MPPT stepdown efficiency

westbranch · March 2010

I am in the process of thinking through the best way to put together a new system for my cabin, probably @ 24 V.. Array planned for ~ 1Kw.

I have an MX 60 MPPT that I will use and have been looking at numerous panels for price and also compatibility with the current Mitsubishi 120W 12v panels I have.

I noticed on one manufacturers site showed panels with VMP of 18 V, 27 V, 30 V, 36 V and 45 V. (The 18 V version is a 12 V, 36V is a 24V nominal panel)

The question that came to mind is:

Would there be a difference in the stepdown efficiency of an MPPT CC, to 12V, using a panel with a VMP of 27 V, 30 V, 36 V or 45 v.

I remember a statement that the closer the panels is to the end voltage the better the efficiency... I am thinking also that there will be a similar relationship with using 24 V as the nominal voltage to panel VMP. Also the slightly lower max absorb V for an AGM battery may impact a choice of panel, but that is another question.

thanks
Eric

mike95490 · March 2010

Re: MPPT stepdown efficiency

Gosh, any chance you would move up to 24V in the future? So, are you putting panels in series, or all in parallel? What's the wire run length, cost of heavy gauge copper? Can you locate a eff % vs Voltage chart, or just assume lower is always better?
And with the 60A Morningstar MPPT @ 99%, how much better do you want?

BB. · March 2010

Re: MPPT stepdown efficiency

Big issue to watch out for is a 48volt bank and an Vmp=48+ volt panels... Two panels in series, hot, may not be enough voltage to charge a battery. Three panels in series, cold, may exceed Voc>150 VDC limit of controller.

High voltage panels are fine--just that combination (Vmp/Voc high, Vbatt=48 volts) is the issue.

Check the efficiency graph/chart for your controller/battery combination... Vmp high (over ~2x vbatt) tends to be slightly less efficient. But if you have a fair distance from the panels to the charge controller, high Vmp/Voc is great to reduce wiring losses/costs.

-Bill

westbranch · March 2010

Re: MPPT stepdown efficiency

Ok Ok Ok, I am also thinking about 48 v too but wanted to try to keep the question simple and extrapolate, generally, to higher voltages.
So, No mike I am not stuck in the dark ages of 12 v,;) just trying to do get ahead of the game in the dead of winter.

Right now I have about a 10 foot run from the panels to the 12 V battery, so there is minimal lineloss regardless of voltage.
I am working on getting a set of newer AGMs of greater voltage for the, to be built, cabin, same great view but with actual insulation, double seal windows, etc... When that is completed I will switch over. Wire run length I can only estimate as we have not 'sited' the foundation yet as > 50
% of it is under the existing cabin...
I took a set of timed pictures last weekend and surprised myself as to 'where' I should site an array... a lot more sun but further than I would like, so as you point out, line loss is a big consideration will try to max it out for the CC. Have to guess at the height and path of the new roof lines shadow

Trying to figure out the logistics of placing 6400 pounds of battery (48V) in a no- door basement.

One idea in another string is outside the wall but slightly below ground level. Helps with access, placement etc.. moderates hi temps, but hi concrete costs $300/ yd...

thanks for the other Questions. will look for a chart.

Eric

BB. · March 2010

Re: MPPT stepdown efficiency

Just be careful of anything below grade--Ensure that no water (winter/roof/local flood from heavy rains) cannot get in your battery box... Drain pipe and/or sump pump?).

-Bill

Cariboocoot · March 2010

Re: MPPT stepdown efficiency

For what it's worth, I went from 72 Volt array on 12 Volt system to 48 Volt array on 12 Volt system and then to 24 Volt system. Same panels, same charge controller.
Efficiency? Differences were nominal in all cases. But my downlead is short (less than 50 feet) and heavy (10 gauge), as are all the other wires so there isn't much line loss no matter how it's configured.
I do miss the low light charging advantage of 72 feeding 12, but the array was going over the MX60's input limit on cold days.

Just my opinion, but the '2X' rule works for me: nominal array Voltage twice the nominal system Voltage. Your actual application may vary (void where prohibited, not an offer than can be made by formal prospectus only, some assembly required, batteries not included, etc.)

Keeping the CC operating Amps below 75% maximum helps too; higher Amps, higher temps, higher Voltage conversion all decrease efficiency.

Frankly, if it weren't for the need to supply the heavy current for the pump motors I wouldn't have gone up to 24 Volt system.

Vic · March 2010

Re: MPPT stepdown efficiency

Hi westbranch,

Personally I'd try to go 48 V battery bank if at all possible.

6400 Lbs of battery, that is some considerable storage ! For those who worry about it, some may say that you need considerably more than 1 KW of panel, as C/10 may equate to approximately 200 A @ 48 v. It is also possible that you plan to do your hi-rate charging from a genset, but still, that is some LARGE charger that you would need if C/10 max charge rate would be your goal.

I have never been in the C/10 camp for every day charging, my usual max charge current is in the range of C/30. In the summer much of the charging occurs fairly early in the day, and is done at current levels considerably below C/30.

I like your effort to couple the batteries to the earth. Just hope that you can make them very accessible for monitoring and service etc.

Good Luck, Vic

audredger · March 2010

Re: MPPT stepdown efficiency

I second that Vic, Besides the smaller wire for higher voltage there is greater efficiency to be had at the inverter. Generally speaking, a 48v inverter will be ~1% more efficient than a 24v inverter.

another point is given the same size battery, a higher voltage bank, will store more current than a lower voltage bank..... thus reducing the number of banks. One large bank is much more preferable than many smaller ones.

boB · March 2010

Re: MPPT stepdown efficiency

mike90045 wrote: »

And with the 60A Morningstar MPPT @ 99%, how much better do you want?

I want 101% !!

Remember that 99% is "peak" efficiency... Best case. That will usually be with highest output power (or current) at the highest output voltage (48V battery) and lowest possible input voltage (48V array) and lowest temperature (unit just turned on.)

This is because the power "lost" in the unit due to heat (and idle power) is a lower portion of the output power in Watts. The MX60 has also been measured at around 99% efficiency during certain tests, but from what I have heard, I have a feeling that the Morningstar is even slightly better.

BTW, the "worst" efficiency will (normally) be measured at the lowest output Wattage (current) because of idle power of the CC.

Step down efficiency always goes down when input voltage goes up. The question is, does that drop in step-down converter efficiency get MORE than made up by the increase in overall system efficiency by lowering the losses in the input PV wiring as well as the advantages of the PV Max Power Point voltage being at least somewhat higher than the battery voltage ?? I think that normally trumps converter efficiency alone, with a moderate to larger sized (Wattage) system.

boB

westbranch · March 2010

Re: MPPT stepdown efficiency

MIKe your comment about the one large bank vs smaller ones (48V vs 2x24v) interests me as I have been thinking on that very configuration (2x24)... can you expand on your statement about storing more current.

Vic, sorry I am thinking of 1 kw per 24 v so if 48 V , double the array at min havent worked that part to the end yet.

boB, thanks for the last para , lots to mull over there.

thanks
Eric

Solar Guppy · March 2010

Re: MPPT stepdown efficiency

The MorningStar is the best shipping controller efficiency wise, for a number of design reasons. He is some technical stuff for the geeks among us:

Lower Tar loss, about half other controllers

Dynamically enabled 3-phase buck switching stages, lower currents only one phase is active for lower switching losses, progressively enables other phase as current increases. This give much better low power efficiency.

Best FETS available, MX60/FM60 and XW use a 8 year old FET model

No relay, uses best in class DirectFET packaging for lowest on resistance

Three phase switching, lowers peak current losses in FETs and capacitors

Very short PCB paths internally, minimizes resistance losses

In nominal 48V arrays to 48V battery bank will operate at better than 99% in a large current range, not just a single picked value.

boB · March 2010

Re: MPPT stepdown efficiency

Solar Guppy wrote: »

The MorningStar is the best shipping controller efficiency wise, for a number of design reasons. He is some technical stuff for the geeks among us:

Lower Tar loss, about half other controllers

Dynamically enabled 3-phase buck switching stages, lower currents only one phase is active for lower switching losses, progressively enables other phase as current increases. This give much better low power efficiency.

Best FETS available, MX60/FM60 and XW use a 8 year old FET model

No relay, uses best in class DirectFET packaging for lowest on resistance

Three phase switching, lowers peak current losses in FETs and capacitors

Very short PCB paths internally, minimizes resistance losses

In nominal 48V arrays to 48V battery bank will operate at better than 99% in a large current range, not just a single picked value.

Yes, the Classic also does "some" of this stuff. For the moment, we keep the relay to provide reverse polarity protection, but hopefully most people are very careful when they connect up their charge controller, or, typically, have a professional wire the system. Reversing the battery leads does happen some times. I imagine that wouldn't be covered under warranty though ??

Like I said, The MX measured higher than 99% many years ago under best possible conditions, but we did not want to actually say that. Personally, I rather round down to avoid any extra confusion and sometimes give people pleasant surprises.

"Direct FET"s, huh ? You mean the IR parts ? I have looked at these over the years but they weren't low enough on resistance last time I checked. Still, a neat packaging idea. This shows that they can work well as long as they don't have switching losses, and they wouldn't in a relay replacement application.

But Henry, what happened to your XW-CC enthusiasm ?? What about its stepdown efficiency ?

boB

AntronX · March 2010

Re: MPPT stepdown efficiency

westbranch wrote: »

I am in the process of thinking through the best way to put together a new system for my cabin, probably @ 24 V.. Array planned for ~ 1Kw.

Why not go for 48V if you are starting from scratch? At 12V you need larger wires for interconnecting batteries and inverter. Also, since you are off-grid, you could experiment with 48V powered lighting and small appliances, bypassing inverter losses.

AntronX · March 2010

Re: MPPT stepdown efficiency

Solar Guppy wrote: »

Dynamically enabled 3-phase buck switching stages, lower currents only one phase is active for lower switching losses, progressively enables other phase as current increases. This give much better low power efficiency.

Do you mean 3 parallel buck switching stages, meaning there are 3 separate buck regulators in the unit?

Can you ponder on the idea of 600V input charge controller with 48V output? I just started learning SMPS engineering from design books, so I am a beginner in this. What I understand, is that all these 150V to 48V units available today are single stage buck regulators. Is it true that trying to down convert 600V to 48V in a buck converter results in high stresses on the FETs and the coil therefor making this topography unfeasible for this application? Does this mean that transformer must me used for converting such large voltage difference? But transformer has fixed ratio, therefor MPPT is not possible without extra buck regulator after it to allow for flexible input/output voltage ratios. Can you comment on design implications of this application?

mike95490 · March 2010

Re: MPPT stepdown efficiency

I don't think there are any 600VDC MPPT input controllers.

AntronX · March 2010

Re: MPPT stepdown efficiency

mike90045 wrote: »

I don't think there are any 600VDC MPPT input controllers.

Yea, I know. That's why I want to make one.

Solar Guppy · March 2010

Re: MPPT stepdown efficiency

boB wrote: »

But Henry, what happened to your XW-CC enthusiasm ?? What about its stepdown efficiency ?
boB

The XW Stepdown efficiency is not as good as MX/FM or the MS units, but we both know why its that way

I still have enthusiasm for the XW-Mppt , its a solid performer in 48V systems and hard to beat the integration for a all XW system. I really like the bullet proof Mppt the MS unit has ( not my work ) the harvest efficiency without all the troubles dynamic tracking can get into.

AntronX wrote: »

Do you mean 3 parallel buck switching stages, meaning there are 3 separate buck regulators in the unit?

Can you comment on design implications of this application?

Yes, three parallel buck switches, very efficient and quite design

Leave the HV to the pro ( I don't even have the back ground for this ), You would need a phd in power electronics, yes you would have a transformer and I'd think likely a resonant class converter with active synchronous on the secondary

Its a really big deal doing 600V stuff, I would say don't mess with it unless you really know what you are doing

AntronX · March 2010

Re: MPPT stepdown efficiency

Solar Guppy wrote: »

Its a really big deal doing 600V stuff, I would say don't mess with it unless you really know what you are doing

Well, I know enough about this stuff to not get killed (learned to respect electricity at an early age). Other than that, I have no experience with building SMPS yet. I want to start with a simple 24V to 12V buck converter with constant voltage, variable current input and variable voltage and current output. It will dump excess power from my primary 24V bank into secondary 12V battery.

I think there is a large hole in the market without 600V capable battery charge controller. You can see that by frequent posts on this forum from people needing to run long distance PV wires.

Solar Guppy · March 2010

Re: MPPT stepdown efficiency

AntronX wrote: »

Well, I know enough about this stuff to not get killed (learned to respect electricity at an early age). Other than that, I have no experience with building SMPS yet.

I think there is a large hole in the market without 600V capable battery charge controller. You can see that by frequent posts on this forum from people needing to run long distance PV wires.

A buck converter is simple stuff and with low voltages can be great project for those that want to learn power design basics. As the voltage increase you get into safety issues that are very serious. The Energy in capacitors is the square of the voltage so when a FET fails, or your control software hick up's, or whatever, the available energy will blow holes in things and that's the minimum it will do.

This is not hobby stuff, to be safe you need literally a phd type back-ground to power electronics to design and debug this class of electronics ... I've worked in electronics for 30 years and I won't touch the stuff hardware wise.

As far as need, consider for what is essentially the electronics of a GT inverter with a high current DC back end ( vs lower current AC ) time will tell. Pricing could be a concern for the purchaser can get a quality mppt 60 amp charge controllers at @ 500 street priced, with an HV unit, you have everything in packaging, electronics and more the GT inverters have and those sell for the 2K range.

AntronX · March 2010

Re: MPPT stepdown efficiency

Thanks for you insight, SG. I am off to get me a phd then (...well, maybe not.)

RWB · March 2010

Re: MPPT stepdown efficiency

Solar Guppy - I still have enthusiasm for the XW-Mppt , its a solid performer in 48V systems and hard to beat the integration for a all XW system. I really like the bullet proof Mppt the MS unit has ( not my work ) the harvest efficiency without all the troubles dynamic tracking can get into.

Solar Guppy can you tell me more about the bullet proof MPPT the Morning Star Charge Controllers have vs others. I use alot of the 15A MPPT Sunsavers and really like them and which Morningstar had a competitor for the FlexMax 80.

How does the MPPT Performance on the MS units differ from the others out there? How does it compare to Bluesky MPPT?

How you don't mind me asking in this thread, I think the 600v ideas are interesting also.

westbranch · March 2010

Re: MPPT stepdown efficiency

AntronX, a quick economic analysis,
my wife is just finishing off her PhD...
4 years study and research, $10,000, for first 3 years at Univ. = ~ $ 30,000 plus a lot of headaches
As SG said GT inverter ~$2,000... And no headaches.... Hmmmm...8)

Eric

westbranch · March 2010

Re: MPPT stepdown efficiency

OK back to the original Efficiency Query.

I made a decision to go with 1320W of 110W panels (rather than 8 - 180W, 24 V, 1442 W) as they offered more flexibility re configuration, 3 x 4 and 4 x 3 and just ~ 120 W less. 24 V system chosen as existing Gen can handle charging if > 3 days no sun.

Used the Outback array tool for what would work, max array size 1600W

When I plotted the interception points for the W vs V on the Outback efficiency graph, there was < 1 % variation between the 12 v panels vs just a bit > 1 % for the 24 v panels.
It is a pretty coarse resolution graph... calling it 1 % for both cases.

So the question is: does it really make a difference to run a 72 v (4 panels x 3) with a 1.85355% Voltage loss Array to battery or a 54 V (3 panels x 4) with a 2.33767% loss array to battery?
Net to Net basically only a + .5% difference here for 72V including a -1% less efficiency in the CC.

I'm thinking it will be other factors than affect the choice more than % line loss
thoughts?

Eric

Cariboocoot · March 2010

Re: MPPT stepdown efficiency

For what it's worth, I had a 72 Volt array on my MX60. In Winter conditions its output exceeded the MX's max input Voltage. That being so, I'd go with the 3 x 4 set-up.

How severe is your line loss?

westbranch · March 2010

Re: MPPT stepdown efficiency

3 panels x 4 4 panels x 3
Pv - CB 1.02333 1.36444
CB - BB 0.16089 0.28603
BB - CC 0.09653 0.11441
CC - Batt 0.5728 0.5728

something is not working here right now

Cariboocoot · March 2010

Re: MPPT stepdown efficiency

You've completely lost me there, Eric.

niel · March 2010

Re: MPPT stepdown efficiency

marc,
he is giving the losses between each segment, but also listed it as it would be for both the 4x3 and 3x4 pv arrangements. now i do hope that the figures he listed for the voltage drops where in % and not volts as he did not specify.

westbranch · March 2010

Re: MPPT stepdown efficiency

Niel has it .... they are % 's

I don't have a DVM good to 5 decimal point either.. Do they even make one?

Tried to insert a table but the formatting gets dropped by the BB program!:grr:grr

EJ

BB. · March 2010

Re: MPPT stepdown efficiency

use the tags {code} text {/code} (use square brackets instead) will not suck out spaces...

Also, Use the Fixedsys Font (select your text, then set font)--that is single space font (no proportional spacing)--makes it easier to keep tables aligned.

[FONT=Fixedsys]Word       spaces    numbers
cat        space     123[/FONT]

If I am cutting and pasting from a web page--I will open "notepad" or other simple text only editor--strips out all of the formatting and graphics from the tables (sometimes, tables are too trashed anyway to cut and paste).

-Bill

Cariboocoot · March 2010

Re: MPPT stepdown efficiency

Eric, you're getting hung up on the fine details! Five decimal points? Take your DVM, set it on a lower Voltage scale, and grab each lead with a different hand. Squeeze. Notice anything? Even if you had such a device, in the real world you couldn't get consistent, dependable readings that fine. Slight increase in resistance to a connection due to pressure variations and you get a different reading.

Your efficiencies between the two panel set-ups are very close. But do take a look at the possibility of four panels in series causing much too high VOC for the MX60. Remember how cold it gets up here?

I had 3 '24 Volt' panels in series. Nominal 72 Volts, but in reality the VOC was over 150 in Winter, despite the panel ratings being 133.2 (44.4 * 3).

westbranch · March 2010

Re: MPPT stepdown efficiency

Marc, thanks for the Vtoc caution. The Outback calculator came up with
112V Vtoc from a 4 in series string and
84V Vtoc from a 3 in series string

with the temp set to - 30 C, mind you the temp button did not seem to change anything when moved.

looks like you were getting > 11% more output. Even adding 15 % to the above would keep me below 141 V max...

the most benefit comes from the ~ 70 foot distance from the panels to the CC. If it turns out to be more I will go with the 4 in series ....

EJ

MPPT stepdown efficiency

Comments

Categories