# how to limit peak voltage from DC solar panels?



## dtbaker (Jan 5, 2008)

I'd like to know more about ways to accurately set/limit the peak DC voltage from a set of PV panels... to be used in an off-grid charging situation where I'd like to avoid using an invertor to standard 110vAC (or 220vAC), and then running thru an off-the-shelf charger to attain typical CA->CV control of charging a pack.

What would be the easiest/cheapest way to limit a PV panal array designed to output slightly higher than desired peak voltage, and restrict voltage to a specific preset max... essentially creating a charger that is always CV at a specific voltage?

A 'fixed' configuration is fine, as long as the preset voltage output is pretty accurate... like maybe +/- 10% or say 1 volt in a typical 120vDC or 144vDC pack. The idea would be to set that max pack voltage, and jut let it charge at CV, assuming amps=0 is full...

possible use might be an off-grid battery pack for home or vehicle to avoid efficiency loss from inverting and then back to DC...

ideas?


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## spdas (Nov 28, 2009)

Aloha, you will need a charge controller, like a Outback FX 80. That will give you steady voltage and will vary the amperage as clouds and sun position changes. But this unit will max out at 80Vdc if you program it right. (so you may have to charge one bank at a time or charge the 2 banks in parallel. Just set up your panels to about 100v-115v average input. They cost about $550.00. Advantage is that you can set an algorithum to suit Lifepo4. 

Francis


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## aeroscott (Jan 5, 2008)

study the "mppt" as used in the Outback FX-80 (others use this too), it's max output , 60 volts LA pack , 80 amps . it adjusts volts/amps to maximize watts into battery and to protect battery . Outback should build a high voltage unit for ev's .


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## dtbaker (Jan 5, 2008)

hhhmm, ok you've given me some info to go read more about... I am hoping that I can dig up some little (less expensive) voltage regulator type thing where I could set up a small set of PV cells that would kick out maybe 1kW at 140v-150v and relatively low amps like just 5a-10a, which could be regulated accurately down to a safe CV for the pack like 136v-138v for a 120v nominal pack, and just let it trickle in...


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## Ziggythewiz (May 16, 2010)

What is your application? Would you be charging for days continuously, or only while at work after discharging X amount? If it is the latter, you could pretty easily size your PV system so given ideal conditions it would only charge to 90-95% while you're at work.

If it's the former, I'd suggest building your own charge controller. There are many commercial charge controllers available, but typically for lower voltage than your requirements. There should be a number of DIY controllers out there, I know Lady Ada with adafruit has done some work on optimized solar charging.


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## dtbaker (Jan 5, 2008)

Ziggythewiz said:


> What is your application? Would you be charging for days continuously, or only while at work after discharging X amount?


I'd want something that cannot be overcharged... i.e. if I drove into backcountry and left a vehicle sit a couple days while hunting or backpacking, etc. Some small panel setup small enough to set up on site, but might take a couple days to charge would be fine.


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## Ziggythewiz (May 16, 2010)

Unless this is your portable PV setup, I don't think an on-site setup will overcharge you in a few days.










Do you know what\how many panels you'll be using? Is this to charge your plublished EV system?


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## dtbaker (Jan 5, 2008)

for a different project... 

probably not using standard panels since I would need too many to get voltage up enough. probably have to go with series of small panels to keep total system size and weight down. I realize it might take days to trickle in enough juice, but that is ok for this project.

technical issue is how to regulate voltage down 'not to exceed' a specific value accurately as inexpensively as possible.

D


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## Ziggythewiz (May 16, 2010)

Just google up some DIY controllers, many people have made them for everything from single cell phone charging to wind PV systems and shared their circuits. You'll have to customize their circuit for your application. Try to find one that is designed similar to the open revolt controllers, where the logic and power segments are separate, and all you have to do is upgrade/modify the components in the power segment to handle the voltage you need.


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## peggus (Feb 18, 2008)

You just need a comparator that turns off a transistor when the pack voltage exceeds some preset value, maybe throw in some hysteresis for bang bang style control, or slow down the gain so the transistor slowly turns off acting like a linear regulator.

This really isn't much different from how a shunt regulator works, but instead of turning on a balancing load you turn off a transistor that is in series with you pack.


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## dtbaker (Jan 5, 2008)

peggus said:


> You just need a comparator that turns off a transistor when the pack voltage exceeds some preset value



interesting... even simpler than I was thinking. I wouldn't mind just clicking to 'done' when pack voltage hits a specific value and stays done till manual reset.

do you have any specific recommendations on an accurate, settable 'comparator' that I can use to open a relay to cease charging? and/or a vendor I can poke thru online?

totally out of my experience here so I don't really know what to look for.


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## DJBecker (Nov 3, 2010)

You should first do a little research about the current-voltage output curve for the panels you plan to use. It's a non-linear relationship, and it changes with the light intensity, color temperature and panel temperature. If you want maximum power out of the panels you'll need a smart controller that dynamically matches your array's output to the load.

Typically maximum power output is loading the panel to 20% lower than its open circuit voltage.
The controller doesn't actually need to know the parameters. It can just draw slightly more or less current and see how the voltage changes. A simple multiply tells which point produces more power, and it can use that as the new basepoint, thus "hillclimb" to the optimal setting.

I'm pretty certainly you'll never need to worry about overcharging your batteries unless the array is immense or the battery pack is tiny.


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## dtbaker (Jan 5, 2008)

DJBecker said:


> I'm pretty certainly you'll never need to worry about overcharging your batteries unless the array is immense or the battery pack is tiny.


or the packs sits for a month filling up.... I would just feel so much better with an auto-off when pack voltage hits a set limit!

...and would I have to add diodes or anything to prevent pack voltage from 'leaking' out the panels when their voltage is BELOW pack voltage?


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## Ziggythewiz (May 16, 2010)

dtbaker said:


> or the packs sits for a month filling up.... I would just feel so much better with an auto-off when pack voltage hits a set limit!


Yeah, if it's not a couple days but a month, you definitely want to control that limit. I fried a battery once just by not watching enough TV (45 watt panels & 27" tube) over the summer with a cheapo controller maintaining it.



dtbaker said:


> ...and would I have to add diodes or anything to prevent pack voltage from 'leaking' out the panels when their voltage is BELOW pack voltage?


If you're using actual solar panels, they'll have the diode built in. If you're making your own panels, you'll need to add it. I've heard it's best to buy 'em made as DIY is only slightly cheaper and takes a lot of time for poor results.


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## tweaker (Apr 7, 2011)

Years ago, it was common to charge batteries with no controller using Arco panels producing 3.?v, meant to be used in either 3 or 4 panel configurations. The 3 panel configuration only produced somewhere around 14v so all was good. Point is, you could configure your panels to produce less than full battery voltage and not need anything.


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## dtbaker (Jan 5, 2008)

tweaker said:


> Years ago, it was common to charge batteries with no controller using Arco panels producing 3.?v, meant to be used in either 3 or 4 panel configurations. The 3 panel configuration only produced somewhere around 14v so all was good. Point is, you could configure your panels to produce less than full battery voltage and not need anything.


in this case, it would have to be in the (narrow for Li) band between operating voltage and max..... Perhaps I could series together panels that would output a MAX above 38x3.4= 129v and below 38x3.7=140v and no control at all would be required.... hhhhmmmm...... not very efficient probably, but cheap and effective given enough time.


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## spdas (Nov 28, 2009)

aeroscott said:


> study the "mppt" as used in the Outback FX-80 (others use this too), it's max output , 60 volts LA pack , 80 amps . it adjusts volts/amps to maximize watts into battery and to protect battery . Outback should build a high voltage unit for ev's .


I have emailed Outback to ask when they are coming out with a 144v version, just to put a bug in their ear. BTW the FlexMax 80 and FX60 both put out 80v max and up to 80 or 60 amps, respectively . (you just have to set it manually and not use the 12-24-48-60v preset functions).  And a charge controller (especially MPPT) is a given essential and cheap. 
I am setting up my 44 lifepo4 cells into two banks of 22x3.2 and will charge at 80v giving 3.63v per cell. I will monitor and see if CV to the end is appropriate or maybe drop it back to 3.55 and CV to the end with max amps and monitor how the Outback tapers off.

Francis


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## dtbaker (Jan 5, 2008)

spdas said:


> I am setting up my 44 lifepo4 cells into two banks of 22x3.2 and will charge at 80v giving 3.63v per cell. I will monitor and see if CV to the end is appropriate or maybe drop it back to 3.55 and CV to the end


I will follow with interest on this. .) I hesitate to split pack on charge because of balance issue in operation, but maybe that will work out ok. I would also hesitate ending at 3.55xcells because the curve is still pretty flat down there, but I guess it would be safe... just with a little less capacity.


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## spdas (Nov 28, 2009)

I can go up to 3.64v but have to experiment to see how many amps I can be pushing in at that voltage to be safe and not overcharge. So I may have to cut back the voltage a bit or trim the amperage down faster so not to overcharge. And yes I wish I had a solar charge controller that could do 160v and not need to split !! But the charge controller is quite precise and the charge should be identical in each bank. Alternatively you could put contact switch in between the 2 banks to isolate them and parallel the charge from the controller. 
I think Morningstar or Xantrex is coming out with a 96v.

Francis


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## peggus (Feb 18, 2008)

dtbaker said:


> interesting... even simpler than I was thinking. I wouldn't mind just clicking to 'done' when pack voltage hits a specific value and stays done till manual reset.
> 
> do you have any specific recommendations on an accurate, settable 'comparator' that I can use to open a relay to cease charging? and/or a vendor I can poke thru online?
> 
> totally out of my experience here so I don't really know what to look for.


Google "voltage sense relay". They're often for AC so you might have to dig a bit to find one that works for DC.


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