is called P&O(perturb and observe),there are many other MPPT algorithms which can be used(eg : incremental conductance etc..),but the above is the most commonly used.
     
  8. tindel

    Active Member

    Sep 16, 2012
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    Solar arrays output power based on the i-v curve (current vs. voltage) of the array. You will find that on the iv curve there is a open-circuit voltage where power is zero and a short-circuit current where output power is zer0. The maximum power is somewhere between the short-circuit current and the open-circuit voltage. My blog post goes into more depth on this topic.

    http://forum.allaboutcircuits.com/xfa-blogs/tindel.178169/
     
  9. Hakimos

    Thread Starter New Member

    Oct 14, 2014
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    0
    Thank you guys for your answer;

    I know the P&O method. I read articles on P&O method in MPPT. Something other is confusing me. When using a battery as a load, do we have to care about the output voltage or the output voltage will be forced by the battery (Output of DC-DC converter) ? In case when we have to worry about the battery, the duty cycle will depend on the input power and output voltage.

    Thank you again and sorry for my english, I have some troubles to explain something..
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,233
    Your English is very good. Where are you?

    You need to worry about both voltage and current.
    If you have a powerful charger you can have to much current in the battery causing it to get to hot. So usually (If it is a lead acid battery) you limit the maximum current to C/10, with C the amp hour rating of the battery. So if you have a 100 amp hour battery no more than 10 amps.
    The voltage is also important when the battery gets fully charged. This number depends on the type of battery and if you are going to leave it charging or stop it when it is "full'.
    Here is a good site that explains it better than I can and with a lot less typing. :D

    http://www.powerstream.com/SLA.htm
     
  11. Hakimos

    Thread Starter New Member

    Oct 14, 2014
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    Thank you for your answer and for the link, thank you all.

    (I'm from Algeria)
     
  12. Hakimos

    Thread Starter New Member

    Oct 14, 2014
    16
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    Sorry, it's still confusing me.

    I know that if I want to save the battery life, I have to watch current (No greater than C/10), temperature and depth of discharge. I know that I have to use 3-Step or Stage charge, there is a lot of articles on the Internet.

    Just one point is confusing me a lot, see the attached (simple structure) file please and tell me : If our PV delivers 5 A @ 18 Volt, the battery voltage is 13.5 and the duty cycle is 0.8 @ 40-50 kHz, what will be the output of the buck converter (in this case), what will I get on the output ? ~ 6.25A @ 14.4 Volts or ~6.7A @ 13.5V (Battery voltage)

    Thank you for answer, and sorry I know that I'm a lot and stupid questions.

    Good night.
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    About 6.6 A. If battery V was 14.4 V then steps would have been taken to limit current to float charge, something in 50 mA range.
     
  14. Hakimos

    Thread Starter New Member

    Oct 14, 2014
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    Thank you Bernard.
     
  15. Hakimos

    Thread Starter New Member

    Oct 14, 2014
    16
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    Hello,

    Another question, when designing a DC-DC Converter (Buck in this case) for a solar panel that delivers for exemple 14-19 Volts, how should I calculate Vin ? I take the low voltage, the high voltage or the mean value between them ? And what about Vout, 12V ? 13.5 ? Thanks.
     
  16. Hakimos

    Thread Starter New Member

    Oct 14, 2014
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