Solar question

RLS7201 wrote:
Mr. pianotuna, I have a question for you and BFL13.

Lets set up 2 systems.

1: 500 watts of 12 volt solar w/PWM and 4 6volt GC batteries, configured at 12 volts.

2: 500 watts of 24 volt solar w/MPPT and 4 6volt GC batteries configured at 12 volts.

Question, will either system stand a better chance of equalizing than the other (15.5 volts) on a slightly over cast day?

Thanks
Richard

Trick question, answer is at the bottom of all this blurb--

OK OP is going to sort his out next year, so to answer this question, first 500w is a tricky number. The limit for a 30a PWM controller is about 400w.

( You need a 20% amp margin with PWM for when the panels go over their ratings so 25a "expected amps" with 20%/5a extra is 30a. Ok, so we know a 12v panel aimed at high sun will do its Isc to the battery, and that a 130w panel has 8.2a Isc. So 25a would be a 25/8.2 x 130 = 396 so call that 400w limit)

Also to do an equalize first you need to get the batts to "full" OK so now you need to do that early enough in the day to do the Equalize or else you have to wait till the next day (hoping it will not be cloudy next day). so firstly it doesn't matter whether you have MPPT or PWM for the same amps (which is what you get per watt more or less depending on temperature of panel--MPPT beats PWM in winter, but does less in summer--average is same over a year)

So lets say you get to "full" by noon and it will be sunny till dark. Now it is all is to the controller settings. Whether PWM or MPPT, you need say 16v for the Equalizing. Some controllers can do that, others can't. EG, my Solat30 PWM can go to 15.5v which will equalize my 6s but not my 12s, which need 16. My MPPT Eco-Worthy gets to 14.8 and then drops to 14.4 for the rest of the day so it can't equalize anything (but my 6s on that never need equalizing if it stays sunny for days and days in a row on that 14.8/14.4 routine)

So now if the controller, whether PWM or MPPT can't be adjusted to Equalize voltage, you can by-pass the controller and go panel direct to get it done (BTW the inverter won't workm over 14.9v and the fridge will be hurt over 15.3v so be prepared to do battery disconnect)

So to answer the question of overcast day MPPT vs PWM for chance to equalize, my answer is MPPT or PWM has nothing to do with it.

And BTW the "urban myth" about MPPT doing something in low light vs PWM has not been seen by me with both running at the same time when I had both set-ups. In fact for most of the summer the MPPT has under-performed PWM due to higher panel temperature lowering panel voltage so get fewer amps with MPPT than would with PWM.

Even if there were a slight MPPT advantage on overcast days it is too small to matter in daily AH compared with daily usage. Anyway from April to October around here it sure can't be seen. Maybe if you did solar in winter, but then solar is hurting then no matter which set-up you have.