Classic 200 Conversion Efficiency

[rwsaustin]

I have a 3900W PV array and a Classic 200 charging a 360Ah LiFePO4 battery. I'm off the grid. The cells are CALB 180Ah in 2p16s. They were installed April 2019. I'm in Central Texas. It's approaching 100F, so I'm using every kWh I can to run the air conditioner. Typical daytime load is 1150W. I'm able to charge my battery to about 50% capacity most days with the A/C running. I am using AUX 1 in Diversion mode to control my A/C. I am tweaking the settings so it will turn off the A/C early evening, then I can run it on a timer for an hour or so at bedtime and leave enough in my battery for my nighttime loads, mostly the fridge.

From the Local App (aka LocalStatusPanel aka Midnite Solar Status Panel) log today, I recorded a peak power reading of 3119W. Ambient temperature was around 95F. PV modules could have been 120F or 125F.

Input voltage 119.6
Input current 31.9
Battery voltage 53.4
Output current 58.1
Watts 3119
FET temp 77.6C
PCB temp 70.2C

If I multiply voltage times current to get power I get:

Input power calculated 3815.24
Output power calculated 3102.54

That's a conversion efficiency of 81%. Should I be getting more? Am I doing the math right?

Robert

[Vic]

Hi Robert,

How is the Input current being measured?  IIRC,   this is not shown on the MNGP,  or on the LA display.

It does seem that that Classic is roasting.   Have never seen any FET temps above 58C,   ever.

What do you suppose is the ambient temp around that Classic?

Previously,  MN Engineering has noted that making accurate measurements of Efficiency with these MPPT CCs is difficult,   for several reasons.   But will let others address that.

As an aside,   we run Priority A/C  in the power room,   although,  we are not in TX.   FWIW,   keep cool !  Vic

[ClassicCrazy]

I think the Classic rolls back the charging current when it gets too hot so that would play into it too wouldn't it ?

Larry

[Vic]

I think the Classic rolls back the charging current when it gets too hot so that would play into it too wouldn't it ?

Larry

Hi Larry,

Yes,  seems that when the Classic limits its output current,  it reduces the loading on the PVs (higher voltage than true MPP V).   Increased Vin  increases heating,   and so on.   IMO,  to a certain extent,  a hot CC is less efficient than one with identical Vin and output current,  but 20 or 30 C  cooler.

73  Larry,   Vic

[ClassicCrazy]

here is data from very hot day from two different controllers captured by My Midnite .
Classic 2 has higher input voltage from PV compared to Hoop PV
The vertical line is positioned on the peak FET temperature and that is the data showing on the left for that time

Larry

[boB]

The Classic can not dissipate 700 watts. It might be able to dissipate 50W to 75W ?   If it were dissipating 700+ watts it would probably go up in flames.

Yeah, it can get pretty toasty though when maxed out.  And the 200 will be less efficient than say, the 150 which is why the 200 is not allowed to put out quite as much current as the 150.  The internal inductors are actually the hottest thing in there.

We don't measure efficiency or publish any figures but it is in the 90% to 97% range depending on input voltage and output power/current.

Because it is more optimized for medium to large output currents, it will be pretty low-ish at low output powers though.  But since the Classic is fairly small, a moderate amount of power dissipation (50W--75W ?) will be fairly hot at high input voltage and high output power.

OK, so that being said, do not trust the input current reading nearly as much as the battery output reading.  Input current is very approximate.  Not calibrated at all.  It is mainly for a relative measurement unfortunately.   I wish it were as good as a Fluke meter.

Efficiency is hard to measure accurately.  You can measure much more accurately with other instruments but will also need to measure the output current at the same time with matched instrumentation.

[rwsaustin]

Thanks to all who replied.

Vic, the input current is displayed on the MNGP. I think you have to push Status button twice to get to that display. I haven't seen it on the Local App except in the graphing section. I was able to export to a CSV to capture the numbers.

So boB, it sounds like the Classic's input current measurement is not to be trusted. I have a spare Whizbang Jr. shunt. I may get my electrician to install it on the PV input so I can measure input current with an external meter. It's certainly a good number to know. So I can trust the input voltage number, the Whizbang Jr. output current and the battery voltage measurements?

Larry, I'll take a look at your numbers and compare them to mine.

Vic, my battery room where the Classic resides is probably around 90 or 95F. I'll measure it tomorrow. The room is about 50 sq ft. It is insulated. The floor is a very thick concrete slab shared with the house, so I am sure that cools it somewhat. The battery room has a small exhaust fan powered by the inverter, but I don't have a make-up vent to supply any ventilation. I want to put in an air intake from the inside of the house where it is cooler. A larger exhaust fan is also an option. As an experiment, I guess I could put in a cooler full of ice with a fan blowing on the Classic and see if that improves the output. If I can get a few hundred more watts out of the deal, it may pay to put in a small air conditioner in the battery room. I am opposed to moving the Classic inside. Any other ideas?

[ClassicCrazy]

Robert - if you have internet set up the my midnite online account  and you can see a lot of  history and also change which parameters you want to see in graphs .
That is how I did the graphs above.

Larry

[Vic]

Hi Robert,

Thanks for reminding me,  just where the Input current can be found on the MNGP.   Looked for it yesterday,   but  paged through the Status screens too fast.

To me,   the input current is of no use.   Just interested in Output power,  current,  Net Ah,   and  WbJr current.   Occasionally will look at last Voc on the MNGP,   but,   that is just yours truly   ...

The Shunt that the Wb uses,   produces only 1 mV per ten amps of current,   so most DMMs cannot accurately measure the levels of current that interest most of us.  A five  digit Lab mV meter could do fairly well directly reading fairly low currents from a 50 mV 500 A shunt.

The WbJr measures current very accurately,   BUT  it communicates with the Classic serially over that single wire.  The code to convert this resides inside the Classic.

A  boB  answered a question,   some years ago,   from Member dgd regarding the data format for the WbJr comms with the Classic,   which is on this site.

The "easy" way to add a second shunt to read the input current to your present Classic,  would be to add a shunt,   AND a second Classic.   This would help with the heating of Cl #1,   if you divided the PV array into two,   one array for each Classic.   Yea,    know that this would be expensive.

And,   still the input current to the Classic  seems to be a passing interest.   The conversion efficiency is what it is.   You could make some changes to improve this number a bit,   but,  perhaps focusing elsewhere  could serve you well.

As you know,   the CC has a given efficiency for a set of parameters.   If conversion losses create internal heat,  this heat causes the CC's temperature to rise above AMBIENT air temp.   So,  for much of the day drawing in outside air that is warmer than the room temp is a net negative   ...   obviously.

Insulation of the power room can help and hurt.   If you have cool night air,   circulating outside air at night  will help,   again obvious.   More insulation in the power room  can hurt without cool nights and outside air circulation   ...

EDIT: Will add that a Classic 150  is a few points more efficient than is the Cl 200.  You may have a good reason for choosing the 200,  distant PVs,   racking convinience,   etc.  Higher input voltage to the CC  has a fairly large negative impact on CC efficiency.

Assume that you might be running strings of four 60 cell PVs,   perhaps,   even  strings of four 72 cell PVs.   A bit hard to guess from your noted input voltage to the CC.   BUT  if possible,   strings of three on a 48 V nominal system  will help CC efficiency quite a bit,   vs   a configuration that yields a higher input V.<

Done stating the obvious,   for now.  Thanks again for the input current readout reminder!     Vic

[boB]

So boB, it sounds like the Classic's input current measurement is not to be trusted. I have a spare Whizbang Jr. shunt. I may get my electrician to install it on the PV input so I can measure input current with an external meter. It's certainly a good number to know. So I can trust the input voltage number, the Whizbang Jr. output current and the battery voltage measurements?

For efficiency measurements, I would use two WB Jr.s One on the PV negative and one on the battery negative.  The battery current from the Classic is OK but not nearly as close as the Whizbang Junior.

I suppose the voltage is close enough, especially if your system is 48V.

You will want to make sure that the voltages are STABLE when making this measurement because it is hard to get a good number when the power is changing.
You can help with this by maybe finding the Max power point voltage and then setting the Classic for HYRO mode to keep that voltage to one preset voltage.
Just a thought.

[rwsaustin]

I have 5 parallel strings of 4 PV modules in series. 3 of the strings are newer than the first two. The newer strings are rated at 195W each but tested at 200 at standard conditions. The older ones are rated at 190W each. I have attached a PDF of the results of the Classic Sizing tool. These will be optimistic by a few percent because my array is fixed at latitude so there is around 15 deg. of tilt error. The tool says I am marginal with 1 Classic 200 because of possible hyper VOC. However, that is at cold temperatures, so that's not the problem today. The most glaring discrepancy I see is in the maximum power point voltage. The sizing tool says I should expect about 150V, but the Classic is pulling at 116V presently (at about an hour past solar noon).

Ambient temperature is 99F and so is the battery room. I could probably cool the battery room to 90F by adding an air supply from inside the house and increasing the size of my exhaust fan. My plan is to leave the battery room door open tonight with a fan running and "cool" it down to nighttime temperature of just below 80F. Then in the morning I'll close the door and add an ice chest on a table with a fan blowing on the Classic and see if i can cool her down some and see how that affects the power output.

So boB, your suggesting is to add a WB Jr. and measure the PV current with circuitry external to the Classic? I'll search the forum for this. Was your suggestion to try Hydro mode to stabilize the voltage or to find a better MPPT than the one the Classic found? By my panel specs, MPPT voltage should be about 150, by my Classic is pulling at 116V.

Vic, you suggested strings of 3 instead of 4. I currently have 20 modules (5 strings of 4), but I might be able to add a module and go with 7 strings of 3. I'll have to ask my installer if this is feasible. It would not be easy to rewire everything, and I'd be mixing two types of modules in some strings. I have two spare modules, and I have room for another module on the roof. I think my combiner box is maxed out. When I run this through the sizing tool, it says I would be better off with a Classic 150 than my 200, but your saying that that would have given me better conversion efficiency?

Robert

[Vic]

Hi Robert,

Those PCs are 72 Cells.    The Sizer's string  voltage calc is  just 4X 37.8V  --  this is STC Vmp,  which is   at  25C (77F) cell temps.

72 Cell PVs are rough on 48 V systems,   but at  least you have Li  batts.

Do you have a spec sheet  for the PVs?
You might be  able   to  get   away with strings of two PVs,    with  your  batts.

At the same input voltage,     the Cl 150 is a few points more efficient  than the Cl 200.   Your input voltage with strings of three PVs is marginal,  even in summer with a Cl 150.   Later...

More  later,  Thanks,  Vic

[Vic]

Just briefly,  Robert,

Strongly consider strings of  two  PVs.   You probably do not  use temp  compensation,  or  EQ,     so using the  130% of highest batt V,  still  gives you margin with strings of two.   A Cl   150  would be a lil bit more efficient,  but if it is not too much work  ...   yea,   lotsa combiner breakers,     etc    ...

Anyway,  I know nothing of LiPo batts.

EDIT:   BTW,  have been wondering about adding a shunt and a separate WbJr  in the negative lead  of  the PV into the Cl 200,   and using a toggle switch to switch twix the violet wire on each Wb.  Do not know how long it might  take for  things to sync,    and perhaps it   could cause some error    ...   anyway, to  me,   the   input current is of little  interest   ...<

Just a  thought.     Vic

[rwsaustin]

Last night I left the battery room door open with a fan in the door. I got the temperature of everything down to 85F/29C. Nighttime low was about 78F. This morning, I put in an ice chest full of 40lbs of ice with a fan blowing air from the ice chest onto the Classic. High temperature inside the battery room today was around 90F/32C, about 10 degrees F cooler than yesterday. FET temperature is about 15C cooler. Peak output did not improve. It was sunnier yesterday.

I think I could get similar cooling capacity by increasing my exhaust fan size and getting the intake air from inside the house. From today's results, I don't think it will help much with my efficiency issue.

So the consensus seems to be that I would have been better off to have strings of 2 instead of 4 and operate at half the voltage. A Classic 150 would have been a better match for this configuration. I'll check with my installer to find out why he chose strings of 4. It may have been a trade-off between cable size, complexity, and combiner box limitations instead of efficiency.

Robert

[Vic]

Hi Robert,

Fine on cooling the Classic and its  room.

Thinking more  about  the string  config  some more    ...     noticed  today,   that you entered your max temperature as 120   F,    and thinking also about your Vin with strings of four  PV as 120 V.    This should equate to about 60 V in with strings  of two PVs and identical conditions.   This may not be quite enough,     as  with even warmer  WX outside  that  Vin would probably decrease some  more    ...

More later,   Thanks for the  update.   Keep cool,   Vic