Replacing PWM solar charge controller with MPPT

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Mark

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#269978 17-Apr-2020 22:24

Hiya,

I've looked at this too long and think I've confused myself :-)

Could someone tell me if my maths/logic looks sound for the differences when swapping a PWM with an MPPT charge controller ?

https://docs.google.com/spreadsheets/d/e/2PACX-1vSadIC_ImX78WUM7BS_7LzmtPAu2h9ZPAqGtcxVE6k0eHmgMYi_wQXyvxQgMdMpOIMIAaCsKX-gmF_M/pubhtml

The numbers are all based on "perfect conditions I would guess) I know I should get better but what I worked out looks to good.

Reason I'm swapping : a cottage I own came with a solar setup and I impulse bought the MPPT controller as it was cheap and I had vouchers :-) Plus now the tenant is out, I get to tidy up a god aweful mess the previous owner "designed".

SomeoneSomewhere

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#2464508 18-Apr-2020 00:19

Your charge power with the PWM controller is wrong - that should be about 185W I think.

You will see a bigger improvement if the battery is at a lower state of charge. But less improvement with lower light, as the peak power voltage will reduce to nearer the battery voltage.

If there's a substantial run between the panels and the charge controller, and the input voltage is high enough, it may be worth wiring them all series.

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gregmcc

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#2464528 18-Apr-2020 06:09

Unless your an Electrician, you legally not able to solar work, this does not fall within the homeowner exemption (been a landlord the exemption would not apply anyway), too top it off this also may fall in to the high risk category which will require an inspection.

raytaylor

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#2464574 18-Apr-2020 10:14

gregmcc:

Unless your an Electrician, you legally not able to solar work, this does not fall within the homeowner exemption (been a landlord the exemption would not apply anyway), too top it off this also may fall in to the high risk category which will require an inspection.

Lol this is well within the extra low voltage range.

Offgrid solar only requires an electrician when you start looking at strings of panels which exceed 120V ripple-free DC or on the 240v side of the inverter.

Looking at the spreadsheet this system runs at even less voltage than the Power over Ethernet standard.

Ray Taylor

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Spreadsheet for Comparing Electricity Plans Here

raytaylor

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#2464612 18-Apr-2020 11:31

Replying to OP:

So I cant see the calculations behind your spreadsheet cells, only the values/answers.

This is what I have gathered

- 2x nominal 12v 60w panels in a series string. 17.5v each = 35v per string.
- 2x strings in parallel.

= 2P2S = 35v 6.86A or 240 watts

The battery array is effectively 24v 232ah

A PWM solar controller will trim off the excess voltage when charging the batteries.
So your input from the solar of 35v 6.86A (240 watts) becomes 27v 6.86A (185w) or approx 77% efficiency less the controller deficiency.

But an MPPT controller will convert those extra volts into amps so you can get close to 100% efficiency less the controller deficiency - which results in usually around 95-97%

However one thing to note is that many controllers are smart 3-stage ones and will charge at full amps up to around 80% where they then start slowing down the charge rate from the 80% to 100% state of charge. Add on an extra hour and you should be sweet.

On a rainy day you will find even more improvement - an MPPT controller will get that extra 10-20% out of the panels when it makes more of a difference.

Ray Taylor

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Spreadsheet for Comparing Electricity Plans Here

Mark

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#2464630 18-Apr-2020 11:59

It's the "efficiency" bit I think I've gotten confused on ... a few places say MPPT is roughly 95% efficient and PWM is roughly 74% so I did "(Charge voltage * PWM Charge Amps)*Controller Efficiency" .... maybe I'm using that value in the wrong place ? Or interpretted it wrong ?

I'll reshare the sheet so people can see calculations I thought it would look like it does on Google Sheets

https://docs.google.com/spreadsheets/d/e/2PACX-1vSadIC_ImX78WUM7BS_7LzmtPAu2h9ZPAqGtcxVE6k0eHmgMYi_wQXyvxQgMdMpOIMIAaCsKX-gmF_M/pub?output=xlsx

Mark

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#2464632 18-Apr-2020 12:03

gregmcc:

Unless your an Electrician, you legally not able to solar work, this does not fall within the homeowner exemption (been a landlord the exemption would not apply anyway), too top it off this also may fall in to the high risk category which will require an inspection.

Nope ... I can work all I like on this side of the solar setup, low voltage DC, the DC-AC inverter connection to the house however is a different matter, that has been connected by a licensed electrician and I'm not going to that side of things.

(though I will admit I'm still paranoid around batteries that could weld screwdrives together with an accident cross!)

raytaylor

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#2464848 18-Apr-2020 15:16

Ahh right I think i see where you may have gone wrong...

The PWM controller is inefficient because of the voltage trimming, meaning the watts-in-from-solar does not equal watts-out-to-battery.

So i wouldnt calculate an efficiency rating or use the 1.00 vs 1.30 ratings. Just convert it to watts, then take off 2% for self consumption of the controller, then convert back to amps.

The only difference in the calculation then is that the PWM has an extra step where you calculate the voltage difference between the solar input and the charging voltage that gets trimmed so 35-27 = 8 volts and then multiply that by the amps going through so 6.86A and that gives you the trimmed 54.88watts

So
PWM is input watts less trimmed watts (54.88) less 2% self consumption = charging watts / 27 = charging amps

MPPT is input watts less trimmed watts (0) less 2% self consumption = charging watts / 27 = charging amps

Ray Taylor

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Spreadsheet for Comparing Electricity Plans Here

Mark

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#2465105 18-Apr-2020 22:21

That makes sense .. I shall redo my logic and see what I get.
On a related topic, I swapped in the MPPT controller today, it has wifi so I'm hoping I can get some interesting stats out of it 😁