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Efficiency of MX60 for 24v to 12v conversion

Posted: Tue Nov 29, 2005 11:49 am
by JPF
I am planning a system for use in my travel trailer with the following components:

Fx2012
Mx 60
Mate
(2) Rolls 4000 series 6v 410 ah batteries wired to provide 12v
3.0 - 3.5kw tri fuel or diesel generator
Either:
(3) Sharp 123 (12v)
or
(2) larger 24v (ie Sharp 208)

I read in previous posts that the losses going through the charge controller are typically 5% and have been told that I can expect to lose another 5% reducing voltage from 24 to 12 volts. What will the losses be if I go with the 24v panels through the Mx60 to 12v battery setup?

I am leaning toward the two 24v panels because of space limitations and price per watt but am wondering about other benefits / drawbacks. I have heard conflicting things about higher voltage panels performing better in marginal sunlight conditions. Could someone speak to this?

Are there other benefits or drawbacks of this type of setup?

As I am very new to this stuff any comments about my proposed setup would be appreciated as well.

Thanks in advance for your help,

Jeff

Posted: Wed Nov 30, 2005 4:03 am
by Kent Osterberg
The efficiency curves in the MX-60 manual suggest that the MX60 efficiency only drops about 1% if you wire the PV array in a 24-volt configuration rather than a 12-volt configuration. The improvement in wire losses between the PV array and the MX60 can easily make that up.

The system that you are proposing operates the MX-60 on the low end were it's effiency is the poorest. Still, the efficiency should be about 95% when in full sunlight.

For such a small system, the MX60 may not be the most economical choice. The price for a Sharp 123 is about the same as the price of a MX-60. Compare option A: two Sharp 123 modules and a MX60 and option B: three Sharp 123 modules and a brand X charge controller. Cost of the two options is similar but you'll get more charging current from option B.

Posted: Wed Nov 30, 2005 5:16 am
by JPF
Thank you for the response.

Option A is actually two higher wattage panels such as the Sharp 208's which have a nominal voltage of 24 volts. So, from what I understand, I would need a charge controller like the MX60 to reduce that voltage to charge my 12 v battery setup. Given that I am trying to maximize wattage in a small space and that loss in efficiency is only 1% for voltage reduction it seems like this is the way to go.

Would wiring the two Sharp 208's together for 48 volts improve the performance of the mx60?

Thanks again

Posted: Wed Nov 30, 2005 8:53 am
by Kent Osterberg
It is a lot easier to find 24-volt modules than 12-volt moduels these days. An old fashion charge controller just won't do the voltage conversion. The MX60 opens up a lot of options including PV modules with strange voltages.

The Sharp 208 does make a lot of power in a small area. It's probably the most power per sq ft that you can get at present.

The MX60 efficiency will probably drop another 1% by wiring the array at 48-volts and again you'll get at least part of that back by reducing the wire losses between the array and the MX60. For better efficiency, the MX60 would like more input power - a larger PV array.

Posted: Wed Nov 30, 2005 9:44 am
by crewzer
The question of MX-60 efficiency when operating with relatively low-power arrays and/or a >>12V to ~12 V mode has come up before. See:

http://www.outbackpower.com/forum/viewtopic.php?t=777 and
http://www.outbackpower.com/forum/viewtopic.php?t=546

As I understand the MX-60, the Vin, Vout and Iout values are measured, but the Iin value is calculated, so we canÔÇÖt use the display numbers to accurately calculate efficiency.

My own systemÔÇÖs specs (725 W STC PV array, 36 V PV array to 12 V battery configuration) and performance has caused me to wonder about this issue as well. For example, on a recent sunny and fairly cold day (ambient temp ~36 F), I saw 42 A going into the batteries. Not bad, but no better than my arrayÔÇÖs total Imp spec at 12 V nominal.

I ordered a 100 mV / 100 A shunt today, and IÔÇÖll install it between my array and my MX-60ÔÇÖs input. I should be able to read the voltage drop across the shunt using my DVMÔÇÖs 400 mV setting. Between this shunt, the MX-60ÔÇÖs display, the MateÔÇÖs display and the Link-10 meter (along with its shunt), I should be able to collect enough data to make a stab at calculating a reasonably close efficiency factor.

More later
Jim / crewzer

Posted: Sun Dec 04, 2005 2:17 pm
by crewzer
The new shunt arrived yesterday afternoon. So did the clouds, unfortunately. So, I installed the shunt and I'll try for some useful readings next Saturday.

In the meantime, here is some interesting data:

36 V array -> 12 V battery bank; numbers I recorded from my MX- and Mate display at ~Noon on November 25, 2005:

Vin 46.0 V
Iin 14.0 A

Vout 13.4 V
Iout 42.1 A

That's an indicated 644 W in and an indicated 564 W out, or 87.6% efficiency.

36 V array -> 24 V battery bank (temporarily rewired); numbers I recorded from my MX displays at ~Noon on December 3, 2005:

Vin 46 V
Iin 12.2 A

Vout 27.4 V
Iout 19.2 A

That's an indicated 561 W in and an indicated 526 W out, or 93.7% efficiency -- quite an improvement!

Posted: Wed Dec 07, 2005 4:21 pm
by crewzer
Following up... I was able to make some quick recordings and measurements today @ ~12:40 EST on my system (nominally 36 V in and 12 V out) with the new PV array shunt installed. Here's the data and calculations:

Indicated (on MX display):

Vin 47 V
Iin 13.2 A

Vout 14.6V
Iout 37.5 A

ThatÔÇÖs an indicated 620.4 W in and an indicated 547.5 W out, or 88.2% efficiency.

Measured:

Vin 47.3 V (DVM measured across the MX-60 PV input)
Iin 12.5 A (DVM measured 12.5 mV across the 100mV / 100 A shunt)

Vout 14.62V (DVM measured across the the MX-60 output)
Iout 37.3 A (Link 10 indicated 37.1 A, plus 0.2 A for Link 10 consumption)

ThatÔÇÖs a calculated 591.25 W in and an calculated 545.33 W out, or 92.2% efficiency -- that would be better, then! :grin:

The local weather forecast for this Saturday looks promising... I'll try again.

HTH,
Jim / crewzer

Posted: Sat Dec 10, 2005 10:03 am
by crewzer
Following up... I was able to make another set quick recordings and measurements today @ ~11:45 EST on my system (nominally 36 V in and 12 V out) with the new PV array shunt installed. Here's the data and calculations:

Indicated (on MX display):

Vin 48 V
Iin 13.0 A

Vout 13.8 V
Iout 39.6 A

ThatÔÇÖs an indicated 624.0 W in and an indicated 546.5 W out, or 87.6% efficiency.

Measured:

Vin 48.4 V (DVM measured across the MX-60 PV input)
Iin 12.2 A (DVM measured 12.2 mV across the 100mV / 100 A shunt) -- cut & paste typo corrected -- thx, boB :oops:

Vout 13.8 (DVM measured across the the MX-60 output)
Iout 39.4 A (Link 10 indicated 39.2 A, plus 0.2 A for Link 10 consumption)

ThatÔÇÖs a calculated 590.48 W in and an calculated 543.72 W out, or 92.1% efficiency -- that would be better, then! :cool:

It would seem that the "Amp in" calulation displayed on my MX' screen is consistently on the high side.

That's all, Folks!
Jim / crewzer :oops:

Posted: Sat Dec 10, 2005 11:27 am
by nsingh
crewzer wrote:It would seem that the "Amp in" calulation displayed on my MX' screen is consistently on the high side.
crewzer -
Yes, that is true. The displayed values do vary slightly with operating temperatures, too. The MX60 meter is a good/useful reference -ÔÇô it was never meant for efficiency readings/measurements.

I must commend you on undertaking the efficiency experiment to prove that the MX60 is more efficient that what the built-in meter was displaying. My efficiency numbers are slightly improved over yours, but as long as our efficiency numbers are above 90% -- weÔÇÖll all be satisfied or would we? :shock:

On Behalf of OutBack, thank you, crewzer, on your diligence and curiosity on the independent efficiency testing and measurements.

Good job! =D>

regards,
nsingh

Posted: Sat Dec 10, 2005 1:44 pm
by crewzer
Nevin,

Glad to help. Although boB has cautioned me and others on this issue in the past, I have to admit that I'm pleased to confirm that my MX-60 operates more efficiently than the display suggested. Allowing for the wide input-to-output voltage ratio (~3.5:1) and other "12 V" factors (relatively low total power, relatively high current), I'd say that a 92% power efficiency is pretty good! :cool:

I may try this again later this month using a 36 V in and 24 V out configuration. :roll:

Regards to all,
Jim / crewzer