Panel to Solar Controller Wire Size
As some of you already know, I'm piecing together a solar system. Controller is a Morningstar 45A PWM. Panels will be two 12V 150W wired in parallel. I'm going to be using a combiner box on the roof, ...
Forum Discussion
The reason for tapering is due to loss of headroom. That's a panel/cable issue.
You'll quickly loose headroom as SOC goes to 80%. That's because battery resistance increases significantly. If there's enough headroom, the charging system can deal with the increased battery resistance without tapering.
On the other hand, when the panel is hot you'll start tapering long before you get to 80% SOC. Looking at the Sharp IV curve, 15.0V is probably a good measure where you're starting to fall off the cliff.
Sharp panel temp coefficient: TC = -0.36%/C
Let's take a reasonable temp rise of 30C.
Panel voltage @ 30C rise = (1 - 0.36%/C * 30C) * 15.0V = 0.892 * 15.0V = 13.38V
If you have a 2% cable voltage drop, you'll see significant tapering when battery voltage is at: Vbat = 0.98% * 13.38V = 13.11V.
If you're charging at C/10, 13.11V corresponds to 40 to 50% SOC. That's where you'll begin to taper.
The OP's panel has better specs. Vmp = 19.1V vs. 17.4V (sharp). It has a 1.7V more headroom. Tapering will be later.
Sal
You'll quickly loose headroom as SOC goes to 80%. That's because battery resistance increases significantly. If there's enough headroom, the charging system can deal with the increased battery resistance without tapering.
On the other hand, when the panel is hot you'll start tapering long before you get to 80% SOC. Looking at the Sharp IV curve, 15.0V is probably a good measure where you're starting to fall off the cliff.
Sharp panel temp coefficient: TC = -0.36%/C
Let's take a reasonable temp rise of 30C.
Panel voltage @ 30C rise = (1 - 0.36%/C * 30C) * 15.0V = 0.892 * 15.0V = 13.38V
If you have a 2% cable voltage drop, you'll see significant tapering when battery voltage is at: Vbat = 0.98% * 13.38V = 13.11V.
If you're charging at C/10, 13.11V corresponds to 40 to 50% SOC. That's where you'll begin to taper.
The OP's panel has better specs. Vmp = 19.1V vs. 17.4V (sharp). It has a 1.7V more headroom. Tapering will be later.
Sal
BFL13 wrote:
Also it gets hard to tell if it is IV curve or just high SOC tapering the amps once batt voltage gets to 14 and up so can't see much point in using fatter wire to keep Isc from tapering given that is indeed possible, when amps will be tapering anyway from high SOC.
