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--- electrical:solar:pwm_tweaking [2023/11/19 21:16]
frater_secessus [PWM's achilles heel]
+++ electrical:solar:pwm_tweaking [2025/04/22 17:59] (current)
frater_secessus [panel selection]
@@ Line 9: / Line 9: @@
 ===== PWM's achilles heel =====
-During charging with a PWM controller **the solar panel will be running at the voltage of the battery bank**.((or close enough for the purposes of this discussion.  Specifically, the panel will be ~Vbatt when the controller is pulsing ON.  Vpanel will be closer to Voc when the pulse is OFF because the panel is Open Circuit (unloaded) at that time.))  Unfortunately, this is almost never the voltage at which the panel makes its rated power (Vmp).  The panel will make less and less power when battery voltage is low.
+During charging with a PWM controller **the solar panel will be running at the voltage of the battery bank**.((or close enough for the purposes of this discussion.  Specifically, the panel will be Vbatt + wiring sag when the controller is pulsing ON.  Vpanel will be closer to local Voc when the pulse is OFF because the panel is Open Circuit (unloaded) at that time.))  Unfortunately, this is almost never the voltage at which the panel makes its rated power (Vmp).  The panel will make less and less power when battery voltage is low.
 This affects charging in all three [[electrical:12v:charging|charging stages]] but it is devastating in [[electrical:12v:charging#bulk_stage|Bulk]].  Let's do some math to see why.((temperature derating is not considered))
@@ Line 15: / Line 15: @@
 {{ http://bestsolarpanelforhome.com/wp-content/uploads/2017/03/57-79-600x600.jpg?200}}Consider the spec sheet for a random 100w panel (right).  100W is made when the panel is run at 17.8v (Vmp). We will use Imp for our math, but the absolute numbers are not important, only the pattern.  Current will usually be less under normal conditions, but can spike to Isc [[https://diysolarforum.com/threads/why-is-mppt-better-than-pwm.24892/page-4#post-295861|under some conditions]].
-.8v x 5.6A = 99.68w <-- rating under laboratory conditions \\
+.8v x 5.6A = 100w <-- rating under laboratory conditions((the math doesn't line up perfectly because the specs are rounded)) \\
 .8v x 5.6A = 94.70w <-- MPPT running at 95% efficiency
@@ Line 35: / Line 35: @@
 Here are steps to take to maximize power from the PWM controller, in order from greatest effect to least effect.
+==== panel selection ====
+Choose panels for your PWM with a [[electrical:solar:panels#specifications|Vmp spec]] as close to your battery voltage as practical;((Vabs + 10% might be ideal.))  this will mean the panels will be running closer to their Vmp, and therefore closer to their rated output.
+Assuming a charging voltage of 14.2v, let's consider two different panels.  Remember that Vmp x Imp = Pmax (rated power).
+  * 100w panel, 20.0Vmp - 100W / 20Vmp = 5.0A Imp.  14.2v x 5A = **71W** max.
+  * 100w panel, 17.5Vmp - 100W / 17.5Vmp = 5.71A Imp.  14.2v x 5.71 = **81W** max.
+In this example, choosing the best-matched panel results in **14% increased harvest for free**. All it costs is attention and a few moments reading specs.

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