mppt vs pwm

jrnymn7 wrote:
I did a search for the 'member's solar installs' thread, but came up empty.

Help, please!

This thread

75c is 167f

Just where do you plan on camping?

Hi Salvo,

I'd love to see an analysis of Mppt vs Pwm at a vmp of 33 with a panel temperature of 75C. To make it a level playing field use 16.5 v for the pwm.

i.e. MPPT configuration series/parallel @ 33 vmp
and PWM configuration parallel @ 16.5 vmp.

Salvo wrote:
Fig 14 pretty much sums up the differences. The RV crowd are for the most part fair weather campers. That means cell temperature is in the range 40 to 80C. In that range, for the most part, mppt gains are between 0 and 10% better than pwm. On average it could be about 5%. Nothing to crow about.

I'm still confused about the effect of partial shading, and which external wiring , series or parallel, will eliminate the entire panel/array from shutting down?

(I do have a pretty good handle, I think, on the use of by-pass diodes in the panels themselves, to stop hot spots from overloading the entire panel)

So, if series/parallel is not an option, which way is best, where partial shade is concerned; series or parallel?

Thanks.

I did a search for the 'member's solar installs' thread, but came up empty.

Help, please!

MrWizard wrote:

another reason..why i keep saying its about watts not amps (but i'm flogging a dead mule) nobody wants to listen to that one

I'm certainly listening.

Yes, you did drop down to 8%.

Fig 14 pretty much sums up the differences. The RV crowd are for the most part fair weather campers. That means cell temperature is in the range 40 to 80C. In that range, for the most part, mppt gains are between 0 and 10% better than pwm. On average it could be about 5%. Nothing to crow about.

I see no real advantage when using a higher Vmp. Temperature still reduces power by the same amount. 90's thinking still apply.

lorelec wrote:

Salvo wrote:
Right, Don needs to review that thread.

I think Rogue dropped his gains after the discussion from 20% down to 8%; but it's been a while.

No, it's entirely possible to get 20% gain with MPPT. Much more than that, actually, if you're using an array with a Vmp far above the battery voltage (which only MPPT controllers will allow you to do with any measure of efficiency). If you've been stuck in the boondocks for a long time, you might not realize that the popular thing to do these days is to use panels meant for grid-tie installations -- which do indeed have a higher Vmp than traditional panels meant for off-grid installs.

For CONSISTENT gain, with an array that has a Vmp closer to the battery voltage (17v Vmp, for example, with a 12v battery), around 10% is typical when you factor in the effects of temperature on Vmp. This is 1990's thinking, though. Time to move on.

i had used mppt blue sky 2000e for over 4yrs, and i am now using, PWM xantrex C40

i can tell you with 17v panels, the daily totals on "clear sunny days" is almost identical, Not worth the extra price of the MPPT

BUT on days of changing weather, days that start out cloudy or over cast or rainy
its the MPPT that will have the higher charge (amp hrs) at the end of the day

there is gain with mppt and nominal voltage panels, but only under specific conditions

but IF you have high voltage panels, you need an MPPT controller
or you are just throwing away watts, that can be converted to amps

too many people in this thread are doing an apples to oranges comparison
and that makes for comprehension errors

another reason..why i keep saying its about watts not amps (but i'm flogging a dead mule) nobody wants to listen to that one

Yes, understood concerning amps tapering in Abs, but I'm surprised to hear they will also taper a little in bulk.

My panels are rated at 8.15a, so x2 = ~16a. I wasn't thinking about how low a C rate that is on my 430Ah bank, so my "first few minutes of bulk" statement was, well, kinda dumb. lol.

Nevertheless, in bulk, as Vbatt rises thru the 13's, amps will taper slightly. But at a C/27 rate, I imagine that will take quite a while.

Thanks BFL.

jrnymn7 wrote:
So, Isc will decline even before amps taper? So I should not expect to ever see 16 amps from my two 140w panels, except for maybe the first few minutes of bulk?

Look at the IV curves for those panels for exact numbers, but say Isc is about 8.8a. (You get twice that with two panels) You get that until battery voltage rises into the 13s, then it starts down slowly so by the time your Vbat is 14.8, Isc might be 7.x amps so you get 14.x.

In fact, once you get to 14.0 Vbat after a hard day's slogging on solar, the voltage rises quickly from there, so it doesn't take long to reach the 14.8 so the time factor is not much for when amps slide a bit. from 14 to 14.8v. Not really an issue in real life.

When amps taper due to SOC is totally separate and has to do with the charging rate but it now depends on whether the amps in play can be accepted by that size of bank at up to 14.8v or whether the bank is so small it will taper that amount of amps at some lower voltage. A larger bank will keep the same amps up longer to a higher SOC than a smaller bank at the same amps.