Hi, I am designing an MPPT solar charge controller to get more efficiency out of my system. At the moment I am at the conceptual stage and I have some questions about MPPT battery charging in general.
First of all, the way I understand it, the implementation of MPPT works via varying the resistance of the circuit such that panel output voltage * current drawn are at the maximum point on the power curve.
Then the battery charger section takes whatever voltage is 'given' by the MPPT section and drops it to within the battery charging voltage band (e.g. 13.5-14.5V for lead acid), maybe using a buck converter. Further, the way I understand it, most MPPT battery chargers use the constant-current bulk-charge method followed by a constant voltage topping method (perhaps followed by a constant voltage float charge).
Now what mystifies me is how the charger is supposed to run a constant-current bulk-charge algorithm while using the power coming to it efficiently. Because, if the charger simply regulates the output voltage via a sense resistor such that the output current remains at some predetermined constant level, that voltage is dependent on the battery characteristics and charge state, so essentially the battery charging power curve is fixed for a fixed current. So what if the MPPT suddenly gives more power? Does it get wasted? What if the MPPT produces less power? Does the constant current become not so constant?
Lastly, if there is such a thing as a constant-voltage MPPT charger (which makes more sense to me for this situation) how would transition from bulk charge to topping charge be carried out during changing conditions (changing current). Plateau detection for instance would be impossible.
Thanks very much for your help.
Billy
First of all, the way I understand it, the implementation of MPPT works via varying the resistance of the circuit such that panel output voltage * current drawn are at the maximum point on the power curve.
Then the battery charger section takes whatever voltage is 'given' by the MPPT section and drops it to within the battery charging voltage band (e.g. 13.5-14.5V for lead acid), maybe using a buck converter. Further, the way I understand it, most MPPT battery chargers use the constant-current bulk-charge method followed by a constant voltage topping method (perhaps followed by a constant voltage float charge).
Now what mystifies me is how the charger is supposed to run a constant-current bulk-charge algorithm while using the power coming to it efficiently. Because, if the charger simply regulates the output voltage via a sense resistor such that the output current remains at some predetermined constant level, that voltage is dependent on the battery characteristics and charge state, so essentially the battery charging power curve is fixed for a fixed current. So what if the MPPT suddenly gives more power? Does it get wasted? What if the MPPT produces less power? Does the constant current become not so constant?
Lastly, if there is such a thing as a constant-voltage MPPT charger (which makes more sense to me for this situation) how would transition from bulk charge to topping charge be carried out during changing conditions (changing current). Plateau detection for instance would be impossible.
Thanks very much for your help.
Billy