Selecting panels

garynappi

Given a choice of 12 volt panels with different VMP specs of 17.5v and 19.5v I assume the one rated at 19.5 would be a better choice but do not know why.  Is it that the higher VMP panel will stay above the minimum charge rate (~13.5v) for a 12 volt battery with less available sunshine?

Would the CC type matter, PWM or MPPT?

Raj174

The higher voltage panel is likely better because high temperature panels cause lower voltage output. Only the PWM controller will work with a single panel or panels in parallel.

garynappi

Thanks Raj174. That makes sense, but an MPPT CC cannot be used with a single or multiple panels in parallel is news and confusing.  

Estragon

It's not quite as simple as it will/won't work with mppt.  An mppt cc needs some overhead pv voltage to buck down to charging voltage.  I've seen suggested minimums of 20-30%.

The actual overhead will depend on the temperature of the panel (lower when warmer) and battery (should be higher when cold).  Panel Vmp is rated at STC, 25°C, but will generally operate at higher temp and lower Vmp in full sun.  There will also be some wire loss voltage drop.

In other words, an mppt may work bulk charging a room temp bank on a breezy cool day, but may not be able to hold absorb voltage on a hot day for a cool bank.  Doing an EQ may not be possible.

mountainman

Imho Even with pwm The 19.5 is definitely the best choice if you have 100f + Summers. 

Photowhit

...and  of course, if your array is close to your battery bank and you use PWM the lower voltage panel will provide enough  voltage for proper charging and more current for the same wattage. Hence the better value.

garynappi

Estragon said:

It's not quite as simple as it will/won't work with mppt.  An mppt cc needs some overhead pv voltage to buck down to charging voltage.  I've seen suggested minimums of 20-30%.

The actual overhead will depend on the temperature of the panel (lower when warmer) and battery (should be higher when cold).  Panel Vmp is rated at STC, 25°C, but will generally operate at higher temp and lower Vmp in full sun.  There will also be some wire loss voltage drop.

In other words, an mppt may work bulk charging a room temp bank on a breezy cool day, but may not be able to hold absorb voltage on a hot day for a cool bank.  Doing an EQ may not be possible.

The overhead issue is what I never read of before, thanks for clearing it up! Gads, this site is GR8!