Mixing solar panels

[PbdBlue]

I am in the process of installing solar on my new MH. I don't full time but do boondock while travelling. We're pretty frugal with power usually keeping the max down to about a 12 amp draw during the evening for about 3 hours + mostly just the minimum level during the daytime. Don't run the microwave or the coffee pot off 12 volts. Probably less than 50 Ah total per day. Also have Honda 2000EU which I don't mind running occasionally. Anyway on to the solar question. I was looking at the Tracer 40A mppt controller but now thinking about using the Midnite Solar Kidd controller due to quality\support concerns. I have two 160 watt panels 22.4 volt max. Can I add a 100 watt panel to utilize more the Kidd capacity? I ran the calc on the MS calculator and it says it's ok. The 100 watt panel I'm looking at has the same Voc and Vop parameters as my 160 watt panels. Should I run all three in parallel or series?

[CountryFit]

batterypro is more authoritive on this, i would do 3 paralell. if you go with serial the current will be limited by the lowest watt one thus you are losing the two higher panels' full capacity.

[PbdBlue]

Originally Posted by CountryFit

batterypro is more authoritive on this, i would do 3 paralell. if you go with serial the current will be limited by the lowest watt one thus you are losing the two higher panels' full capacity.

I'm thinking parallel also if for no other reason than to reduce shading effects but does series have an effieciency advantage other than wire size?

[BatteryPro]

Originally Posted by PbdBlue

....I have two 160 watt panels 22.4 volt max. Can I add a 100 watt panel to utilize more the Kidd capacity? I ran the calc on the MS calculator and it says it's ok. The 100 watt panel I'm looking at has the same Voc and Vop parameters as my 160 watt panels. Should I run all three in parallel or series?

Hi,

If the Vmp of different size solar modules is close, I usually specify 0.5 volts or less, then they will work together fairly well in parallel. What you should never do is put different size solar modules in series. The lowest current module will act as a load to the higher current modules in that string.

Originally Posted by PbdBlue

I'm thinking parallel also if for no other reason than to reduce shading effects but does series have an effieciency advantage other than wire size?

The primary benefit of series wiring is that higher voltage means less loss for a given power. This means you can use smaller wire which costs less and is easier to run. However, MPPT charge controllers are less efficient converting high voltage than lower voltage. So while wire loss may be less, conversion losses will be higher. See the attached chart. Notice how different Wattage modules affect efficiency? Now notice that the higher voltage you use, the lower the efficiency across the entire power range.

So if you use the right size wire, lower voltage, parallel connections will perform better.

Larry

[PbdBlue]

Originally Posted by BatteryPro

Hi,

If the Vmp of different size solar modules is close, I usually specify 0.5 volts or less, then they will work together fairly well in parallel. What you should never do is put different size solar modules in series. The lowest current module will act as a load to the higher current modules in that string.




The primary benefit of series wiring is that higher voltage means less loss for a given power. This means you can use smaller wire which costs less and is easier to run. However, MPPT charge controllers are less efficient converting high voltage than lower voltage. So while wire loss may be less, conversion losses will be higher. See the attached chart. Notice how different Wattage modules affect efficiency? Now notice that the higher voltage you use, the lower the efficiency across the entire power range.

So if you use the right size wire, lower voltage, parallel connections will perform better.

Larry

Thanks. The panels match each other within 100 millivolts. The total Iop for the panels is about 23 amps. The run to the controller will be ~15 feet. Would 8 ga wire be a reasonable choice or should I jump down to 6 ga?

[Bumps]

I was looking at the Tracer 40A mppt controller...

You may have a different experieince, but I tried two of the Tracer MPPT charge controllers and they both failed. They did fine while the RV was stationary, but did not tolerate road vibrations very well. I switched to the MorningStar TS-MPPT60 and have had no problems. \ken

[PbdBlue]

Originally Posted by Bumps

You may have a different experieince, but I tried two of the Tracer MPPT charge controllers and they both failed. They did fine while the RV was stationary, but did not tolerate road vibrations very well. I switched to the MorningStar TS-MPPT60 and have had no problems. \ken

Read a lot. Quality and support after sale were big factors in my decision not to go with the Tracer.

[chboone]

Here is some good reading on Solar, especially about STC and NOTC ratings.

AM Solar's Educational Pages for RV Solar Systems

[PbdBlue]

Originally Posted by chboone

Here is some good reading on Solar, especially about STC and NOTC ratings.

AM Solar's Educational Pages for RV Solar Systems

Thanks. I already read that and it was helpful. I had a simpler solar setup on my old MH so this isn't my first go at it but the first with multiple panels and MPPT. Looking around it seems if I stick with the supplied MC4 connectors that it's difficult to find cables larger than 8 ga.

[chboone]

Originally Posted by PbdBlue

Thanks. I already read that and it was helpful. I had a simpler solar setup on my old MH so this isn't my first go at it but the first with multiple panels and MPPT. Looking around it seems if I stick with the supplied MC4 connectors that it's difficult to find cables larger than 8 ga.

AM Solar cuts off the MC4 connectors, butt splices, and uses epoxy fill heat shrink for 10 ga on the roof. My panels didn't have MC4, just had a junction box each panel.

[PbdBlue]

I can get 8 ga with MC4 connectors. The max current of the panels is 23A (which I doubt I will see very often if ever). I think 8 ga will probably be adequate.

[CountryFit]

handybob recommends 6g between panels and controller and 4g from controller to batteries. Here his blog https://handybobsolar.wordpress.com/...ging-puzzle-2/

[BatteryPro]

Originally Posted by CountryFit

handybob recommends 6g between panels and controller and 4g from controller to batteries.....

Contrary to what Bob or anyone says, the PV wire size and controller to battery wire size should always be calculated, not guessed at. To say one needs to use 6 AWG without knowing the voltage or distance is wrong.

If you are using a PWM only controller, you don't need two different sizes of wire. The current flowing through the whole circuit is the same at the battery as it is at the PV solar modules as it is at the controller. Likewise, the charge controller can be placed anywhere in the circuit because it is simply an on-off switch. While Bob has a passion for telling people what to do, it is important to know why.

For example, [moderator edit] a 327 Watt PV module. To run 30 feet and have less than 1% voltage drop, I can use 14 AWG wire! That is a tiny wire compared to #6. How do I do it? The operating voltage is at 54.7 volts and the current is at 6 amps. If someone told you to use 14 gauge wire for 327 Watts you would think that's WAY too small. (FYI, I never run any PV wire with less than #10)

Now, let's compare that to using 2 160 Watt CTI modules. That's 320 Watts. To have under 1% drop I would need to use #4 AWG. That's bigger than the "recommended" 6 gauge. Why? Because the PV modules are operating at 17.5 volts and 17.72 amps. A very big difference in the voltage and current.

So it should be clear that wire size should always be calculated, not guess at.

[BatteryPro]

What is the operating current (Imp) of each PV module?
What is the operating voltage (Vmp) of each PV module?
These are different than the Isc, short circuit current and Voc, open circuit voltage.

If you let me know I will show you some calculations.