Pwm vs mppt amps

mountainman

With 12 volts 400 watts on pwm the most amps I've ever saw was 16 on a 50% soc unless I was running a load then around 20 amps. In the real world on mppt how many charge amps should I see?

Photowhit

Trick question?

16 amps...

If you are getting more amps when running a load the batteries are limiting the input.

Photowhit

Might describe your system with battery type and size.

mcgivor

mountainman said:

With 12 volts 400 watts on pwm the most amps I've ever saw was 16 on a 50% soc unless I was running a load then around 20 amps. In the real world on mppt how many charge amps should I see?

MPPT controllers  do not magically improve performance in all cases, but in certain conditions they can extract energy from the panels higher voltages caused by low temperatures, high altitude, or both. In a side by side comparison in a tropical location where temperatures are always ~30°C there would be negligible difference in performance. Move to the mountains where temperatures often drop below freezing the PWM is unable to do anything with the elevated voltage but waste it, MPPT will convert it to current, hense more efficient, in the right conditions.
 https://www.solar-electric.com/learning-center/batteries-and-charging/mppt-solar-charge-controllers.html

mountainman

Gc fla 208 ahr

mountainman

I'm in the mountains 3000 feet. With clear skies and 10f what should I gain with mppt ?

Photowhit

If the batteries are capable of only absorbing 16 amps that's all you will get. Are they older batteries?

The general idea is that if you have 17.5 volts coming in up to 18.9 volts, that the wattage above the voltage that the batteries are charging at the amperage they are providing will be converted into amperage at the lower voltage. Clear as mud?

So if you are in bulk near absorb or close to absorb voltage say 14.8 volts, at 4 amps and your panels are providing 17.5 volts, you would gain about  1.7 volts x 4 amps or about 7 watts per panel, or 28 watts for the 4 panels. This pretty much only applies in bulk, once you hit absorb the batteries start accepting less current. 28 watts is about 10% of the 300 watts you should expect from your panels.

If they panels can produce 5 amps then a few more watts, but apparently your battery is accepting a maximum of 16 amps. Most MPPT CC work at a very high rate of return around 95%. You would also want to reconfigure your array so that you had strings of 2 panels. Most/All MPPT type controllers need a bit of head space above the charging voltage to operate properly.

mountainman

Batterys are only  3 or 4 months old. Don't understand the 230 watts 14.4 volts 16 amps peak charging. If I run the furnace and tv while in bulk 308 watts 21.89 amps 14.1 volts. Panels are 5.29a 18.9v. So normally 18.9-14.8=4.1×5.29= 21 more watts per panel 

Photowhit

If your charge controller says you are getting 21.89 amps from the array when you have a load, but only 16 amps when you are just charging, then it's not a problem of available current/amps. The batteries are only accepting 16 amps....

Do you have a single unit charger/inverter? perhaps it's a mis-understanding of the values?

mountainman said:

Don't understand the 230 watts 14.4 volts 16 amps peak charging. 

It's the way PWM - Pulse Width Modified, charge controllers work, the voltage must be above the charging voltage for the panels to begin with. Then the charge controller "Pulses the current to the system and limits the voltage by switching on and off many times a second to limit the voltage. 

Before MPPT type charge controllers, panels were made in voltage ranges to charge batteries. The higher STC VMP is for several reasons, one of the major reasons is the loss of voltage in Normal Operation. I couldn't find a panel in the 100 watt range that had published NOCT values, but here's specs of a Astroenergy panel;

You can see that the voltage drops from a VMP of 35.77 to 32.67 or about 9%, this is at the panel. Note the over all drop of wattage from 305 watt to 213 watts, about 70% of the rated power vs the Normal operating power. 

Just want to give you a feel, 21 watts is "sort-of" right, but the loss at the panel has already been taken into account using the PWM controller. 

In addition, the times when a MPPT type really make a difference is during bulk charging and over cast days. In normal operation, you may well see zero benefit from MPPT type charge controller. In general systems are designed to discharge only the top 20% during a normal daily cycle. Well the top 20% barely gets you out of bulk. Once a system reach absorb or shortly after the batteries limit the amount of charge they can take in and an array if designed to charge at 13% of the battery capacity will not rely on the 'extra' available since the batteries are already limiting the current.

Hard concept to explain. If you have the capacity an additional panel might be cheaper than a MPPT charge controller, though keeping an even number of panels would be advised as if/when you switch you will want to run strings of panels so you can provide about 30% more voltage than the charging voltage so a MPPT charge controller can work properly.

mcgivor

mountainman said:

Batterys are only  3 or 4 months old. Don't understand the 230 watts 14.4 volts 16 amps peak charging. If I run the furnace and tv while in bulk 308 watts 21.89 amps 14.1 volts. Panels are 5.29a 18.9v. So normally 18.9-14.8=4.1×5.29= 21 more watts per panel 

The state of charge in the morning will influence the charge profile, say for example the battery voltage in the morning is 12.5V, as the sun climbs the voltage will gradually rise as will the current, but by 10 am the voltage is near or at the PWM stage so the battery can no longer accept current so it modulates the width of pulses to hold the voltage at the setting, let's use 14.4V and lowering current . The only way to increase the current going into the battery would be to raise the charge voltage, which is in effect overcharging, not good unless required to equalize unbalanced cells.

Now let's consider the battery was heavily used one night, the morning battery voltage is 11.5V, the weak morning sun can only bring the voltage up to 12.0V when the prime sun hours begin at 10am (approximately), now is when the current will start to climb, because the battery is accepting all that can be produced, the maximum production would occur when the sun is directly overhead, if the voltage at that time is below 14.4V, then the maximum output would be observed.

What's being seen is the battery nearing full charge, therefore reducing current, adding a load is offering somewhere for the otherwise wasted energy to go. As an experiment disconnect the PV at the end of the day, reconnect the next day at noon, this will demonstrate what's being explained, the battery will take everything available, until the voltage set-point is reached, you need to discharge the battery overnight to some degree of course. 

littleharbor2

Photowhit said:

Before MPPT type charge controllers, panels were made in voltage ranges to charge batteries. The higher STC VMP is for several reasons, one of the major reasons is the loss of voltage in Normal Operation. I couldn't find a panel in the 100 watt range that had published NOCT values, but here's specs of a Astroenergy panel;

You can see that the voltage drops from a VMP of 35.77 to 32.67 or about 9%, this is at the panel. Note the over all drop of wattage from 305 watt to 213 watts, about 70% of the rated power vs the Normal operating power. 




Here's a 120 watt STC rated panel showing a mere 87 watts at NOCT level output.

mike_s

What gauge and length wire between the controller and battery? Remote voltage sensing? Too much resistance can limit the battery current, too.

BB.

With high charging current (good sun, low battery State of Charge, and/or DC loads on battery bus), what is the voltage at the charge controller output terminals and at the other end of the cables at the battery bus connection terminals? Ideally you would want a maximum of 0.05 to 0.10 volt drop for a 12 volt battery bank (2x that for a 24 volt bank, and 4x that for a 48 volt bank).

Looking for voltage drop for both wiring and connections (loose, dirty, poor connections can cause problems too).

-Bill

mountainman

Cc only accepts 8 awg so I used 4 awg and trimmed to fit the cc lugs. On calculator .net 5 ft at 12 volts .052 v drop. No temperature compensation. All connected tight no extremely hot terminal. I think what mcgivor said could be the answer. I get great east and south unobstructed sun. The battery bank slowly fills earlier on. When the sun is at peak the batteries can only apcept 16 amps. Just my opinion I'm still very much tring to learn. I do appreciate all of your help. In cold Mountain air my arrays putting out more amps than stc 21.89 (rated at 21.16) as for the question of mppt amps maybe theres to many variables for an answer short of hooking it up to see. But if any one knows how to do the math what peak amps could be possible with mppt x 4 22.5 voc 18.9 vmp 5.29 imp 5.75 isc temperature Co ef of pmax -.44%/°c in 10f temps? Again thanks very much to all that are helping me understand my curiosity on solar.

mountainman

Thought this might be of help

BB.

Wrong thread....

-Bill

mountainman

Bill wasn't this intended for some one else

BB.

Yes, thank you MountainMan.

I am posting to correct thread now.

-Bill

mountainman

Mcgivor. weather app for Virginia says we'll have sun next Wednesday think I'll try your experiment. I'll post results

Estragon

If my arithmetic is right (25°C STC to -12°C ambient is 0.0044 x (25+12) = 0.1628, so Vmp of 18.9 + 18.9 x .1628 = 21.98v) about a 3v higher voltage?

In theory, a ~16% potential better output with mppt, but not likely in real world use. Ambient temp may be -12°C, but the panel will be much warmer than ambient in full sun. Using ambient with the temp coefficient is really only useful in sanity checking string config so as not to overvoltage a controller on a cold morning (when panels are near ambient before being warmed by the sun).

mountainman

Mcgivor you were right. I Did the test as you instructed unhooked my solar array.yesterday left my tv on all night. Noon today I reconnected it to see 21 amps going into the 60% soc batteries.