Why can't I use a Xantrex C-Series Controller with higher voltage panels?

bajajoe@hughes.net · January 2013

When looking at 200 plus watt solar panels it references needing to use a MPPT type controller with them. Since the C Series C30 I have states it will handle up to 125VDC open circuit input and if I am running a 24VDC system, why can't I use a panel that is rated at 36VDC Open Circuit?

inetdog · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

bajajoe@hughes.net wrote: »

When looking at 200 plus watt solar panels it references needing to use a MPPT type controller with them. Since the C Series C30 I have states it will handle up to 125VDC open circuit input and if I am running a 24VDC system, why can't I use a panel that is rated at 36VDC Open Circuit?

You could. The advise that an MPPT controller is required was probably given on the assumption of a 12 volt system. In a 12 volt system there would not be any damage to the panels or to the CC, but more than half of the panel's potential output power would be wasted when using a PWM CC.

Cariboocoot · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

No, you can't. Voc means nothing for charging batteries, really. Vmp is what counts.

A panel with a Voc of 36 will have a Vmp of about 29 which means it will not produce enough Voltage before system loss to bring a 24 Volt system up to its normal absorb charge Voltage. As for equalization, forget it; that requires even higher Voltage.

Add some heat and wiring losses and the 28.8 Volt normal Absorb point will not be available from a panel that has only a 29 Volt Vmp to begin with.

When using these "odd Voltage" panels on a battery system the only way to ensure you have sufficient charging Voltage is to place them in series to get a string Vmp above what is needed and use an MPPT type controller to convert the 'extra' Voltage into current.

Photowhit · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

If the cost difference is enough, you can use 2 in series and the "waste" will be near half, but might be cost effective since 12 and 24v nominal panels have become rarer and more expensive, panels with VMP of about 17.5 and 35 volts respectivly.

If you keep an eye out you can sometimes find true 24V panels at or near the cheap prices, i had seen some just the other day but they are sold out now.

bajajoe@hughes.net · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

The panel I am looking at has a Max Power Voltage at 30.4 volts and a VOC of 37 volts.. What voltage do you need for equalization? It looks like they are more than adequate for normal float voltage.....

Cariboocoot · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

bajajoe@hughes.net wrote: »

The panel I am looking at has a Max Power Voltage at 30.4 volts and a VOC of 37 volts.. What voltage do you need for equalization? It looks like they are more than adequate for normal float voltage.....

Float Voltage is not the same as Absorb Voltage (which is higher) and is not what counts for charging. Typical 24 Volt system Voltages are Absorb @ 28.8, Float @ 27.6, and Equalize @ 31. The trouble is that even though the panels has a Vmp rating of 30.4 you won't necessarily get 30.4 Volts at the battery: resistance in the wiring will drop Voltage, as will inevitably high panel temperatures. Normally a 24 Volt system would have an array Vmp of 35 in order to have enough "Voltage overhead" to compensate for these inevitable losses.

If you put two of these panels in series as Photowhit said the Voltage will be "pulled down" to battery Voltage but the current will remain the same. Effectively reducing the maxim power of the two panels to 28.8 Volts * Imp (around 8 for a 240 Watt panel) or 230 Watts total instead of the 480 Watts you'd expect from 2 * 240 Watts.

Photowhit · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

Cariboocoot wrote: »

If you put two of these panels in series as Photowhit said the Voltage will be "pulled down" to battery Voltage but the current will remain the same. Effectively reducing the maxim power of the two panels to 28.8 Volts * Imp (around 8 for a 240 Watt panel) or 230 Watts total instead of the 480 Watts you'd expect from 2 * 240 Watts.

Yes, I didn't do a very good job of explaining why I would even consider doing such a thing, if the 24V nominal panels (VMP>35) cost 2x as much or close, and you already have a C series charge controller and want to expand... You could do it in 2 stages, buy the cheaper panels and later buy an MPPT charge controller...

bajajoe@hughes.net · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

I keep looking at various sites on what voltages are required to charge a vented lead acid battery. The general answer I seem to find is that for a 24 volt system is bulk charging up to 30 volts, absorption 28.4 to 31 and float 25.6 to 26.4. And that Equalization should be at 10% over full charge. Since full charge for a 24 volt system is less than what? 28 volts, you can equalize at 30 or 31 volts if you can maintain that level.

My understanding of the operation of a "typical" solar panel is that Currect Short Circuit would be the maximum current at zero volts (i.e. short circuit), the maximum voltage would be at zero current (i.e. Voltage Open Circuit). That you then have a point on the curve that graphs voltage versus current of Current Max Power and Voltage Max Power, which is the point on the curve just before current heads to zero and voltage max's out.

So therefor if I have done an adequate job of wiring and minimized loss between panels and regulator, and it is not real hot outside, and I have a panel that is at 30.4 volts at max power voltage I should be able to obtain that or even 31 volts with Voltage Open Circuit at 36 or higher volts. Where am I misunderstanding this.

Cariboocoot · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

Three things:

1). Actual charging Voltages are determined by the specific battery. The Voltage points will be for Absorb, Float, and Equalization (when necessary - it is not a regular part of charging). I gave you the "typical" numbers for these points. There really isn't an "Bulk Voltage" in RE, although some controllers use the term. Usually that's the same as the Absorb set point.

2). Current at short circuit (Isc) an Voltage at open circuit (Voc) have no bearing on standard panel operation. The current at maximum power (Imp) is available with full illumination from the lowest practical (real) Voltage up to Voltage at maximum power (Vmp) providing the circuit resistance calls for it. Above Vmp it will drop off. Therefor the power rating of the panel is said to be Imp @ (times) Vmp. This is a number that rarely comes up in the real world do to #3.

3). Panels heat up in sunlight. There is no getting around this. As they do their Voltage will decrease at any given power point. Nominally panels will put out about 80% of their rated power, and most of that drop is due to the decrease in Voltage from heating. Panel temperature is affected by ambient temperature but is not the same as it; the panels will be warmer even on a cold day. Furthermore any and all wiring, no matter how well done, will have additional resistance value that will take away more Voltage. It is absolutely impossible to negate these effects.

When you'd need that 30 Vmp most would be during the Absorb stage when when battery Voltage is at its peak. Inevitably this will also be when the panels are hottest and their Voltage lowest.

Conversely, really cold temperatures will allow the panels to "superconduct" enabling them to output higher than normal Voltage. The additional power provided is lost with a PWM type controller, however, for the same reason as with two panels in series. Again an MPPT type controller can make use of the higher Voltage.

Pick a panel and look up the NOCT rating as well as the STC rating and the temperature coefficient.

BB. · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

bajajoe@hughes.net wrote: »

I keep looking at various sites on what voltages are required to charge a vented lead acid battery. The general answer I seem to find is that for a 24 volt system is bulk charging up to 30 volts, absorption 28.4 to 31 and float 25.6 to 26.4. And that Equalization should be at 10% over full charge. Since full charge for a 24 volt system is less than what? 28 volts, you can equalize at 30 or 31 volts if you can maintain that level.

Yep, all correct... Except that Vmp is rated at ~25C ... Which only occurs at two times in a panel's life... One for markting (and MFG) test (i.e., a bright light simulating the sun is turned on over a room temperature panel for a few seconds)... And on a Zero Degree C day (or colder) in full sun.

Typically a solar panel "cell" operates at 10C to 30C (or more?) over ambient temperature. And the cell voltage falls by ~ -0.5% per ^oC above 25^oC. So if you have a hot day (35^oC or 95^oF) and a 20^oC rise (no wind, lots of sun), the Vmp~30 volts at 25C becomes:

30 Volts Vmp - (30 volts Vmp * (35C+20C-25C) * -0.5% per ^oC) = 30 volts - 4.5 volts = 25.5 volts Vmp on a "hot day"

While the Vmp*Imp curve is not a peak but fairly rounded (perhaps + 5%)--That still means that on a pretty hot day (on your roof top), that your Vmp is low enough that you will loose significant charging current when you try to finish the charge (absorb) at 28.4 volts. Without allowing for wiring voltage drop (typically 1-3%) and another volt or so drop at the at charge controller.

That is why we use the "marketing" Vmp specification of ~17.5-18.6 as the "optimum" Vmp point for charging a 12 volt battery bank (or 35-37.2 volts for a 24 volt bank) to take into account all of these issues.

Even then you can see that it is difficult to get Vmp~35 volts and equalize charge a "cold" battery bank (say high desert and the batteries are in a cool basement). It sort of works because we usually recommend a 10% rate of charge for solar charging, and you only need ~2.5 to 5% rate of charge for equalization (another of those rules of thumbs that bury lots of little hidden nuggets like this).

My understanding of the operation of a "typical" solar panel is that Current Short Circuit would be the maximum current at zero volts (i.e. short circuit), the maximum voltage would be at zero current (i.e. Voltage Open Circuit). That you then have a point on the curve that graphs voltage versus current of Current Max Power and Voltage Max Power, which is the point on the curve just before current heads to zero and voltage max's out.

Note that solar panels are, more or less, "current sources"... This means that once the panel's output voltage is at around Vmp, the lower the output voltage/load (say a 24 volt Vmp panel charging a 12 volt battery) will simply be outputting Imp all the way to near zero output volts (yes, Isc is a bit larger than Imp--for for our example here--they are close enough to equal).

Once the solar panel voltage rises much above Vmp (5-10% of Vmp higher) the current rapidly begins to drop off and hit "zero" amps at Voc (again, Voc is also voltage sensitive--on on very cold days, you have to be careful that Voc-cold-array is not higher than the maximum input voltage for the charge controller).

So therefor if I have done an adequate job of wiring and minimized loss between panels and regulator, and it is not real hot outside, and I have a panel that is at 30.4 volts at max power voltage I should be able to obtain that or even 31 volts with Voltage Open Circuit at 36 or higher volts. Where am I misunderstanding this.

As soon as you bring that panel out on a nice 68F day (~18C)--Your solar cells are going to be ~20C (36F) higher--Or higher than the 25C/77F used to generate your "paper" specification.

And, remember, if you are in cold weather, there is a -5mV per degree C (per cell) in battery charging voltage set point... Say your battery is at 0C (32F). For a 24 volt battery bank your 29 volt set point becomes:

29 volts + (-0.005V per C * 0C - 25C STC * 12 cells) = 29 volts + 1.5 volts = 30.5 volts

So, your Vmp rating in cold weather under full sun is not at OC but ~20C, and the battery is now really cold and need that extra 1.5 volts to properly charge.

These is no easy way around this with current equipment.

There are switching power supply topologies that can "boost" solar array voltage ("boost" and "buck boost"). However, at this time most (all?) MPPT controllers only use a buck type power supply--Which can only "drop" voltages across the power supply (boost can only raise voltages, and buck-boost can do both--but both cost more money to build and are not as efficient).

-Bill

Photowhit · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

bajajoe@hughes.net wrote: »

I keep looking at various sites on what voltages are required to charge a vented lead acid battery. The general answer I seem to find is that for a 24 volt system is bulk charging up to 30 volts, absorption 28.4 to 31 and float 25.6 to 26.4. And that Equalization should be at 10% over full charge. Since full charge for a 24 volt system is less than what? 28 volts, you can equalize at 30 or 31 volts if you can maintain that level.

Not sure where you've been checking, but you should check reputable sites like this one!

If you've been shopping for cheap panels down on the boulevard, you'll find some sites that incorrectly label panels as "24V" even though they will NOT properly charge 24 volt systems. When selling some sites will tell you anything!

Here's a link to some Trina panel specs, that will show the NOCT (normal operating Cell Temperature values just below the Specs, They show about a 2 1/2 volt drop at normal operating temperatures, and that's before you have some voltage drop across the wires and through the charge controller.

If your looking to use an inexpensive C series charge controller for a small system, Northern Arizona Wind and Sun sells a Kyocera KD315GX-LPB for $378 it has a VMP of @39 volts and will allow you to use a C series charge controller.

bajajoe@hughes.net wrote: »

So therefor if I have done an adequate job of wiring and minimized loss between panels and regulator, and it is not real hot outside, and I have a panel that is at 30.4 volts at max power voltage I should be able to obtain that or even 31 volts with Voltage Open Circuit at 36 or higher volts. Where am I misunderstanding this.

If you have 27 cents it just doesn't ever add up to 31...

Panels use to only come in 12 and 24 volt nominal voltages, for a while a couple manufacturers made 16 volt VMP panels, as self regulating, claiming you could use them with out charge controllers, but it didn't work out and the practice stopped almost as quick as it started.

I'm a big fan of PWM charge controllers, they are simple and simple tends to last longer, I have a 20+ year old SCSS MarkIV(?) charge controller that I keep in storage, it was still working when I pulled it from service 6 years ago. If panels are the same cost per watt and you use only the top 15% of your battery capacity, I'll gladly argue a PWM charge controller works as cost effectivly as a MPPT until you reach 2Kw+ of array. It's just harder to find panels in 12 or 24Volt nominal ratings at close to the price of the Grid tied type panels that only work well with MPPT charge controllers. Though there are options of using auxilary loads with some of these 'smart' MPPT controllers.

waynefromnscanada · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

You can certainly wire things up as you suggest, and it will work for a time, but that time will very likely be short. If you keep a very close eye on the battery SG, you'll very likely soon find your batteries are not coming up to full charge, and at that point, irreversible battery sulfation will be occurring and the end of the useful life of your batteries will be in sight unless you VERY quickly upgrade your charging system. To be honest, you are cutting things way too close, and extremely likely, under what's needed to expect reasonable life from your batteries. But it is your money.
You'll also find, hopefully not in hindsight, that the guys on this form have been honest with you, trying to point you in the right direction so you'd end up with a well working and satisfying system.
Good luck with whatever you decide.

bajajoe@hughes.net · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

I would like to thank all of you for your responses. I started my initial question because I was offered a 230 watt panel delivered to my home in Mexico for $225. I was then told by one of our local "experts" that it was a grid tie panel and could only be used with a MPPT controller. As I've read these responses I spent a little more time looking at what was available on the internet and I find for example a new panel that is advertised as a 24 volt panel at 230 watts and similar Voc, Vmp, Isc, and Imp ratings that says nothing about needing a MPPT controller. What I have gathered is that the MPPT type controller is only necessary if I am trying to bring higher voltages off the roof and then convert them to a lower voltage to the Inverter. Also I guess that if I have problems with equalization I could always run my generator for a while when equalizing. But it appears to me that a Vmp of 30.4 volts should be more than adequate for my needs. I'm currently reading less than 36 Voc at my regulator so I shouldn't have any problems there by installing a panel with a Voc of 36.4 volts. I decided to add additional capacity when I added a 7 cu.ft. freezer to my system.

Once again, I would like to thank everyone for their input: Bajajoe

Cariboocoot · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

bajajoe@hughes.net wrote: »

But it appears to me that a Vmp of 30.4 volts should be more than adequate for my needs.

For a 24 Volt system? Nope. You've completely missed the point. Your 30.4 Vmp panels will not produce 30.4 Volts under normal circumstances.

If you are reading 36 +/- Volts "at your regulator" there is zero load on the panel, and it means nothing.

niel · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

the term 24v panel can be misleading. a true 24v panel, that is one that can deliver proper charge voltages to a 24v lead acid battery, will have 72 cells. a 72 cell pv will typically have a vmp (voltage at max power) in the mid 30s with around 36v to 37v being quite typical nowadays and open circuit voltages that go much much higher than that up into the 40v+ range. by this you see that the deeper the loading on the pv the lower its voltage shall go. the vmp point will be the optimal amount of voltage and current to give you the best overall power from the pv.

BB. · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

niel wrote: »

the term 24v panel can be misleading. a true 24v panel, that is one that can deliver proper charge voltages to a 24v lead acid battery, will have 72 cells. a 72 cell pv will typically have a vmp (voltage at max power) in the mid 30s with around 36v to 37v being quite typical nowadays and open circuit voltages that go much much higher than that up into the 40v+ range. by this you see that the deeper the loading on the pv the lower its voltage shall go. the vmp point will be the optimal amount of voltage and current to give you the best overall power from the pv.

At Standard Test Conditions--Which almost never apply to a solar panel installed in the "wild" except during winter (very cold weather). Which is why we always derate panels so they will work the 9 months of the year it is not winter.

-Bill

Photowhit · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

bajajoe@hughes.net wrote: »

I was then told by one of our local "experts" that it was a grid tie panel and could only be used with a MPPT controller.

So you got good advice! Please listen to him!

bajajoe@hughes.net wrote: »

As I've read these responses I spent a little more time looking at what was available on the internet and I find for example a new panel that is advertised as a 24 volt panel at 230 watts and similar Voc, Vmp, Isc, and Imp ratings that says nothing about needing a MPPT controller.

Yes, We know that's out there. Reread my #11 response!

Read what Northern Arizona Wind and Sun, a responsible Seller, says;

"few solar panels over 140 watts are available in 12 volt versions. Most are intended for grid-tie applications, with panel voltages between 22 and 75 volts. Not suitable for battery charging when used with a standard charge controller. You must use an MPPT charge controller such as..."

Here's what Backwoods Solar (another reputable dealer, been around 30 years) says;

"Most higher wattage solar panels are designed for the grid-tie market where array voltage is not as critical as compared to an off-grid battery based system. A 200+ watt panel with an operating voltage in the range of 28 to 30 volts will not properly charge a 24 volt battery..."

You might check AltE store, who correctly identifies the 'nominal' voltage of panels and will call the panels with less than 35VMP, as 20 volt panels!

Three examples from 3 reputable sellers who have been in business for a long time! Not wholesalers trying to sell you something today, but people who want your business for years to come.

bajajoe@hughes.net wrote: »

What I have gathered is that the MPPT type controller is only necessary if I am trying to bring higher voltages off the roof and then convert them to a lower voltage to the Inverter.

No, where did you get that idea? My ground mount 24volt system has the charge controller 10 feet from the combiner box.

bajajoe@hughes.net wrote: »

Also I guess that if I have problems with equalization I could always run my generator for a while when equalizing.

Yes, if you charger will do equalizing or you wish to do it manually, but...

bajajoe@hughes.net wrote: »

But it appears to me that a Vmp of 30.4 volts should be more than adequate for my needs.

If you could get that to the battery it would charge the battery properly. We have told you that won't happen, and I've provided a link to show that the voltage, at the panel will likely be 2.5 volt less, and that is before you run through wire and you charge controller(yes there will be a voltage drop across a C-series charge controller!)

In addition you are in Mexico where your likely to already be very warm and will have an even greater voltage drop.Here's a chart with the basic relationship, likely you can find one for your panels;

Attachment not found.

If you want to see the math it's here!

bajajoe@hughes.net wrote: »

Once again, I would like to thank everyone for their input: Bajajoe

Why on earth would you thank us? you appear to be ignoring us! And lots of other people who have put information out there. Have you found the NOCT values for the panels you intend to buy?

bajajoe@hughes.net · January 2013

Re: Why can't I use a Xantex C-Series Controller with higher voltage panels?

Once again I would like to thank all of you for your responses. I have learned a lots. I am now much more knowledeable about 24 volt panels. I have looked at other 24 volt panels out there on the internet and see that most of them do have Vmp of 35 or so volts, and agree that if I could put a system in from scratch it would not be the system I operate here on the Beach in Mexico. The system I actually run here at my home in Mexico in one that most of you would say would never work, or is totally horrible. But: I've been retired for over 15 years on a fixed income, and have built a system that has grown over that time frame. My current system is actually 48 volts, not 24, using an Outback inverter, that I switched to when my Trace, serial number 045 finally failed, a 24 volt 4000 watt inverter. I have panels that go from 12 volt, 24 volt, and grid tie. I have 24 Sams club 5 volt batteries, two C40 inverters, analog and digital meters, and run two 18 cu.ft. regrigerators, with ice makers, 7 cu ft freezer, tvs, three computer normally on line 23/7, ceiling fans, microwaves, etc. And it all works. If you are operating at 24 or 48 volts and have done a good job of making sure all your series panels match, you have system voltages of some of the strings much higher that you need, you can add a panel that while lower than ideal, is still more than adequate to reach float and everything will work fine. I am norally at float by noon, run my home like I would in the States, except AC and life is good. Mexico is land of second chances and you can get by just fine with cheap grid tie panels and long as you insure you overall voltage is high enough. And by the way, those that had comments about my comments on battery voltages, the voltages listed were from this web site. I normally get 5 years or more on my "golf cart" batteries. That was after I could no longer get the 2 volt batteries I could obtain when I worked for a living. You can use panels that are lower than what you might want as long as you overall system voltage is high enough and you are willing to run a generator for a couple of hours a month. Thank You all again, once more for helping me on my question on the need for MPPT type regulators. In closing, if you run at 48 volts, the MPPT type regulators will buy you very little, if anything, especially given their cost, if you can run 72 + volts off the roof. Thankyou again: Bajajoe

Why can't I use a Xantrex C-Series Controller with higher voltage panels?

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