newbee needs help

Re: newbee needs help

What is the Vmp/Imp rating of the 2+1 solar panels?

Basically, MPPT controllers are not magic. You have to match Vmp's (~within 10% of each other) when connecting in parallel, and Imp's (again within ~10%) when connecting in series.

If you have more miss-match than ~10%, the system total output will, mostly, end up being from what ever either parallel array has the higher Vmp or the lower Imp (when connected in series).

Of course, whatever you connect to the charge controller must be less than the Vnever-exceed / Inever-exceed ratings.

And, yes, for your MorningStar MPPT controller, you can add ~280 watts of solar panels on the input (12 volt battery bank) and be relatively cost effective (more panels, and the controller will simply current limit in the middle of the day).

For the MorningStar (and a few other MPPT controllers), you can actually add a huge amount of solar panels to the input if you install a series protection fuse. In most cases, it is a waste of money to do this (controller will simply limit its output to 15 amps)--but it can be done (one configuration is to use the MPPT controller to connect to a 24/48 volt battery bank and charge a small 12 VDC battery bank for 12 volt appliances off of a higher voltage system).

Just as a hat-tip to our host, Northern Arizona Wind & Sun, they usually have pretty good prices and much better pre- and post-sales support than the "discount houses" out there.

This forum is just one side of their support (forum is funded by NAWS, but pretty much everyone here, including us two moderators, is volunteering their/our time to help others).

-Bill "Volunteer Moderator" B.

Re: newbee needs help

Assuming you are charging a 12 volt battery bank--Your best bet to use all three panels is to just hook them all together in parallel (with a series protection fuse of ~10 amps in each line from each panel--check the solar panel spec. sheet for exact fuse rating).

And watch the output when charging a well discharged battery. Try charging with three panels, then again with the 80 watt panel removed.

If you get more power with the 80 watt panel--then probably go ahead and use it .

However, it is possible for the MPPT charge controller to MPPT track at either the 17 or 23 volts or somewhere else when you have miss-matched panels (>~10% difference in voltage) and you will get less than optimum voltage.

Otherwise, just sell off the 80 watt panel and use the funds to get another 100 watt panel and parallel them.

If your solar panel to charge controller wire run is a relatively long distance, it would make sense to run the two 100 watt panels in series--this reduces the current by 1/2 (vs two parallel panels) and makes it easier to run longer cables and/or use smaller gauge wire (less $$$).

If you have a 24 volt battery bank, then your only option is the two x 100 watt panels in series and forget the extra 80 watt panel. The Vmp-array needs to be > Vbatt-charging+one to two volts (~29 volts + 2 volts = Vmp 31 volts or higher) for proper charging of your 24 volt battery bank (almost all solar charge controller can drop voltage, they cannot increase it).

I hope I am clear here...

-Bill

Re: newbee needs help

How much a controller will output... Simple question, complicated answer.

For an PWM (Pulse Width Modulation) charge controller, their output current is simply whatever the solar panels' output current is. The PWM is, more or less, simply a switch (electronic, some older ones are probably a relay) that simply turns on or off... On current flows, off current stops. Run the switch at 100's or 1,000's of Hz -- A little more on vs off, more current, etc. A PWM controller cannot limit maximum current as it has no method to do that. So, a PWM controller should not have more solar panel (current) than it is designed to manage or it will over heat. For PWM, current in equals current out.

A MPPT controller is actually a buck mode switching power supply. It uses switches (FETs or Transistors) and inductors to efficiently down convert the high(er) voltage low(er) current on the input to lower voltage/higher current on the output... In someways, very similar to a variable AC transformer (Variac/etc.).

An MPPT can take, for example 24 volts and 10 amps (240 watts) and down convert that to 14 volts and 17 amps (less about 5% or so for losses).

In your case, your controller can output 15 amps... So, depending on what the battery voltage is, the charge controller will adjust the Volts*Amps to match:

• 12 volts * 15 amps = 180 watts into battery
• 14.4 volts * 15 amps = 216 watts into battery
• 28.8 volts * 15 amps = 432 watts into battery

Now, what does a solar panel output...

• 100 watts (sw100-c) is 23.0 volts rated 4.3 amps voc 28.5

23 volts * 4.3 amps (Vmp*Imp) = 98.9 watts

Now, a few things to understand... A solar panel is not like a storage battery. An "ideal" battery will hold 12.7 volts no matter what load or charge current to applied to the battery (obviously a real battery is not so perfect).

A solar panel, on the other hand, is a current source. Under STC (Standard Test Conditions), your solar panel will output 4.3 amps with the output voltage anywhere between 0 and 23 volts (there is a little change, but not much to worry about for our example). As the Voltage goes over 20-23 volts or so, the current starts to drop until you reach Voc (Voltage open circuit) of 28.5 volts--then no current flows.

But, remember we want "power" from a solar panel--Power=Volts*Amps. We could take 4.3 amps from the panel into a dead short (zero volts) and get lots of current but no useful power because there is no voltage. (note Isc, current short circuit, is a bit higher than Imp--and used to size wiring/fuses/etc.).

The MPPT charge controller actually figures out the P=V*I equation by varying the current and measuring the voltage until it finds Pmp (Power maximum power).

And, back to the solar panel... Vmp (and Voc), once you have some minimal direct sunlight across the whole panel (no shade), Vmp/Voc is pretty much set by the temperature of the solar cell (higher temperature, lower Vmp, and less power from the solar panel).

So--To shorten this post--Voltage output is based on temperature of cell, and current is pretty much proportional to the amount of solar energy hitting the panel. And the STC condition is around 1,000 watts per square meter (high noon sun).

In real life, the sun is often less bright (not noon, dust/humidity in air, dirty solar panels) and the panels are hotter than STC (panels get hot in sun)--so we typically derate a solar panel + charge controller by 0.77 as being a good "typical" average maximum power you get from your solar panel system. With clean panels, very clear/cool/cold day, and under some scattered cloud conditions--it is possible to get more power.

So, if we go back to the 14.4 volt 15 amp battery charging and 216 watts:

• 216 watts / 0.77 = 281 watts

So--it is perfectly OK to "over panel" your MPPT controller by 1/0.77 because of all the typical inefficiencies between Lab/Marketing specs. and real life use--that 99% of the time, your 281 watts of solar panels will drive the 15 amp MPPT charge controller to its typical maximum. And that 1% it goes higher, the MPPT charge controller will safely limit its maximum current to 15 amps and you will loose that little bit of power from the solar array (no damage, no problem).

I hope that this is clear and makes sense--please ask if something does not make sense...

-Bill