Can I modify these 24v panels to output 12v?

neuroneuro Registered Users Posts: 8
I am working on an RV setup with two 24v panels.

Panel specs:

Suntech STP165S-24/Ab-1

Open - Circuit Voltage (Voc)- 43.6v
Optimum Operating Voltage (Vmp)- 34.8v
Short - Circuit Current (Isc)- 5.04A
Optimum Operating Current (Imp)- 4.74A
Maximum Power at STC (Pmax)- 165Wp

I have 2 of these panels for a total Max Power of 330Wp.

These panels have a junction box on the back that looks like this inside:

Attachment not found.

Is it possible to modify these panels to output in the 12v range (17-18v) instead of the 24v range (34-35v) so I can use a 12v charge controller in order to charge my battery bank?

I'm having trouble finding an affordable (<$200) MPPT charge controller that will accept 24v input and output 12v to my battery bank.

Thanks!

Comments

  • petertearaipetertearai Solar Expert Posts: 464 ✭✭✭✭
    Re: Can I modify these 24v panels to output 12v?

    Unfortunately the short answer is No.
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Can I modify these 24v panels to output 12v?

    It looks like your solar cell strings are broken into 1/3rd's--Which means there is no good way of "breaking" the panel into halves of 12 volt (Vmp=17.5 volts) each.

    If you have a volt meter, you can double check. With the meter set to 200 VDC full scale (or >20 VDC), put the black(-) wire on the Negative solar panel connection, and then with the positive meter lead, check the voltage on each of the other connectors--I would guess you will see 1/3rd, 2/3rd and full voltage as you go across the three terminals).

    MPPT controller wise, some very good smaller units that do both 12 and 24 volt systems:

    Morningstar SunSaver 15 Amp MPPT Solar Charge Controller
    Price: $217.00

    www.roguepowertech.com/products/mpt3024.htm
    Rogue 30 amp 12/24 volt MPPT controller $325
    • High efficiency: 98.0% peak (34Vin/24Vout) - 97.2% peak (17Vin/12Vout) - 96.1% peak (34Vin/12Vout)
    • Ultra-low standby power consumption: 53mW typical in sleep
    • 30-amp output, 12-volt or 24-volt
    • Use with 12-volt or 24-volt nominal PV arrays (60-volt Voc maximum)
    • Fully automatic maximum power point tracking - no adjustments to make
    • Capable of charging a lower voltage battery from a higher voltage array
    • 20x4 backlit LCD display with intuitive, easy-to-use interface
    • Setpoints can be retrieved, viewed, and adjusted at any time with the front panel controls
    • 30-day data logging records vital parameters for assessing system performance
    The first 15 amp controller is a bit on the small side for a 330 watt PV system, unless you are mounting the panels flat on the roof (less than ideal tilt)--Then you may be OK. If you get this guy, highly recommend getting the Remote Battery Temperature Sensor option for optimum battery charging.

    The second controller would be fine with those two panels connected in parallel (Vmp=35 volts) and a 12 volt battery bank with 30 amp maximum battery charging current. And you would even be able to add a 3rd panel on the same controller.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • neuroneuro Registered Users Posts: 8
    Re: Can I modify these 24v panels to output 12v?

    Thanks for the replies guys.

    Is it not typical that MPPT controllers will accept 24v input from the panels and then "step down" the voltage to charge a 12v battery bank.

    I've been asking questions of sellers on eBay and the general response I get is that 24v controllers work with 24v panels and output 24v to the battery bank. Likewise, 12v controllers work with 12v panels and output 12v to the batteries.

    I guess I'm looking for a "hybrid" controller that can accept 24v input from the panels but output 12v to the batteries, which seems hard to find for under $200. With the ones I do find, the maximum open circuit voltage the controllers can handle is well under the output of my panels. I'm not "electrically versed" and do not understand the implications of this discrepancy. Would a panel such as mine with a Voc of 44v damage a controller whose specifications state a maximum Voc of 30v, for example?

    Here is one on eBay:

    20A 12V/24 MPPT Solar Charge Controller- Item No. 320762936368

    That Morningstar controller looks pretty good. Can I also connect one of the LCD type monitors to it so I can see all the specs of what's going on under the hood?

    Thanks again for the responses.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Can I modify these 24v panels to output 12v?

    If a charge controller fails then it could mean that your batteries are overcharged which will lead to premature failure. So it really pays to invest in a known brand good quality charge controller like the morningstar or Rogue units.

    To make a fair price comparison to the ebay no-name brands, add the cost of replacing your battery bank to them ;)
  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Can I modify these 24v panels to output 12v?
    neuro wrote: »
    Is it not typical that MPPT controllers will accept 24v input from the panels and then "step down" the voltage to charge a 12v battery bank.
    It depends, many smaller/low cost MPPT controllers have a limited input voltage range. There are a few 12 volt models than only take Vmp/Voc of 18volts/25 volts range on the input range.

    Usually, the newer/battery MPPT charge controllers will accept higher input voltage ranges.
    I've been asking questions of sellers on eBay and the general response I get is that 24v controllers work with 24v panels and output 24v to the battery bank. Likewise, 12v controllers work with 12v panels and output 12v to the batteries.
    In general, a good 12/24 volt MPPT controller will, at a minimum accept a 24 volt solar panel (Vmp~35 volts STC rating) and charge either a 12 or a 24 volt battery bank just fine (and yes, the 12 volt battery bank will get ~2x the charging current because of the MPPT conversion).

    When you have the spec's saying a "24 volt panels" (really Vmp~35 volts) for a 24 volt battery; and a "12 volt panel" for a 12 volt battery--those are usually PWM charge controller specifications (and PWM controllers are much less expensive).
    I guess I'm looking for a "hybrid" controller that can accept 24v input from the panels but output 12v to the batteries, which seems hard to find for under $200.
    Yes they are hard to find... The MorningStar 15 amp 12/24 MPPT is something like $217 (and is a very good MPPT charge controller).

    The Rogue 30 amp 12/24 controller is $325 or so--and is also a very good charge controller.

    The more expensive MPPT charge controllers (~$500+) will have an maximum input voltage of 150 VDC (or more). A 150VDC max input usually works out to around a maximum of Vmp~100 VDC for most people.
    With the ones I do find, the maximum open circuit voltage the controllers can handle is well under the output of my panels. I'm not "electrically versed" and do not understand the implications of this discrepancy. Would a panel such as mine with a Voc of 44v damage a controller whose specifications state a maximum Voc of 30v, for example?

    Probably would cause damage.
    I don't know if that is a MPPT or a PWM charge controller. Not enough information and no internal pictures of electronics (looking for large inductors if MPPT).
    That Morningstar controller looks pretty good. Can I also connect one of the LCD type monitors to it so I can see all the specs of what's going on under the hood?
    Yes, MorningStar makes lots of money for their remote meters.

    Rogue already has an LCD digital display as part of the base price.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FrxddyFrxddy Solar Expert Posts: 113 ✭✭✭✭
    Re: Can I modify these 24v panels to output 12v?

    I did not read every reply, but have you given thought of selling the 24V & buying 12V ones?
  • neuroneuro Registered Users Posts: 8
    Re: Can I modify these 24v panels to output 12v?

    I would have preferred to buy 12v panels, but I got these 24v grid-tie panels at an extreme discount from a Westinghouse warehouse sale in San Jose, CA. I bought them for $.86 cents/watt not knowing about the charge controller issues. So, now it looks like I'm going to spend what I saved on the panels for the appropriate charge controller. Lol.

    BTW, the eBay import charge controller I posted is an MPPT controller.
  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Can I modify these 24v panels to output 12v?

    The reason I am holding judgment on the MPPT from EBay is that we have seen lots of very nice advertising and when the box is opened up (or Vin/Iin:Vout/Iout is measured, they usually turn out to be nothing more than PWM controllers with a MPPT label on the front.

    It certainly could be a MPPT charge controller, but the few specifications there seem to point towards a PWM (as you say, a 12/24 MPPT controller should be able to run a "24 volt" solar panel and charge a 12 volt battery bank with almost 2x current increase at the battery voltage... 20% is almost within the margin of error of power measurements with home tools--and would only be seen on a 12 volt panel/12 volt battery bank on very cool days (near freezing and below). You would not see any current increase on hot summer days.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • neuroneuro Registered Users Posts: 8
    Re: Can I modify these 24v panels to output 12v?

    Wow, I didn't know there was such blatant false advertising and misrepresentation going on. I guess unscrupulous sellers count on customers never actually checking out the specs with testing equipment?

    Can I use a basic multimeter to test out any controller I eventually purchase?
  • rollandelliottrollandelliott Solar Expert Posts: 834 ✭✭
    Re: Can I modify these 24v panels to output 12v?

    I didn't read every reply, but it looks like that panel has 4 strings then thus could easily be divided into two 12v units.
    Just remove the central diode
    two left metal tabs should be 12v nominal and
    two right metla tabs should be 12v nominal

    get a $15 DMM from homedepot and it takes 2 minutes to test.
  • bmetbmet Solar Expert Posts: 630 ✭✭
    Re: Can I modify these 24v panels to output 12v?

    Don't you believe it. The Chinese translation has conflicting specs, plus there is this little blurb.... "Control mode for chargingPWM"


    neuro wrote: »

    BTW, the eBay import charge controller I posted is an MPPT controller.
  • neuroneuro Registered Users Posts: 8
    Re: Can I modify these 24v panels to output 12v?

    I thought PWM was the method all controllers used to actually charge the batteries?
  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Can I modify these 24v panels to output 12v?

    Sort of...

    Pwm without any inductors is just an on/off switch and can have high losses with poor voltage mismatches.

    Pwm with inductors are switch mode power supplies which can efficiently down convert higher voltages/lower currents to lower voltage/higher current.

    Sort of comparing a one speed bicycle to 15 speed bicycle. The switches plus inductors are the equivalent to a transmission.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • neuroneuro Registered Users Posts: 8
    Re: Can I modify these 24v panels to output 12v?

    I checked the voltage across the different strings with the diodes removed in direct sunlight.

    Across each of the two left tabs, two right tabs and two center tabs I read a consistent 12.5 - 13v. Across the two outer tabs I read 38 - 39v.

    Even as an electrical novice I know that a battery requires a higher voltage to charge it, so it looks like the two 12v panel theory won't work due to the output across any individual string being too low?

    As an aside, I read approximately the same voltages with the diodes returned to their respective places. And yes, I made sure to check their polarity.

    So, it looks like I'm still looking for an MPPT 24v to 12v controller. I got in touch with one of the eBay sellers who said they could modify one of their current line to work with my panels and output 12v. They were simply going to upgrade the circuit that handles incoming voltage so it could accept my panels' 35v output. So, waiting to hear back from them.

    I'll be sure to inform them that I will be doing rigorous testing to ensure that the controller is in fact an MPPT unit and is performing to specifications.

    They don't know that I'm an electrical idiot! :cool:
  • ArmyVetArmyVet Banned Posts: 20
    Re: Can I modify these 24v panels to output 12v?

    Tecnically Yes ...You can change the output. for 12 volt. I believe these have 72 cells. Usually they take and wire 2 set of 36 cell in series. As you can see you have terminal 1 2 3 4 Usually Terminal 1 is + 2-3 tied together 4 for the ground.
    What you should find with a Multimeter is 1 + 2 - 3 + 4 - Which explains the diode config. recheck and you can jumper the way you need it.

    But then again Ill get slammed for telling the secret of panels..
    :P

    Your voltages are reading right it seems. You just have to do the proper wiring. In full sun yo should see 17 or higher volts easy.
  • neuroneuro Registered Users Posts: 8
    Re: Can I modify these 24v panels to output 12v?

    How do I jumper the panels to achieve the proper 12v charging voltage (17V +/-)? Do I insert jumper wires across the terminals? If so, could you tell me which ones I should jumper?

    Here's the pic again, so you don't have to keep opening my first post:

    Attachment not found.

    Thanks!
  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Can I modify these 24v panels to output 12v?

    I don't think you can... Those panels appear to be broken into 1/3rd's (one bypass diode per string). Your panels are rated to ~Vmp of 34.8 volts.

    Going from the left, the Vmp rated voltage would be (call it common ground):
    • 1:1 = 0 volts
    • 1:2 = 11.6 volts
    • 1:3 = 23.2 volts
    • 1:4 = 34.8 volts
    For a "12 volt panel" you are looking at ~17.5-18 volts.

    1:3 at 23 volts will be, roughly:
    • 17.5 volts / 23.2 volts = 0.75 = 75% efficient (if used with a pure PWM charge controller on a 12 volt battery bank)
    You will have not used 1/3rd of the array (and the array wattage), and the remaining 2/3rds of the array will be used on 75% efficiency (assuming the charge controller does not over voltage--probably close, but it should be OK).

    If the charge controller is a working MPPT controller--it should be an interesting test. You should see a substantial increase in output current into the 12 volt battery bank with 1:3 terminals used.

    If the controller can handle the higher voltage, then just use 1:4 like a normal MPPT controller would (assuming within rated input voltage).

    Anything else would require you to cut and hack the panel to get other mixes of voltage/current combinations. Which may make sense if this is for pure lab testing and you don't care (too much) about weather sealing of the panel.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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