Need a way to limit or regulate PV voltage to the CC

[Ray by-day]

Hello. I am looking for a way to limit the PV input voltage that my PWM Charge Controller will see.

I have 80W solar modules with Vmp = 32.3V, Voc = 45.2V, and temp coeff of Voc =  -0.150 V/degC
Am in cool/warm Northern Calif. where 11 to 13 deg C (low 50s F) mornings are common. Zero degree C during daylight is possible, but rare.

I already bought a charge controller with Max Input Voltage of 47V.  At the time I did not account for panel's actual Voc being higher when outside of STC. I'd like to make this work.

How can I regulate the array's output Voltage going to the Charge Controller?

-- I've read that diodes "...isn't going to work well here... To really take the Voltage down you need to "preload" the panel so there is no Voc."  (from: http://www.wind-sun.com/ForumVB/showthread.php?16658-Panel-volts-exceeding-MPPT-controller )
-- I found Ghurd's Dump Controller, and thought that my be a solution. "When assembled for use as a Dump Controller, when it reaches the set voltage, power is sent to a dump load until the voltage falls some, and then the dump load is turned off. "
However, the standard connection method is directly to the battery bank.

Can I connect a Ghurd dump controller to the array in some way to buffer voltage going to the charge controller? 

Or, what other possibilities here...? 

I'll provide more specs/details in next Post to make it simpler to follow.
Thanks in advance.

[Ray by-day]

Here is what I am planning, and the specifics for the major components.

Plan:
Three (3) 80W solar panels in parallel. Nominal output 24V ( Vmp of 34.3V ) and 7.5A (2.48A per module).
Charge Controller = 30A PWM
24 volt battery bank = two 12V 80Ah DC AGM batteries. 
Off-grid. Simply use for powering a small freezer and when there is power outage, can have a little light.
Inverter not purchased yet (probably a modest size PSW 24V).
---------------------------
Modules:  BP Solar 980  80W
Maximum power     (Pmax) 80W
Voltage at Pmax     (Vmp) 32.3V
Current at Pmax     (lmp)  2.48A
Warranted minimum  Pmax   75W
Short-circuit current   (lsc)   3.0A
Open-circuit voltage  (Voc)   45.2V
Temperature coefficient of lsc   (0.033± 0.005)%/°C
Temperature coefficient of Voc   -(150 ±20 mV)/°C
Temperature coefficient of power   -(0.36±0.05)%/°C
NOCT                           51±2°C
Maximum system voltage          600V
NOTE:  I am told "These are also bi-facial modules which means they collect energy from the diffuse radiation on the back so an 80 watt modules will put out 90-100 if diffuse radiation is allowed to enter the back of the panel."
I am assuming that extra power will show up in extra amps, but that the upper limit of Voc will also be pushed up beyond 45.2V (spec Voc for the 90W is 46.4V).
-----------------------------
Charge Controller:   (similar to) Steca PR-3030
System voltage     12 V (24 V)
Own consumption    12.5 mA

DC input side
Open circuit voltage solar module    < 47 V
Module current    30 A

DC output side
Load current      30 A
End of charge voltage liquid    13.9 V (27.8 V); gel 14.1 V (28.2 V)
Boost charge voltage          14.4 V (28.8 V)
Equalization charge            14.7 V (29.4 V)
Reconnection voltage (SOC / LVR)       > 50 % / 12.6 V (25.2 V)
Deep discharge protection (SOC / LVD)  < 30 % / 11.1 V (22.2 V)

Operating conditions
Ambient temperature          -10 °C … +50 °C

Terminal (fine / single wire)    16 mm2 / 25 mm2 - AWG 6 / 4
Degree of protection           IP 32
Technical data   at   25 °C / 77 °F

[Mary B]

How many feet of wire between the panels and the charge controller? The voltage drop in the wire will take up some of it maybe. After that a voltage dropping resistor could be used.

[Ray by-day]

How many feet of wire between the panels and the charge controller?

Probably a run of 28ft linear (2 wires = 56ft). I think that will drop less than 1 volt (if 10ga wire), correct?

[Larsmartinxt]

The wire will not drop the voltage unless under load. Therefore it will not work to prevent high VOC

[Watt]

You may not want to hear this but, you may be better off buying a better suited charge controller as most anything else will eat up valuable wattage.  A MPPT controller if you do buy something else.  Diodes would be another option if wattage is not a concern. 

[ghurd]

Best bet would be to get a different controller.
I don't like how the controller was discribed as "similar to..."  Sounds like an ebay type unit.

I doubt an MPPT will make a lot of difference due to the small battery bank and charging at almost C/10 at Imp already.
For the gain of an MPPT, could add another 80W PV cheaper, but that wouldn't make much difference either!

Ghurd controller would do it.  For solar, can set it up with no dump load.
G-

[Ray by-day]

Thanks for the replies. I know it's not a perfect set-up and I just wanted something to get started. I am not concerned about maximizing wattage, I only need a modest system for very limited purpose.

I'm actually glad to hear wire resistance will not solve the particular issue -- I'd like to get the wiring and safety right.

My home/site is not the best-suited for solar. Main roof slopes East-West, and we get high winds sweeping in from N and NW which complicates installing panels with tilt. But I will deal with this at some future time when I would upgrade to a bigger grid-tied system.

Best bet would be to get a different controller.
Ghurd controller would do it.  For solar, can set it up with no dump load.

I'm glad to hear that. It sounded promising when I first read about it.

For the Ghurd device, do I just need one unit for the whole array (or one per panel)?  I will have 3x 80W in parallel, possibly 4x which would be 10A total at spec.

Also, where do I install it?  e.g. assuming one Ghurd.... use it between the PV combiner and the Charge Controller? (exactly how?) ...or on the battery bank (w/ 24V kit)?

Thanks.

[boB]

It may be that the 47 volt max is very conservative.  It may very well be able to withstand higher input voltage than this.
What kind of controller is it ?

I see from your list that you have a 12 volt system but basically are running a PWM controller with a 24 volt PV input ?
That won't work nearly as well as wiring the PV up for 12V nominal input to your PWM controller.

 boB

[Ray by-day]

boB,
Thx.
>>>>I see from your list that you have a 12 volt system
No, the controller is auto-detect to cover 12V and 24V. I'll set up batteries as a 24V bank.

>>>>What kind of controller is it ?
It's a pretty basic PWM type, should be a step better than low-end. Can find specs under Steca PR3030 as mentioned. Its specs for 24V operation are shown in '( )'.

>>>>It may be that the 47 volt max is very conservative. 
No, it's not conservative IMO. Once I read the details on the technical site, they advise to "account for" increased Voc due to conditions that differ from STC. So in this case, getting ambient temp down to say 50 degF while having good sun exposure can raise the voltages if the panels cool a bit.

Hopefully, most days it will be a non-issue. But I think Glen's design for a Dump Controller may just be what I need to buy some margin.

[ghurd]

For the Ghurd device, do I just need one unit for the whole array (or one per panel)?  I will have 3x 80W in parallel, possibly 4x which would be 10A total at spec.

Also, where do I install it?  e.g. assuming one Ghurd.... use it between the PV combiner and the Charge Controller? (exactly how?) ...or on the battery bank (w/ 24V kit)?

Only need 1 kit.

Depending on the situation, may need 1 fet and 1 blocking diode per panel.
Would not hurt your situation if you used #14 outdoor romex, one for each panel.

The ghurd circuit monitors the battery voltage, and controls a fet (or fets) in the charging wire(s) before the blocking diode(s).

This is the typical 12V version of it,



For future reference, here is another version.
This one leans toward 'overkill', but I like overkill, especially when the price difference is only $1 or 3.



You would want to greatly oversize the blocking diodes, meaning something bigger than a 6A diode.
Never parallel blocking diodes, meaning 2 x 6A diodes is a bad idea.
I am sure I have a few suitable options here if you order a kit too.
G-

[Ray by-day]

Glen,
Thanks. I am reviewing and thinking, and will get back here soon.

I am sure I have a few suitable options here if you order a kit too.

Good to know. Again thx.
I've gotten back on your site and am digesting all the info and components. I am pretty sure I'll be ordering at least one Ghurd kit, and whatever extra ICs/devices needed.

I might go ahead and get a 2nd one also (I have 5 panels. Currently thinking I'll use 3 of these on the available south-sloping garage roof (limited area though). That is the 24V system I am planning. I have the option to make that 4 modules instead of 3. Then in the future, with the remaining 1 or 2 panels, I'll assemble a small trellis-mounted 12V system at the other end of the house (use it for emergency lighting, gadget charging, etc). I might want a Ghurd kit here too, or might opt for a MPPT charge controller.)

Back soon.

[wooferhound]

Ray
I think you are misunderstanding Ghurd, He is saying that you can use 1 controller kit to control your battery voltage. At that point you will not need the charge controller that you already have.

[Ray by-day]

I think you are misunderstanding Ghurd, He is saying that you can use 1 controller kit to control your battery voltage. At that point you will not need the charge controller that you already have.

Thanks for the heads-up. Well that changes things if true. I can see that the diagram leaves out a PWM Charge Controller, but I did not interpret that to mean use the dump controller in place of a CC.

I certainly want a three-stage charge controller for my DC AGM batteries, and I was expecting to just attach the CC in addition to what Ghurd's design lays out.

I hope that what was proposed in the above solution is that the combination of the Ghurd dump controller and the individual FETs on each pair of feed wires from the PV modules --- would sense and dump excess voltage above say 46V or max 47V. (I expect I would need to start a little lower by approximation and monitor the trigger level on a scope, adjust the pot.).

Again, we're talking about a PV array putting out 32.3V nominal Vmp (minus volts dropped in the wire run).

[ghurd]

A dump controller IS a CC.

A ghurd controller, in the standard format, is a PWM charge controller.

Not sure why you would want a 3 stage controller with AGM in a RE system.
Myself, with RE and AGM, I would only want a single-stage.
RE is different than UPS systems, from the concept.
(yes, I am aware some people like the 3rd stage with SLA/AGM in RE systems, and some even like the 2nd stage, but I do not, because it always toasted my battery)

What 'we' are worried about is Vcc at Voc under certain conditions being higher than something similar to a not very popular controller can accept.
I know a few options for the situation.  The best by far is to get a better controller.
G-

[Ray by-day]

Not sure why you would want a 3 stage controller with AGM in a RE system.

I guess the "in a RE system" is the key here. Most of what I researched about deep-cycle and AGM batteries led me to folks on a wheelchair/mobility forum with some thorough discussions of the battery technology and proper maintenance. (can link if desired). In those uses/applications, the expert advice was 2-stage, or possibly 3-stage modern chargers. Also it seems that most solar charge controllers provide 3-stage charging (some adding periodic equalization if applicable).

I suppose when you say RE system, that the implication is that the battery bank will always be brought up to good recharge level whenever the sunshine returns (unless the array is undersized), and in this application/system a float stage is unnecessary?

A dump controller IS a CC. A ghurd controller, in the standard format, is a PWM charge controller.

Thanks for clarifying that. Part of what got me thinking adding the Ghurd device along with the CC, was Sparweb's post on your early development and beta devices -- he had a Morningstar Ts60 in the mix. His need and application was different than mine (he had solar + wind, and wanted to put the excess power to good use). But I latched onto the idea that the dump controller was an add-on.

I know a few options for the situation.  The best by far is to get a better controller.

Ultimately I may have to cave and just go for a Morningstar or other 'popular' CC.
As for other options or solutions, I do want to see what else is doable and practical. Linear Technology has some ICs and modules gear to overvoltage protection. (LTC4365, Lt4363, etc) Other semiconductor co's as well. I just do not have the expertise to do it myself, and even if I found the right App-note, there are a number of things to get right in the execution -- and I probably wouldn't.

Thanks for your replies and help.