Lowest OCV An MPPT Is "Worth It"?

[MEXICOWANDERER]

There has to be some experience here on this forum to share. When I look at the MPPT manufacturer's data specs, the incredo-hyperbole gives me diarrhea.

[NinerBikes]

MEXICOWANDERER wrote:
Hi Señor. This is for my secondary system. The 1st 150kWh down here costs the eqvt of 4.6 CENTS USA per kWh so I use it and the 2nd tier 6.7 cents kWh 50 kWh allowed for a total of 200kWh. After that, the electricity costs soar to unbelievable heights.

It's when we have zero power, or 22.8 VAC available that I have to rely on alternative energy. We have giant scorpions down here that love the get underfoot, hide in rolls of tissue, towels, shower curtains.

Bathroom is separate from house. Rigging up high power UV emitters that will illuminate 1 minute before the lights come on. Decided to go fluorescent tube rather than LED because of the short run time. Scorpions glow bright green under UV light.

But rather than operate a DC / DC buck off the 24 volt system, I went to a separate 12 volt system. The 24 is for the 4024 Trace and primarily for the 32 cubic feet of freezers, and a PSW small inverter for sensitive loads.

Take your pick.


Hand Held Scorpion nightime alarm system

We use these in conjunction with led lights for cool night summer night time mountain bike rides on single track in the desert of AZ. They do indeed glow green in the dark. Very cool to use solo on moonlight rides.

[MEXICOWANDERER]

Hi Señor. This is for my secondary system. The 1st 150kWh down here costs the eqvt of 4.6 CENTS USA per kWh so I use it and the 2nd tier 6.7 cents kWh 50 kWh allowed for a total of 200kWh. After that, the electricity costs soar to unbelievable heights.

It's when we have zero power, or 22.8 VAC available that I have to rely on alternative energy. We have giant scorpions down here that love the get underfoot, hide in rolls of tissue, towels, shower curtains.

Bathroom is separate from house. Rigging up high power UV emitters that will illuminate 1 minute before the lights come on. Decided to go fluorescent tube rather than LED because of the short run time. Scorpions glow bright green under UV light.

But rather than operate a DC / DC buck off the 24 volt system, I went to a separate 12 volt system. The 24 is for the 4024 Trace and primarily for the 32 cubic feet of freezers, and a PSW small inverter for sensitive loads.

[NinerBikes]

MEXICOWANDERER wrote:
400 watts at 17.5 volts? I am trying to clarify my question 🙂

So 400 watts at 17.5V is about 23 amps. Realistically, you'll need something like panels that will claim perhaps more amps at 17.0 or 17.1 V, if you decide to go with PWM. Although my 120W panel puts out about 7.0 amps, by the time it goes through my PWM device and gets adjusted to what ever voltage I decide, the end results is I only get 6.67 Amps, regardless of peak set voltage for bulk charging.

I don't know if the amp loss is proportional to the output in amps of the panels, or if it's a flat .3 Amp lost regardless of if it's 15 amps, 20 amps or a 30 amp controller.

As has been said many times before, when you're looking at PWM, you are looking at Amp output, and a slight loss from panel ratings once through the charge controller. I too bought the Solar 30 for $34. Plenty for 400 watts. Voltage programmable. How many amps at what voltage do you need to charge your battery banks based on estimated usage and SOC in the morning when the sun comes up?

At 400 watts, with a 30 amp PWM handling the chores... and you can get 2.3 amps more, times hours per day, with MPPT... are you setting and forgetting the panels, not aiming them, not chasing the sun?

Too many variables... On a budget... PWM... if you are designing it for Big Daddy WarBucks... and it's cost plus.... MPPT.

Solar Blvd sent me a Black Friday special with 100 watt 21.8V panels for $95 each, and 120 watt panels, same voltage, for $115 each.

At under a dollar a watt, you can replace 2 amps lost without MPPT and using PWM, if your system uses under 30 amps, for under $30 with just more or larger panels. $30 vs $300 for MPPT... and the price per watt keeps on dropping.... your bell shape is moving, constantly, due to variables.

[BFL13]

MEXICOWANDERER wrote:
400 watts at 17.5 volts? I am trying to clarify my question 🙂

You get panel Isc with PWM pointing at the sun with sun high enough for STC. Simple. It is all proportional too so you can figure your "expected amps" from one known amount. I use 8.2a for a 130w panel (my Sharp poly-- watch out for monos with high Voc but low Isc. You want Isc)

So 400/130 x 8.2 = 25a. You want a 20% margin in controller amps over that to meet times when panel does above rated. So you need another 5a so use a 30a controller.

My Solar30 (cost $34) with the fancy readout for v, a, ah, etc, and adjustable voltage --not found on the $13 jobs) is 30a and would do you perfectly. I really like mine anyway! 🙂

If you go MPPT with either 12s or 24s that total 400w you get an extra 10% on average, so now you get 25a + 2.5a = 27.5a. But you can still use a 30a controller since most/all MPPT controllers 'clip' the amps to their ratings, so you don't have to worry about the 20% margin rule.

My Eco Worthy 20a MPPT ( cost $102) controller is limited to 42v input Voc, so it can only do one serious sized 24v panel to maybe 250w. Works great with a 230w

So 400w using 12v panels is four 100w panels that do 6.3a each Isc. cost is say $150? (Mr Wiz is right again--watch for sales--maybe find them for $100 who can say?) so four is $600 plus $34 for the controller =$634

Two 200w panels say find a great deal on them at $180 each is $360 plus two $102 Eco Ws for $204 comes to $564.

So here we have crossed some sort of line on price where now the MPPT is a bit cheaper at 400w. If you insist on high priced USA controllers instead of Chinese, then obviously the cross-over point will be different "depending" but then you also need to know what the panel prices are on sale at the time whether 12s are that much more than 24s.

When I got my panels here last year, the 12s and 24s were the same price at $1.08/w. Prices are all over the map then and now. Some quite outrageous, picking on the unwary at hardware stores for "kits" at eg, 60w for $399 and all that kind of thievery. 😞

EDIT here is that thread showing my actual PWM vs MPPT results in the OP

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27667574.cfm

EDIT some more: What is the size of battery bank and daily AH draw? What is worth it can depend on how far up in SOC you want to get daily or whether you just want to stay above 50% and then recharge with gen and solar as required.

Also with that whether you do your say, monthly overcharge to "True-Full" on solar or on shore power

Also what kind of batteries? 12s take a lot of work to get their SG up compared with 6s. My 6s can get by with high 14s and the odd 15.5, but the 12v Trojan 1275s (true deep cycles) need frequent 15.5 to keep their SG up and 14.4 all day is just a joke to them.

[MEXICOWANDERER]

Thanks for the link Bend

I visited and like what I read. Being isolated it was news to me that OEM battery also expressed concern with "controlled overcharge" being missing in action with existing controllers. I can only imagine how much money existing "Smart" controllers are costing OEM in unneeded warranty claims.

How frustrating it's been to see a simple controller like a Trace C-30 which did a perfectly acceptable job after being tweaked, degenerate to a county fair barker convention with integrated circuits that wouldn't know a battery from a boutique.

[Bend]

MEXICOWANDERER wrote:

...

That being said, there is a lot of space between say 14.8 volts and lower OCV "optimum working voltage" of say 17.5 VDC. Got it? Damned near three volts. Is the excess supposed to run around in circles and yell "Help! Help!"?

...


Where are they getting these idiots that design controllers? Companies like Outback have genuine engineers, but I am not going to have a 7 piece Oneida sterling silver place setting administering a Chinet plate. I do not have that kind of money.

Bogart Engineering might be working on near the same idea (due out soon):

"The SC-2030 is designed for use with a 12V panel (36 cells) and a 12V battery, or a 24V panel (72 cells) and a 24V battery with efficiency comparable to that of MPPT charge controllers, but without the high cost of MPPT."

Of course, we did not have the unused watt problem 2 decades ago. When installers were running 14-16 gauge wire from the panels we were lucky to get 15v at the controller. 8>)

[MrWizard]

That's a theoretical 28.98 amps at 13.8v
But that is a reality check of 22.85 output from panels at 17.5 volts

Panels tilted for max power, A 25amp PWM controller will give you max 20~22 amps if the batteries can use it and the Sun is right

But to get the full 400w aka 29 amps charge into low batteries you will need mppt

[MEXICOWANDERER]

400 watts at 17.5 volts? I am trying to clarify my question 🙂

[MrWizard]

Mexi
The bell curve is not a wattage point, it's a voltage point
Mppt will convert any voltage higher than the the. "Charge set points" to extra amps
The higher the voltage difference applied to the controller, the more amps gained at the max charge set point

Take the number of watts you want, then price the system cheap panels with pricey controller

More costly low voltage panels, with cheap controller

There is no break over point (bell curve) that , I am aware of, for figuring out which one too use
Too much fluctuation in the market in pricing of panels,

[MEXICOWANDERER]

I'd better explain what I'm up to, here......

Mexican ADUANA, the Customs folks at the border lick their chops when they chance across a solar panel the size of half a ping-pong table. Residential grade high voltage polycrystalline panels. Yet these panels are heavy as hell, cumbersome, and tend to complain when a 20 lb coconut drops 15 feet on them. Yet again, these panels, watt for watt, are considerably less expensive per watt than <21 OCV panels.

That being said, there is a lot of space between say 14.8 volts and lower OCV "optimum working voltage" of say 17.5 VDC. Got it? Damned near three volts. Is the excess supposed to run around in circles and yell "Help! Help!"?

I guess I have to design my own controller. A PWM that I can adjust the maximum time at absorbsion limit before it defaults to float. A controller that can pass MAXIMUM available panel amps at float to remain in float until battery voltage sags to a point where absorbsion is appropriate.

Where are they getting these idiots that design controllers? Companies like Outback have genuine engineers, but I am not going to have a 7 piece Oneida sterling silver place setting administering a Chinet plate. I do not have that kind of money.

[pianotuna]

Hi Mex,

Blue Sky 3024 derates the amperage when using higher voltage. I would be unable to increase my wattage by much, but I am operating pretty much in the "sweet spot" for MPPT conversion of volts to amps.

Some folks have suggested that the charging day is effectively longer with higher voltages. I don't know if this is so or not.

[BFL13]

Mex, you have to be careful with percentage gain and absolute gain as to what is worth it.

So if you got 20amps at high noon with PWM you would get 22amps with MPPT. Earlier and later when amps are lower it is still 2 more amps but the percentage gain goes UP. 2 over 10 is 20% (battery voltage is also lower earlier in the day which gets you more for MPPT so it is still 2 amps)

The percentages sound terrific, but it is still only 2 amps more for the day of say 8 hours--16AH. Do you even notice 16AH out of your day? I don't.


It makes no difference in MPPT if you have 12v panels in series to get 24v or one 24v panel except when you start looking at things like controller Voc input limits. A BIG DEAL!

There are all sorts of other factors to do with physical panel size and weight, controller read-outs and adjustable set points etc.

A factor often ignored is that you can mix and match by having more than one controller on a bank. I can have PWM 12v and 24v MPPT on the same bank and that is no different in daily AH from if I had matched panels and a very expensive bigger controller.

The limitation there is my PWM controller cannot do the 24 panel and I can't put the 12 and 24 on the same controller. I could if that one 24 panel were two 12s though. 12s give you more options for set-ups.

[MEXICOWANDERER]

In one example, I am looking at panels with 21 VOC and optimum 17.5 WV

Head-to-head, how much or how little will MPPT offer over PWM?

Discharged every day, at first the battery bank has zero chance of 100% replenishment with the first batch of panels.

There is a massive tradeoff comparing high voltage high wattage panels at low cost with an expensive MPPT -VERSUS- higher cost lower OCV panels using PWM. My interest is knowing the parameters of the "Value Bell Curve". How "much" is minimum? At what WATTAGE point of value does high OCV panels with MPPT offer a clear advantage?

Pardon my lameness. We engineers like to have things explained to us minus buzzwords, and emotional bias. I hope you understand. Thank you.

[NinerBikes]

Mex enjoys tinkering with batteries and charging them, as much as he enjoys tinkering with the finest minds here in the tech section, and making people, newbies included, actually think, and analyzing their actual needs, with an emphasis on fine tuning the equipment to the specific application.

I'm fine with making people think for them selves. Plant some seeds, sit back, and watch them grow, just like this thread.