Parallel with SLIGHTLY different Voltage

KhellSk · December 2018

Hello all, bear with me and read completly.

Ive done research and I know that putting panels with different voltages together isnt advised. BUT every time I find an exemple, its with really big differences.

So
I have 5x 48w panels = 3.02A 15.9V rated ( 3.35a 19.8V max)
but 1x 100w panel = 5.9A 17V rated ( 6.5a 21.6V max)

I run a 12v system ( 12v Charge Ctrl ) so i got them all In parallel

Questions;
1 - Its not 18v with 36v OR 12 with 24v ( double the V diff ). We re talking about 1.1V. In my case, Is it as bad as everybody says on the internet ??

2 - The real reason of my post is that I am thinking about buying an other Panel to the lot; 1x 90w 5.17A 17.4V rated ( 5.58a 22.58v max)
--- which would make a 1.5V difference with my 5x 15.9V. I am scared its going to draw down the New 17.4 making it see too many Amps and
wearing it down faster

3 - I feel limited to that one panel because all others are around 18v rated making 2.1v higher, which scares me. Should I worry ?

4 - am I wrong to think that the charge control being PWM so my understanding was that it limited the panels to what was needed thus making okay to
mix my voltage a bit???

Thank you all for your help!!

jonr · December 2018

Don't worry about it.

littleharbor2 · December 2018

Within 10% you'll be fine. they are all 12 volt nominal panels. The lower voltage ones are older cells. PV cells have gradually gotten more efficient, resulting in higher voltage and current. Without even asking I'll bet they are all 36 cell panels.

KhellSk · December 2018

littleharbor2 said:

Within 10% you'll be fine. they are all 12 volt nominal panels. The lower voltage ones are older cells. PV cells have gradually gotten more efficient, resulting in higher voltage and current. Without even asking I'll bet they are all 36 cell panels.

10% of which, the lower ones 48w 15.9v or the new one 90w 17.4 ??

Yes they are all 36cell which is why i felt compelled to buy an other 36cell panel.

Also I really neen the Question 4- answered. Does a pwm ctrl only regulate the V output or what the panel makes too ?

mcgivor · December 2018

The voltage will find a balance point somewhere between the two, perhas ~16.5V connected in parallel
Question 4. The PWM controller will pull the input voltage down to a voltage slightly above battery voltage, as long as the input voltage capability is higher than the maximum charge voltage required it will work. Say maximum battery voltage requirement is 15V and the input is 16.5V this would work, usually the battery voltage during charging is 12-14.4V as the voltage reaches the absorption stage, the current will drop, at 15V (equalization voltage not regular voltage ) there should be very low current so voltage is not generally an issue on healthy batteries.

littleharbor2 · December 2018

KhellSk said:

littleharbor2 said:

Within 10% you'll be fine. they are all 12 volt nominal panels. The lower voltage ones are older cells. PV cells have gradually gotten more efficient, resulting in higher voltage and current. Without even asking I'll bet they are all 36 cell panels.

10% of which, the lower ones 48w 15.9v or the new one 90w 17.4 ??

The voltage difference between your panels. 15.9 vs.17.4. Besides, the max rated voltage is just that. Once you connect your panels to the controller and batteries, the batteries will clamp the output voltage down to something just above the actual battery's voltage. It is the controllers job to hold the maximum voltage to the proper pre set, or user set levels appropriate for your batteries charging needs. Without the controller your batteries voltage would continue to rise to dangerous levels.

With your panels the only time you will have an issue would be if you were to series wire them due to a large difference between current output. since you are using a PWM controller and parallel wiring all your panels, you're fine as long as your controller is rated to handle the current output of your array.

KhellSk · December 2018

mcgivor said:
Question 4. The PWM controller will pull the input voltage down to a voltage slightly above battery voltage, as long as the input voltage capability is higher than the maximum charge voltage required it will work.

Still, while full sun shining on it, My understanding is that would bring down the voltage WITHIN the panel thus raising the amperage? This would wear down the panel faster.

If not this would mean I could buy a 60cell @ 20v rated and get more for my money instead of buy a 36cell

mcgivor · December 2018

Stay with the same 36 cell format is the best advice I can offer, a 60 cell module generally has a VMP of around 30V which may overcome the voltage output of the 36 cell modules creating feedback, thus reverse biasing the smaller modules into becoming heaters. Personally in the past I've connected various modules together in parallel, but they were always the same format, never tried your proposition, so not saying it wouldn't work, just have some doubts on the outcome.

littleharbor2 · December 2018

KhellSk said:

Still, while full sun shining on it, My understanding is that would bring down the voltage WITHIN the panel thus raising the amperage? This would wear down the panel faster.

Where are you getting these ideas? The panels current rating is set. It will never put out any higher current than it is rated at. Solar modules don't wear out as a mechanical object will. You can actually short circuit a module by plugging the pos, and neg. plugs together and it wont damage it. You can parallel wire two panels together and short them out and still do no damage because the current wont ever be high enough to do damage to them.

KhellSk · December 2018

littleharbor2

mcgivor said:

Stay with the same 36 cell format is the best advice I can offer, a 60 cell module generally has a VMP of around 30V which may overcome the voltage output of the 36 cell modules creating feedback, thus reverse biasing the smaller modules into becoming heaters.

Thats where i get this idea. Ive seen this concept being told at different places. This is why i came here to get an answer.

On one hand I people you saying that Amps cannot be exceded even if shorted. On the other hand what littleharbor2 said.

So yes. I will stick to 36cells modules but they are pretty rare and most of them a lot newer with a higher voltage like littleharbor2 said in his 1st post. 18v and 19v. I guess needed to know if 18 19v would fine(giving me more buying options) or

if I should sacrifice watt and money to find one closer to 16v

Estragon · December 2018

I wouldn't pay more for 16v. Slightly higher should work fine IMHO.

You do want PWM, as hot panels on a still day may not have enough voltage headroom for MPPT.

littleharbor2 · December 2018

KhellSk said:

littleharbor2

mcgivor said:

Stay with the same 36 cell format is the best advice I can offer, a 60 cell module generally has a VMP of around 30V which may overcome the voltage output of the 36 cell modules creating feedback, thus reverse biasing the smaller modules into becoming heaters.
Thats where i get this idea. Ive seen this concept being told at different places. This is why i came here to get an answer.

On one hand I people you saying that Amps cannot be exceded even if shorted. On the other hand what littleharbor2 said.

So yes. I will stick to 36cells modules but they are pretty rare and most of them a lot newer with a higher voltage like littleharbor2 said in his 1st post. 18v and 19v. I guess needed to know if 18 19v would fine(giving me more buying options) or

if I should sacrifice watt and money to find one closer to 16v

My comments are based on mixing panels of the same format. If you want to use 60 cell panels and 36 cell panels to charge the same battery bank then use separate charge controllers and DEFINATELY use an MPPT controller for 60 cell panels.

mike95490 · December 2018

KhellSk said:
Still, while full sun shining on it, My understanding is that would bring down the voltage WITHIN the panel thus raising the amperage? This would wear down the panel faster.

How do you come to this conclusion ? Regardless, that's not the way it works. The lower voltage panels actually limit any higher voltage panels, and differential voltage is just "lost".
Current cannot increase beyond the Isc spec on the panel sticker (Isc = Amps Short Circuit)

If not this would mean I could buy a 60cell @ 20v rated and get more for my money instead of buy a 36cell

No. You have made an incorrect assumption based on your imagination and not electronic engineering realities.

BB. · December 2018

A 60 Cell panel would be Vmp~30 volts.

Vmp-cell is roughly 0.5 volts per cell... times X cells in series gives you Vmp-panel

The difference in current between Imp and Isc is something like 10% more current for Isc.

However, remember the definition for power is Power=Voltage*Current...

Power = V*I = 0 Volts * 1.1 Imp (more current with shorted panel output) = 0 Watts

So, no matter how much current you can get with Isc, you will still end up with zero useful power.

And when you look at a 60 Cell vs 36 cell group of panels... Say they are both 200 Watts, and the 60 Cell panel will give you 5% more current at 18 volts Vmp (of a 36 cell panel)...

200 Watts / 30 Vmp = 6.67 Amps @ 30 volts
18 Volt Vmp battery/Vpanel * 1.05x6.67 Amps (running high voltage panel at lower current) = 126 Watts

So, your 200 Watt (standard test conditions) still lost a bunch of "real" power (watts) by running at a non-optimum voltage.

And, the way solar panels work, the "excess current" that is not used, actually "leaks" back through the cell... Remember that silicon solar cells are just diodes, and when Vcell (open circuit) exceeds ~0.6 volts per cell (Voc-panel~21.65 volts for your 36 cell panel), the current simply flows through the "forward biased" junction.

Panel costs are, mostly, based on $$$/Watt. And at a first estimate, a 200 Watt "12 volt" panel would cost the same as a 200 Watt "24 volt" panel.

However, there are other issues... One is that the higher the current in a panel, the more copper you need to conduct the current without getting high losses... So a lower voltage panel (all same wattage), will have more copper in it (not a big factor?).

The other $$$/Watt driver has been volume/automation/process improvement/product demand. "Grid Tie" Solar panels (Vmp typically 30 or 36 volts, but other voltages have been used), are cheaper because they are more popular (supply and demand).

For GT solar power systems, panel voltage is not a big deal... Nominally strings are around 200-400 volts for central inverter systems. Roughly 10 * 30 Vmp panels in series is Vmp-array~300 volts).

For battery systems, if you match (tend to be more expensive) "12 volt" (aka Vmp~18 volt) panels to a PWM controller (PWM controllers are very cheap compared to MPPT controllers), this tends to be less expensive for smaller systems (tends to be 400 Watts or less).

For larger systems, using "cheap" GT Solar panels (about 1/2 the $$$/Watt vs 36 Vmp~18 volt panels), the cheaper solar array brings the system cost down and "covers" for the more expensive MPPT controllers.

There are other reason that MPPT controllers plus "higher voltage" Vmp-arrays (longer cable runs from Array to charge controller) that can save a bunch of money on copper cable runs, and some other advantages.

20+ years ago, panels where probably in the $10-$20 per Watt range... Today they are in the $2 to $0.50 per Watt.

Saving 5x 48 Watt old panels (that are probably still good), vs just buying one 240 Watt panel (20 years ago, worth 2,400-4,800 new; today worth around $120 new--It does "hurt")--Saves a bunch of wiring and mounting--But at the cost of new panel+MPPT charge controller+shipping/taxes vs what you already have...

We have people all the time trying to figure out how to mix and match new/old panels, or how to find a replacement for an old/broken panel--And it is usually a pain. What was available/purchased 20 years ago is not the same as today. Many times, it was better to find a new home for the "orphan" panels (small 12 volt RV system, friend that needs some panels, Craig's List) and just move forward with a new 2018 system.

-Bill

KhellSk · December 2018

BB.

Thanks for your effort
Yes i agree it would have been worth going new of if didnt have much to begin with.

But i been given 3x 48w-15.9v
And bought 2 more for for 20$ each and then found a 100w-17v for 60$. Pretty cheap i think for the wattts and siemens quality.

First, I thought i would be fine with 240w for my application but the. I started consuming more.and having less and less sun as we went through fall. So i bought the 100w. Now i know i still have room on my chrg ctrl and cloudy weeks are a b*tch. Plus all my panels are facing SE. So i wanna move 1 and add 1 to the SW corner.

My issues is resolved now. Thanks to mcgivor and littleharbour2 i know the information about not mixing diff voltage panels are meant for higher differences like 60cells with 72. I now know that by staying within 10ish % and with 36cells for the new panel, everything going to be alright.

Nowhere else could I have gotten more help and that technical. So i want to thank you all for your help!

what motivates you guys to help us noobies ?

mcgivor · December 2018

@KhellSk asks,
what motivates you guys to help us noobies ?

Helping others is a positive thing to do, sharing information is a two way street, at some time we were all at the beginning, the recipients of information passed on by others, so it's only fair to reciprocate with the passing the baton on to others. Perhaps someday you'll be in the position to share your aquired experience with other beginners.

mike95490 · December 2018

Tired of seeing folks ripped off by slick sales persons, which gives solar a bad name.

littleharbor2 · December 2018

It's a two way street for me here. I learn, as well as help and share information. It's good for the industry as a whole that people understand how this works as opposed to getting frustrated and giving up, thinking that solar won't work due to a bad experience.

jonr · December 2018

You can put a 60 cell panel in parallel with a 36 cell. But it will waste watts and would require a blocking diode on the 36 cell panel.

Parallel with SLIGHTLY different Voltage

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