> Response... > If the solar panel is putting out less than 14V there likely is very little > power available anyway, and the MPPT controllers don't seem to function > there anyway, as their internal losses may be more than the small amounts > available. > If the solar panel is putting out less than 14V *UNLOADED* or that is it is not connected to the battery, I would agree as with that unloaded voltage when it is connected its voltage would drop to an insignificant amount above the battery voltage or maybe not any above it at all, but the point is that when you connect the panel to the battery thereby 'loading' its output (when for example its unloaded output might be say 18V), its voltage may still drop below what can effectively charge once it is connected but in fact there may still be current available, but not enough voltage to push it into the battery. By boosting the voltage the solar panel can then charge the battery. In point of fact this can add as much as over 20% during a charge cycle. > You speak of switch mode, not sure if we are thinking of the same thing. I > see switch mode as a DC to high freq AC converter useful in very efficient > power supplies that can adjust voltage and current, which I guess is the > same impedance matching that you refer to. >
"Switch mode" refers to a circuit where the input is 'chopped' into bits, then 'converted' by inductors, semiconductors, and capacitors to a different voltage and current level, and re-integrated back into steady dc. There are also feedback circuits to assure the output stays where it is wanted and to protect the device. Because the device uses 'switches' that are always either all the way on or all the way off they don't waste much power, unlike more traditional designs that use transistors that function much like a rheostat or 'electronic throttle' that spend most of their time only part way on so waste a lot of power, much like a slipping clutch would. > Correct me if I am wrong, but the way I read design on the MPPT controllers > is that it runs the panels at a higher voltage which will still give the > same current which results in more wattage, which is then more (output) > current at a lower voltage. Yes, *IF* the panel can achieve a significant boost in efficiency by operating at a higher output voltage while at high output power levels than that could result in more power stored. I don't know if this is true of most panels, that is that they become significantly more efficient at higher voltages for the same power output nominal levels. I am a bit skeptical that there would be enough of an increase to be usefull. I think most gains are to be had with this type of controller if there is NOT sufficient light to cause the loaded panels output to rise enough to come above the resting battery voltage, then there may still be power available to be stored but it cannot be put into the battery because of this minimum 'threshold' voltage needed which the panels do not have at that point. THEN the controller can boost the voltage and store this power. How?? The internal switch mode circuits convert the DC to AC to DC and match the solar panel's maximum output wattage point to the charging voltage needed. In theory this might gain 25% but only if the voltage difference is great, ie. very low battery charge. Also, quite a lot of heat is made in the controller circuits and this is lost energy. Reply; Not necessarily. A switch mode design can be over 90% efficient overall. Even a cheap one can approach 90%. Older designs did well to get above 70%, 50-60% was not unusual. I have a dc-dc converter I made that is 96% efficient. I think the main reason the controllers of this type are used is to be able to capture some of this 2/3'rds of the potential power that could not be otherwise stored, that is a much larger pie. The other reason has merit too, but I think it is a 'smaller pie'. If a panel is mounted flat or angled with no tracking, most will only be able to capture about 1/3 of the energy they could get if they always faced directly at the sun. When the sun is at a lower angle their output voltage can drop below the 'threshold' needed to be able to push current into the battery. There is still energy available but it can not be stored. By using such a controller, the output of the panels is very efficiently 'boosted' to a value that allows the energy to be captured. Also, the controller functions as charge regulator. he result is that less solar panels are needed for a given amount of energy capture. It may well be worth it on a boat where space is limited. It may not be cost effective in applications where space permits large panel arrays and solar tracking. -Ken > _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [email protected] To unsubscribe send an email to [email protected] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
