Are any MPPT controllers buck+boost?
Apparently some MPPT controllers can boost voltage up when Vmpp is near or below Vbatt. Or do you just wire the panels serial-parallel to ensure the voltage is always above Vbatt is Vmpp can get close...
Forum Discussion
It is hurricane season again so time to take it to the next level.
Absolutely solar panels are current sources over most of their range but the max power point is right around where they stop being a near ideal current source and the current falls off a cliff. Once Vbatt is even a few tenths past Vmpp the current and therefore power produced drops to minimal levels.
I have read that some controllers will run at the max power point and step the voltage up to charge the battery when needed, but I cannot find any examples. For me with either PWM or MPPT, if I run my panels in hi temps but in the shade, I get good power production due to the good temp profile of the Unisolars for power. But once the charging voltage gets up into productive territory of 14.0+ volts, there are situations where the Vmpp is there too. So I'm getting better power production from the panel than MPPT in that case (no DC-DC losses with PWM), *but* that low Vmpp is limiting the controller's ability to run the charge up to 14.5v or 14.6v. An MPPT controller with Buck+boost converter circuitry could do it.
I could also wire it up to be able to switch the panels from parallel to serial at that point and skip the problem, since it may not need all the amps once it wants to charge at over 15 volts, and that would give the controller 28v to work with even in the worst conditions. But it could be that best shade output could really be had by an MPPT that can step voltage up or down. I have a converter I can test this with - one panel on buck and one on boost into the same battery.
For my usage to run just fridge I have excess watts on sunny days and plenty of battery for many rainy days in a row, but yield in rain and shadow are actually at a premium and could conceivably cover a significant portion of my needed 20-24 amp hours a day from just the 136w of Unisolars.
It may be academic though. When the batteries really NEED a good charge, the panels dropped down to a Vmpp of 14v in the shade means a partially discharged battery will be drawing very nearly the max power out of the panels anyway under PWM. It's not to uncommon for me to see 4 amps charging on a cloudy day depending how thick the clouds, and 20+ amp hours back in go a long way toward break even, even on a day with no sun.
I need to figure out what Vmpp and amps produced at Vbatt are on the panels under various rain conditions.
Jim
Absolutely solar panels are current sources over most of their range but the max power point is right around where they stop being a near ideal current source and the current falls off a cliff. Once Vbatt is even a few tenths past Vmpp the current and therefore power produced drops to minimal levels.
I have read that some controllers will run at the max power point and step the voltage up to charge the battery when needed, but I cannot find any examples. For me with either PWM or MPPT, if I run my panels in hi temps but in the shade, I get good power production due to the good temp profile of the Unisolars for power. But once the charging voltage gets up into productive territory of 14.0+ volts, there are situations where the Vmpp is there too. So I'm getting better power production from the panel than MPPT in that case (no DC-DC losses with PWM), *but* that low Vmpp is limiting the controller's ability to run the charge up to 14.5v or 14.6v. An MPPT controller with Buck+boost converter circuitry could do it.
I could also wire it up to be able to switch the panels from parallel to serial at that point and skip the problem, since it may not need all the amps once it wants to charge at over 15 volts, and that would give the controller 28v to work with even in the worst conditions. But it could be that best shade output could really be had by an MPPT that can step voltage up or down. I have a converter I can test this with - one panel on buck and one on boost into the same battery.
For my usage to run just fridge I have excess watts on sunny days and plenty of battery for many rainy days in a row, but yield in rain and shadow are actually at a premium and could conceivably cover a significant portion of my needed 20-24 amp hours a day from just the 136w of Unisolars.
It may be academic though. When the batteries really NEED a good charge, the panels dropped down to a Vmpp of 14v in the shade means a partially discharged battery will be drawing very nearly the max power out of the panels anyway under PWM. It's not to uncommon for me to see 4 amps charging on a cloudy day depending how thick the clouds, and 20+ amp hours back in go a long way toward break even, even on a day with no sun.
I need to figure out what Vmpp and amps produced at Vbatt are on the panels under various rain conditions.
Jim
