Modifying 24 volt solar panels

littleharbor2
littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
I ran across a question posted online which was asking about converting 24 volt output to charge a 12 volt battery bank. I know this is a bit odd and unconventional but has anybody ever inquired about converting a 24 volt solar panel to 12 volt? I have been playing with an extra 24 volt BP panel and found converting it to 12 volt output is pretty easy. It started out producing about 41 volts open circuit and about 4.5 amps, now it produces about 20.5 volts and 9 amps. Of course this voids any MFG. warranty but this was just an old unused panel I had lying around and I wanted to see if I could convert it.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

Comments

  • WillBkool
    WillBkool Solar Expert Posts: 35 ✭✭
    Re: Modifying 24 volt solar panels

    While that can work, I would just use an MPPT charge controller, which will do the conversion for you, and not mess with the panel itself. Just my $0.02
    1220 Watts, 4 Evergreen 120 watt, 1 Eoplly 190 watt; 1 Sungold 200 watt; 2 175 Watt; M-Star 15A MPPT; C40 PWM; 6 105 AH AGM Configured to 315@24V
    Cotek 1500 watt/24v
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Modifying 24 volt solar panels

    Welcome to the forum.

    It's not as unusual as you might think: there used to be panels that were configurable at the junction box for either 24 or 12 Volt. Basically a difference of having the individual cells all in series or dividing them up into two parallel strings.

    It is not very practical, though, as it not only voids the warranty to do this on a panel which is not designed for it but it also 'opens up' the panel increasing the risk of moisture contamination and premature failure of junctions.

    The way things are going even conventional 12 Volt panels are disappearing from the list of choices as GT style panels become the main offering (due to the greater number of grid-tie installs making them more financially viable to manufacture). When it comes to that, MPPT is your friend.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Modifying 24 volt solar panels

    Did you do that by splitting a connection in the Junction Box? That can work.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
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    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Modifying 24 volt solar panels

    Back 7 years ago when I changed to a MPPT controller, I had a bunch of identical "12 volt" pv which I wired in series pairs for a "24 volt" pv array to feed the controller. But I had one odd 12 volt panel. All was not lost. Was easily changeable to 24 volts in it's junction box, and the whole works have been living happily together ever since. BTW, when I got those 12 volt panels a dozen years ago, the price was around $7 per watt.
    Last Summer I purchased more panels (which feed a second MPPT controller) and they were GT panels that cost me less than $1 per watt.
    It doesn't take many GT panels to pay for a good MPPT charge controller. It's really the only way to go for any serious system.
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    Re: Modifying 24 volt solar panels

    What I did is remove the bottom section of frame and warm up the backsheet so I could slice it open just enough to access the buss tabbing. I cut the buss in the center, basically cutting the bank of 72 cells into two banks of 36 cells. I rolled the bussing back away from the surface enough to solder wire on to it. At this point I now have created a positive and negative lead which I brought back up and connected (soldered) in parallel with the regular output wires. I verified numerous times with a multimeter which was the neg. and pos. sides. The only issue I had was accessing the bypass(?) diode which was encapsulated in a clear amber-like substance. This diode was causing me a high or low voltage reading depending on weather I connected the banks in parallel or left them unconnected to one another. Once I clipped that center diode I was able to meter normal voltage expected from a 12 volt 160 watt panel. Of course this surgery probably compromises the weather resistance of the module I was especially careful to not cut the backsheet or encapsulant over any of the cells. If a quality sealant were slathered over the affected areas I think you have a chance this modification will hold up to the elements.
    This was just an experiment to see if I could do this. I'm not advocating cutting into perfectly good panels. That being said, those of you out there who enjoy tinkering with solar panels might find this to be a cool thing to do with the end result being a high power 12 volt panel where there once was none.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    Re: Modifying 24 volt solar panels

    If that was an onboard blocking diode, then no problem as your charge controller more or less duplicates that function.

    However, you may want to consider wiring in two "bypass" diodes now that your panel is two in parallel. The reason being is that like if a cell(s) are shaded, then the entire panel current will now pass through that "reversed bias" shaded cell(s), and can do damage long term. By separating the two interior panels with bypass diodes, if part of the panel is shaded, only half of the panel current will go through the shaded cells. This shading can be an issue from bird-droppings, sticky leaves, or other debris than hangs on to the glass and shades cells underneath.

    My Sanyo panels have specially matched bypass diodes like these encased in amber.

    For DIY, you may want to look into high-efficiency SCHOTTKY diodes as opposed to generic rectifier diodes. All you need are two of them, but make sure you not only have them rated I'd say at least twice what flows into them, not only to handle the current but to make sure the leads themselves don't get too hot. It pays to beef these up and not just meet the minimal spec. A search for "solar schottky diodes" should turn up plenty of sources. If they offer them in matched sets, so much the better.
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    Re: Modifying 24 volt solar panels

    Thanks for the recommendation. This panel has what appear to be bypass diodes as well incased in the "amber". One thing I definitely don't want to do here is lead someone into a potentially dangerous situation.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.