Vpmax

dmiller · August 2010

Adding a panel in parallel, how close does Vpmax have to be to the existing panels? Thanks.

Edit: My existing has a Vpmax of 16.5 . I can run pairs in series at a vpmax of 33. I want to add a panel in parallel (on the same charge controller). How close to 33 or 16.5 should I be. Within 10%? How do I quantify the loss by voltage differences?

This is on a camper, so limited flexibility.

mike95490 · August 2010

Re: Vpmax

It's pretty much a linear loss, putting a 16V panel parallel with a 33V panel, will reduce the array to the lowest voltage panel, and the 33V panel will produce about half it's wattage (at only 16V)
You mileage may vary

BB. · August 2010

Re: Vpmax

What is your charge controller? MPPT (where you can efficiently connect a 33 Vmp panel to a 12 volt battery bank) or a PWM type where your Vmp should be ~17 volts when connected to a 12 volt battery bank (PWM cannot efficiently down convert very high voltage panels to a low voltage battery bank)?

The answers are significantly different depending on your setup and what panels you are using.

-Bill

dmiller · August 2010

Re: Vpmax

Thanks for the responses

I'm upgrading to a flexmax, unless I'm convinced otherwise. This is a mobile application. I need to use uni-solar as my "always on" panels to avoid physical damage to the collectors. But I want an additional conventional panel to be deployable when not moving. The uni-solar have an unusual voltage.

BB. · August 2010

Re: Vpmax

For systems where you have two different Vmp panels--you may end up needing two different charge controllers--One for each "set" of Vmp's.

Understand your wanting to use thin-film / plastic faced solar panels to have less chance of damage from a rock/etc.

One thing you may want to look at is that thin film are typically around 1/2 as efficient per sq.ft/sq.meter of panel size--So if you have limited roof space, a glass crystalline solar panel will give you about 2x as much power for the same roof space.

By the way, if you set out "glass" solar panels when parked--make sure they are mounted/staked in such a way that the wind cannot blow them over--More than a few people have lost their $$$ panels from a simple wind guest.

-Bill

dmiller · August 2010

Re: Vpmax

"Identical" individual panel have different voltages in the real world. There has to be a both a "rule of thumb" as to what's an acceptable variance, and a way to quantify what happens when a Vpmax of 33 is combined with a Vpmax of 34.
Mikes explanation indicates the above example is an acceptable design.

BB. · August 2010

Re: Vpmax

The "knee of the power curve" is reasonably flat at its peak...

If you have two Vmp=17 volt panels in series for Vmp-array=34 volts... The "other panel" can be 34+10%=37.4 volts or 34-10%=30.6 volts and still be within 10% or less "losses" vs an "ideal" setup. One panel will be running on the "right side" of the knee, and the other panel on the "left side" of the knee.

Similar with adding a second panel in series--match the Imp current within 10% (ideally).

If you parallel a 17 volt panel with a 35 volt panel on a PWM controller charging a 12 volt battery bank--Roughly the working voltage will be around 15 volts... Remembering that solar panels are, more or less, current mode devices... The 35 volt panel will be supplying ~ 1/2 the power because Pmiss-match=Vmiss-match*Imp where Vmiss-match is ~1/2 Vmp of the 35 volt panel.

If you put a 17 volt + 35 volt array in series behind an MPPT controller--the "power knee" will probably have a double bump--One ner 17 volts and another local maximum at 35 volts. And possibly even a third one somewhere in between.

Since MPPT controllers are not designed to support miss-matched panels, there is no guarantee which of the two or three local power maximums the controller will pick... And in any case, none of the local power maximums will be actual maximum power of all panels added together.

To get an optimum setup--you would need two charge controllers. One for the 17 Vmp array and a second for the 35 volt array, all charging the same 12 volt battery bank. And the second 35 volt array charge controller would have to be a MPPT type charge controller to optimally down converter the 35 volt array's high voltage/low current into the low voltage/high current for charging the 12 volt battery bank.

-Bill

dmiller · August 2010

Re: Vpmax

Thanks for the reply.

Optimal for a mobile setup doesn't have a whole lot to do with maximizing potential energy capture from each panel. The system is frequently degraded, say and antenna shadow across a panel. And while there are both panel and system designs to minimize these types of issues, as I've said I find it highly unlikely that all the panels are running within a few tenths of a volt of each other.
I guess if I add a 34.5v 200 watt panel to my 33v unisolar and I give up 20w that's fine. 100w probably not fine.

I've never had multiple charge controllers. I assume they are designed to be interconnected if charging the same bank?

BB. · August 2010

Re: Vpmax

Technically, the panels will be running at the same Vmp when connected in parallel...

If the Vmp of the panels are reasonably close (~10% or less difference), then the loss in total output power is probably less than 10%.

The charge controllers will each be set to their proper Bulk/Absorb/Float charge settings and they will each make a decision as to when the battery bank is charged--The controllers, even if identical models and settings, will not switch modes/agree on exact charge voltage and current... One will always switch charging states before the other--but that will not be a issue in the overall scheme of life.

Some high-end MPPT charge controllers can be networked together (at least Outback and Xantrex???) and they will switch states together and allow you to read the charging/operating conditions from one digital readout.

-Bill

Vpmax

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