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--- electrical:solar:pwm_tweaking [2021/06/26 15:46]
frater_secessus [the controller'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**.  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))
-{{ 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).  The current will be around 5.6A (Imp) when the panel is in the usual operating range.
+{{ 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 \\
+.8v x 5.6A = 94.70w <-- MPPT running at 95% efficiency
+PWM harvest \\
 .8v x 5.6A = 82.88w <-- power at a high Absorption voltage, as with flooded lead acid \\
 .4v x 5.6A = 80.64w <-- power at a lower Absorption voltage \\
@@ Line 24: / Line 26: @@
 .7v x 5.6A = 70.61W <-- power at 100% state of charge (SoC) \\
 .4v x 5.6A = 69.44W <-- power at 75% state of charge; bulk charging \\
-.1v x 5.6A = 67.76W <-- power at 50% state of charge;  bulk charging
+.1v x 5.6A = 67.76W <-- power at 50% state of charge;  bulk charging \\
+.0v x 5.6A = 56.00W <-- battery bank accidentally run flat
 This last number shows how far off PWM output can fall when battery voltage (Vbatt) is at its lowest.((The last number is also the source of the "30% boost!!!" claims some MPPT makers make.  Yes, at that point MPPT at 94.69w (after 5% MPPT efficiency penalty) is 40.2% more than PWM's 67.76w but that is when the disparity is the widest.  Most of the time the MPPT charging advantage is lower, perhaps 15%.))  It's also when the battery needs the most current to get out of that deep cycle trough.
@@ Line 32: / 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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