Author Topic: Re wiring/ configuring Panel Voltage.  (Read 3954 times)

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george65

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Re wiring/ configuring Panel Voltage.
« on: November 08, 2016, 02:58:49 AM »

I'm wondering if I can re configure 250W panels which are rated at Voc 37.25 and  Vmp 30.08 to an effective 12V configuration, IE, 15V Vmp?

I have looked this up and seen where the question has been asked before and every respondent has suggested Mppt controllers but that is NOT what I am asking or have found a clear and informative answer for.

What I would like to know is if there is provision to re wire the diodes and tap into the junction box so I get half the voltage and double the amps?
My concerns are:
On modern panels, are the diodes in the boxes generally accessible so the cell strings can be reconfigured, and solar cells seem to be funny things so is there any reason if I can re configure the wiring,  the cells will have some sort of electrical hissy fit or other weirdness?

And no, I'm not worried about voiding warranty's or stuffing them up, no reason that should happen unless they have some weird properties someone can fill me in on.

And just to prempt the " Why not get 12V panels" question that always seems to get asked on the topic....Because i can buy 250W conventional panels for a FRACTION of the price I can buy 12V panels for and for my purpose, a cheap PWM controller would be more than fine rather than going to the expense of Mppt  When I would want multiple setups like this.

Any info appreciated

OperaHouse

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Re: Re wiring/ configuring Panel Voltage.
« Reply #1 on: November 08, 2016, 09:58:49 AM »
I remember some older panels being configurable.  They would have an internal jumper bar.  But, I highly doubt any of those have been made in a long time for cost reasons. The panel world keeps going to higher voltages.

george65

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Re: Re wiring/ configuring Panel Voltage.
« Reply #2 on: November 09, 2016, 05:38:48 AM »

Seems the only way I am going to find out is get some and see for myself. Might be some brands of panels set up and more suitable to convert than others.

OperaHouse

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Re: Re wiring/ configuring Panel Voltage.
« Reply #3 on: November 09, 2016, 06:58:39 AM »
I think that would be like leaving the porch light on for Jimmy Hoffa.  Higher voltage is the way to go What is your target wattage for your array?  Previously you mentioned some really high numbers.  Eventually you will have to go to higher voltages if that is the target. At that price those used panels are likely to have some issues.  I suggest building a tester before final installation.
« Last Edit: November 09, 2016, 07:06:31 AM by OperaHouse »

george65

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Re: Re wiring/ configuring Panel Voltage.
« Reply #4 on: November 09, 2016, 08:16:58 AM »
No, this is for another application.

I want to charge 12V single batteries in multiple locations.  A 50w panel would probably do but I can get a 250 cheaper.  A LOT cheaper. I also don't want/ need an Mppt controller when an $8 Pwm will be more than enough even on a cloudy or wet day. Unfortunately it seems I can't have a 30V panel feeding into one to charge a 12V battery.
That being the case, If I could re wire the panel to give twice the amps at half the volts, I'd be right.

I saw a vid showing the diodes in the box at the back of a panel. They had one for each string and on the panel shown had 6 strings.
It would seem to me that if these strings were split and paralleled, they should work with the tolerance of the cheap controller.
If they still do panels or I can get to the string connections like I saw, should be easy to re wire and bridge them to 12V which I suspect is what is done with 12V rated panels.

What would the tester be you mention?

OperaHouse

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Re: Re wiring/ configuring Panel Voltage.
« Reply #5 on: November 09, 2016, 08:48:00 AM »
For a simple low power application a cheap buck converter for about $8 can be used to convert down to the lower voltage.  Normally I would suggest some added circuitry to operate the panels at power point.  With an oversize panel that wouldn't be necessary. The charge method would be a fixed voltage simple bulk charge mode like a car.  Operating it at 13.9V would likely be a good compromise.
Something like this could work, even two in parallel for more current and less heating. Sometimes the spec is a bit too optimistic.  Isolation from battery with diode probably a good idea just in case they short out.

http://www.ebay.com/itm/DC-DC-CC-CV-Buck-Converter-Step-down-Power-Supply-Module-7-40V-to-0-8-35V-8A-F5-/302133509875?hash=item46588f7ef3:g:QOIAAOSwLmVXEF4a

I just bought three of these after posting.  In a month when they show up I'll tell you what I think of them.  A tester could be as simple as a 4 ohm resistor.  A 300W one would be expensive.  A 2000W 125V water heating element is about 7.5 ohms for $10.  That in a bucket of water would give at least a 4A test at 30V.  Just record the voltage you get.  In future applications you will likely put at least two in series and it is important that these numbers for each string match pretty closely.   Weaker ones could be used for applications like the above.

Are these the ones from ML Solar?
« Last Edit: November 09, 2016, 02:22:30 PM by OperaHouse »

george65

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Re: Re wiring/ configuring Panel Voltage.
« Reply #6 on: November 09, 2016, 08:11:45 PM »

Thanks for the info. Very helpful.

I was looking at those converters last night but trying to find ones that did more amperage.  Never dawned on me just to use 2. Sometimes the things I miss and over look that I should pick up on just frighten me.

I'll get the panels and see how they are set up. If I can't " hack" them, these things will be a great and workable solution.

One of the things I want to do is keep some batteries on a tractor, ute and a bunch of other machinery charged. I wanted to use multiple chargers off the one panel as the batteries won't be connected and will basicly be floating them anyway. I can run these converters before the PWM's on the separate circuits and batteries I want to connect to.

I have been testing the cheap charge controllers for a couple of months and am happy with them.  I'm sure this will be a big thing for my father who is getting too old to be carrying batteries round every time he wants to start something. I bought him a couple of those Lithium Jumper packs that fit in your shirt pocket and they are great but they don't stop the battery from being damaged through going flat and sitting from lack of use.
I'll put breakaway type plugs on the charging leads and wire the vehicles so he can just reverse out without having to Disconnect them or rip everything out. He's real good at remembering to take the terminals off the battery when he's finished, usually jumps on or in without remembering to reconnect however.

All the water heater elements I have are around 2400-3600w ( have a couple of 4800W units) @240V.
For a load tester I can just use my Hydrogen cell. Only thing they are good for is load testing.  Just add in the amount of solution to get the load you want.  If the test is going to be a while, I stick it in a bucket of water to help cool the thing other wise it can boil like a kettle.
You can sink some serious amps into these things while keeping them quite compact but you are in fact making an electric Jug.

Got to order a couple of those RC type load meters as well.  60V@ 100 amp with watt hour measurement would be real handy especially for the price.

OperaHouse

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Re: Re wiring/ configuring Panel Voltage.
« Reply #7 on: November 10, 2016, 04:53:02 AM »
I had a long reply but Microsoft wiped it out doing an update.  Why don't they put the notice in the middle of the screen instead of off in the corner where you don't look?

Short answer, if cheap enough just tap off at the half voltage point and waste the  other half.

DamonHD

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Re: Re wiring/ configuring Panel Voltage.
« Reply #8 on: November 10, 2016, 04:44:52 PM »
My 36V (60-cell) panels have 3 diode-bypassed strings internally.  There is no half-way point.  And I'm not sure it's easy to get to the taps.

For the same price per W I can buy 12V-nominal (ie 17V+ Vmpp) panels.

Rgds

Damon

george65

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Re: Re wiring/ configuring Panel Voltage.
« Reply #9 on: November 29, 2016, 08:00:25 AM »

To follow up.....

I got a good, programmable PWM controller  for $17 from fleabay and put it on my 30.5V nominal panel.  The thing worked fine charging a 12V battery.
Most current I saw was 10A which was predictable but not shameful. Perhaps if I had another controller on the same panel I'd get more in total across both controllers.

I popped the  panel off the back of the junction box and head easy access to teh 3 Diodes. I measured about 11V across 1 diode, 21 across 2 and the normal panel voltage off the 3.  Didn't touch anything because the cheap controller works fine on a 37V oc panel charging a 12V battery.

OperaHouse

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Re: Re wiring/ configuring Panel Voltage.
« Reply #10 on: November 29, 2016, 08:50:10 AM »
Using PWM at higher panel voltages the current will limit to the panels max.  I did see on another board someone had replaced three "MPPT" controllers because the battery voltage was up around 16V and his batteries were being damaged.  Pictures of the inside of the controller proved that it was only a PWM controller from CHINA.  Once battery is near full charge it can easily go over voltage with as little as 10A pulses.  I think it is worth while monitoring the battery for a while to see if this over voltage condition exists in full sun.  If it does, you should try the 21V tap.

I just received a couple of those $4 buck converters and they look pretty good if you have other projects for these panels.
« Last Edit: November 29, 2016, 09:11:24 AM by OperaHouse »

george65

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Re: Re wiring/ configuring Panel Voltage.
« Reply #11 on: November 29, 2016, 03:40:08 PM »

The controller I got has settable cutoff voltages.  I have only seen it overshoot .1 of a volt for a very short time.
I originally had it set for 14.5v but the battery was getting a bit warm so I set it back to 14V and it seems fine.  I like a fraction extra to allow for drop off and inaccuracy etc.

The controller I have is definitely PWM, it was never made out to be anything else but geez it works well. I has an LCD screen where you can set all the functions so I guess under more ideal parameters would be not too far away from a MPPT.  I also have some real cheap controllers and was using them on a cheap battery charger for an automatic cut off. They were a little high on the voltage but must have only put out low amps at that rate as the battery was quite happy and not getting warm or gassing.

I'm real happy with this controller and think I'll get another similar one. I have seen them where they display all the info on one window so you don't have to scroll through it all. For my purposes of messing around that would be handy although I have ordered some other meters I can hook up as well for monitoring.

Just for fun I'm running the outside bar fridge off the setup. I have a UPS hooked up that is the type that will cold boot just off battery power so I'm using it as an inverter. seems to be working fine. Had a smaller one hooked up but it wouldn't start the Fridge motor when it was hot so I put on the larger one.
I disconnect it at night to avoid taking the batteries lower than I want to go but yesterday was cloudy and the 2 160 panels had the batterys at the 28V I had set  wether the fridge was running or not.

Once the sun comes out I will try hooking up a bigger fridge!  :0)

ghurd

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Re: Re wiring/ configuring Panel Voltage.
« Reply #12 on: December 05, 2016, 07:33:33 AM »
I tinkered with quite a few buck switchers.
Not exactly what I expected.
2 X 12v 1A panels in series, 12v battery, set for a 12v battery.
When charging, 1A from the panels equaled 1A into the battery. Straight through. 50% efficient.
They do work efficiently for other things, like a 12V LED bulb from a 24v battery.

Warning. Remember to set the output voltage higher than the battery before connecting the battery or they blow up violently. Some probably have protection from that but I know 1 didn't!

If you are trying to experiment with grid backup charging, there is a variation to my controller that can do that.
G-
www.ghurd.info<<<-----Information on my Controller

Warpspeed

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Re: Re wiring/ configuring Panel Voltage.
« Reply #13 on: December 26, 2016, 01:17:58 AM »
I don't see any reason why you need to specifically reduce the voltage of the solar panel. 
Sure, it will be pulled down below its most efficient operating point, but so what ?

All you need to do is disconnect it when the battery voltage reaches some preset maximum, say 14.2 volts, and re connect it when the battery voltage falls below some preset minimum, maybe 12.6 volts.

The battery will very quickly come up to 14.2, then very slowly fall back to its resting voltage.  A resting voltage of 12.6 is considered fully charged, and it should hold that voltage for a long time if the battery is good.

The charge, rest, charge, rest cycle is MUCH better for the battery than floating it at a constant continuous 13.8 volts.

I have been doing this for almost two years on a vehicle I very rarely drive.  Power comes from a 15v 2A wall pack through a 12v 18w filament lamp to limit current. you probably will not need a current limit with a solar panel, but that is up to you.

After a while the charge cycle gets shorter and shorter, and the rest periods get longer and longer.  Its very good for the battery and the battery has used zero water over two years.  In fact the battery seems to be in much better condition than it was two years ago.

You can buy a programmable voltmeter that will do all of that. It will read battery voltage to four digits, say 12.34v and there are two independent one amp relays that you can program for exact pull in voltage, and exact drop out voltage in 10mV steps.  That is what I am using and its probably all you need except maybe a more grunty external relay or possibly a lamp to limit charging current.

http://www.lightobject.com/Programmable-4-Digit-Red-LED-ACDC-Volt-Meter-with-Dual-Control-Good-for-HHO-System-P408.aspx

Just connect up a few wires, program the voltmeter for whatever voltages you like, and the job is finished.  An occasional glance at the battery voltage will tell you exactly how the battery is doing.
« Last Edit: December 26, 2016, 01:39:06 AM by Warpspeed »

OperaHouse

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Re: Re wiring/ configuring Panel Voltage.
« Reply #14 on: December 26, 2016, 08:42:10 AM »
That looks interesting.  You would never get that it is a controller from the spec.  Like one reviewer said, "the instructions are near to useless. There is nothing to tell you where to attach the wires."

It always confounds me that sellers want you to buy stuff and provide no information on how it is used. There is absolutely no information that it even has outputs or can control anything,  just that it has a lot of terminals.  If I was looking for this item I would have given it a quick look and moved on.  Any online data sheet for this?  In that price range there are certainly other options.  Where I once worked they didn't want to confuse the customer with extraneous information on data sheets like how to hook it up.
« Last Edit: December 26, 2016, 10:08:36 AM by OperaHouse »

Warpspeed

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Re: Re wiring/ configuring Panel Voltage.
« Reply #15 on: December 26, 2016, 01:14:45 PM »
I have found nothing on line, but there is an eight page A4 sized operation manual that comes with it that is not that bad.
See below for some abbreviated  specs.

Have to agree that the information available on line is absolutely atrocious.
But the unit itself is well made, accurate and works very well.
I can provide a wiring diagram and help to anyone might need assistance, but its all there in the manual.

As with many of these things, programming it can be a bit of an initial hurdle the first time, first there is the password, (all units are the same) then you scroll through to select  function of which there are many, then each "on" or "off"  voltage can be incremented or decremented.
Its hopefully something you ever only need to do once.
From then on, it just goes on working without needing any further attention.

Four input voltage ranges (ac or dc) 1V  10v  100v  500v full scale (separate input terminals provided for each range).
It has selectable low pass input filtering to reduce noise.
Can measure + or - input voltages with a - indicator on display.
An offset voltage can be added digitally to move display zero away from true input zero.
Operating power dc 9v to 30v 2 watts
Two relays with changeover contacts rated ac220v 3A or dc30v at 3A

Quoted accuracy is 0.8% with dc input, and 1.0% with ac input.
 
It would be perfect for battery under voltage and over voltage alarms and cutout.
Or to control an on/off battery charger.
Or make a nice battery discharge tester.

In spite of the woeful information on the internet, its actually a rather useful instrument.
I originally bought one, then later bought two more for future projects.
« Last Edit: December 26, 2016, 01:41:49 PM by Warpspeed »

littleharbor2

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Re: Re wiring/ configuring Panel Voltage.
« Reply #16 on: January 03, 2017, 10:51:13 AM »
If you want to attempt surgery on a larger grid tie panel to get 12 nominal voltage out of it you need to start with a 72 cell panel, something in the 300+ watt range. You need to split the buss tabbing between the 36th and 37th cell. Then parallel the positive side with the positive in the junction box or whatever is on the back of the panel. Ditto for the Neg. side of the buss tabbing. It can be done, I have done it and ended up getting half the voltage and double the amperage. It's not pretty but it is doable. And you can forget about any warranty on the panel.
 Attempting this procedure on a 60 cell panel will result in too low voltage for practical 12 volt charging.
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