MPPT vs standard PWM?

[jimindenver]

in really good conditions, you could see Isc from the panels. Don't forget the first 10 years, Panels produce MORE than spec, so that they will be within 20% spec after 20+ years.

MPPT circuit produces the "extra power" only in bulk, and drops back to PWM mode in absorb and float, so it's helping you in the first 3 hours of charge.

I know I'm new here and don't want to step on any toes but the "extra power" from the buck converter is always there, it is only maximized for fastest charging in bulk mode. My system with a Schottpoly 230w panel rated at 8 or so amps Isc can produce the same 16a in PWM as it does in MPPT. With my twin 245w systems, I can run loads up to 34a at 12v before the battery starts loosing voltage and the controller switches to MPPT. This allows our bank to stay at float for most of the day while the panels handle the loads except hair dryer and microwave use. Even then at 34a it doesn't take long to get back to float.

[PNjunction]

One thing to check:

If you are not getting the results you think you should and amperage is lower than expected on a very good day, are you discharged past about 20-30% DOD in the first place?

If not, then the battery may already be self-limiting the current because it is in the absorb stage, and not bulk. (The battery does the current limiting towards the end of charge; the controller only limits the voltage).

I had a friend pack all his stuff up in anger and return it, when upon inspection, the battery was just not discharged enough. He was expecting full current charge throughout the whole cycle, not knowing about absorb limiting from the battery itself.

[Jemplayer]

Dave with PWM INPUT CURRENT = OUTPUT CURRENT 365 days a year. With PWM the panel voltage has to be 18 volts for every 12 volts of battery. So for a 24 volt system you are looking at a panel voltage of 34 to 36 volts. A 300 watt 24 volt battery panel should have a spec of roughly Vmp = 36 volts and Imp = 8.3 amps. With two in parallel should get you roughly 36 volts @ 16.6 amps. Assuming you battery is demanding full charge current and sun conditions are noon with panels facing directly into the sun they should recieve 16.6 amps and at 24 volt battery is 16.6 amps x 24 volts = 398 watts.

With MPPT Output Current = Panel Wattage / Battery Voltage. Using the same two panels we can wire them in series, or parallel if we wanted to waste money on copper, that same battery would receive roughly 23.75 amps or 575 watts assuming 95% efficiency in the MPPT.

You are claiming 20 amps which makes me think your meter is not accurate.

Quick question in calculation above i.e. 16.6 amp x 24v = 398 watt.... I don't find that the calculation works on my mppt. Example, I am making 51.2amps x48volts should them = a wattage of 2457 watts, however my wattage on my mppt is showing I am making more than that actually 2900 watts. So is here a problem with my mppt. Is this loss?

[inetdog]

Quick question in calculation above i.e. 16.6 amp x 24v = 398 watt.... I don't find that the calculation works on my mppt. Example, I am making 51.2amps x48volts should them = a wattage of 2457 watts, however my wattage on my mppt is showing I am making more than that actually 2900 watts. So is here a problem with my mppt. Is this loss?

Are you looking at the actual PV output voltage as measured at the inverter, or are you just taking the nominal voltage of the panels? The use of 48 makes me think it is the latter.

Panels which are cooler than the STC or PTC conditions will produce a higher voltage (both V_MP and V_OC.) If you really want to measure the amps to independently calculate the array output power you need to measure the actual voltage too.

[paulcheung]

The voltage is not 48 volt. 48 volt battery bank being charge is lot more than 48 volt, if the mppt report 2900watts and is 51.2 amps, the voltage should be 56.64 volt.

[Jemplayer]

The voltage is not 48 volt. 48 volt battery bank being charge is lot more than 48 volt, if the mppt report 2900watts and is 51.2 amps, the voltage should be 56.64 volt.

Yes, this is the problem, the amps do not match up with the wattage, The amps are lower than what they should be in relation to the wattage. So, any idea as to why?

[mapmaker]

Yes, this is the problem, the amps do not match up with the wattage, The amps are lower than what they should be in relation to the wattage. So, any idea as to why?

Where/how are you measuring the amps? Is it with a battery monitor? --mapmaker

[Jemplayer]

Where/how are you measuring the amps? Is it with a battery monitor? --mapmaker

The MPPT shows me power off the panels in watts and and the boost charge in amps. Its the relation between these two values that does not lined up 100%. The gap gets bigger as the wattage increases. As shown in my previous post.

[mapmaker]

The MPPT shows me power off the panels in watts and and the boost charge in amps. Its the relation between these two values that does not lined up 100%. The gap gets bigger as the wattage increases. As shown in my previous post.

Very few controllers can measure their output current with great accuracy. Those that can, use a current shunt to do so.

Also, do you have any loads (including an inverter) connected while you are making your measurements? A load will pull down the voltage, so that the product of amps X volts does not equal the watts produced.

To explain what you are seeing I would need a lot more details of your system... brand & model of your controller, for starters.

--mapmaker

[Jemplayer]

Very few controllers can measure their output current with great accuracy. Those that can, use a current shunt to do so.

Also, do you have any loads (including an inverter) connected while you are making your measurements? A load will pull down the voltage, so that the product of amps X volts does not equal the watts produced.

To explain what you are seeing I would need a lot more details of your system... brand & model of your controller, for starters.

--mapmaker

Hi Mapmaker - Thanks for the interest, here is a breakdown of my system:

x12 200watt (2400watt total) PV panels Spec:

Voltage (Vmp) = 24.88
Current = 8.19
Open circuit = 31.29
Short circuit = 8.73
Min bypass diode = 3.00

They are installed in 3 strings of 4 panels each. Each string has its own breaker in the DC db board.

I then have a Microcare 60amp MPPT, and a Microcare Sine wave 5KW Off Grid Bi-directional Inverter.

[paulcheung]

The MPPT shows me power off the panels in watts and and the boost charge in amps. Its the relation between these two values that does not lined up 100%. The gap gets bigger as the wattage increases. As shown in my previous post.

What is the voltage, amps and wattage reading at any given time?

[Sunking]

What voltage are you using for the battery voltage? I suspect you are using nominal 48 volts which is an error on your part.

For example lets say you are inputting 2600 watts input and output current = 44.8 amps. If you assumed incorrectly 48 volts you would calculate only 2150 watts. Bu tif you actually measured the real voltage of 58 volts, now you get 44.8 amps x 58 volts = 2590 watts.

[Jemplayer]

What voltage are you using for the battery voltage? I suspect you are using nominal 48 volts which is an error on your part.

For example lets say you are inputting 2600 watts input and output current = 44.8 amps. If you assumed incorrectly 48 volts you would calculate only 2150 watts. Bu tif you actually measured the real voltage of 58 volts, now you get 44.8 amps x 58 volts = 2590 watts.

Ok, so what you are saying is that you actually need to multiply the boost amps by the current (i.e. real time) battery voltage not the nominal system voltage of 48v. Do I understand you?

[paulcheung]

Ok, so what you are saying is that you actually need to multiply the boost amps by the current (i.e. real time) battery voltage not the nominal system voltage of 48v. Do I understand you?

That is correct. the voltage and the current are changing constantly too.

[Sunking]

Ok, so what you are saying is that you actually need to multiply the boost amps by the current (i.e. real time) battery voltage not the nominal system voltage of 48v. Do I understand you?

You almost got it right, but stated it incorrectly.

Power (watts) = Voltage x Current.

What you want to know is does Power Input = Power Output.

To do that that you need 3 pieces of information. Input Power, Output Current and Output Voltage.

OK so as an example you have 2600 watts input to the controller. On the output you measure at the same moment in time 58 volts @ 42.6 amps. What is the output power? 58 volts x 42.6 amps = 2471 watts. So with an Input of 2600 you have 2471 watts output. That is a conversion efficiency of [2471 / 2600] x 100 = 95% efficiency which is really good.

What I think you were doing to determine ouput wattage was using nominal battery voltage fixed at 48 volts which is an error. Under the same example 42.6 amps x 48 volts = 2046 watts which is incorrect.