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  • This is the most simple and affordable solar battery charger that the hobbyist can make. It has a few drawbacks over other similar controls, but offers numerous advantages. It is intended for charging lead-acid batteries, but may also be used for charging any battery at a constant voltage. Voltage output is adjustable.

    Advantages & Disadvantages of this solar charger

    • + Simple, small & inexpensive
    • + Uses commonly available components
    • + Adjustable voltage
    • + ZERO battery discharge when sun is not shining
    • – High drop-out voltage—may be marginal for 6V application
    • – Current limited to 1.5A
    • – No LED indicators—no bells or whistles

    Solar battery charger specifications

    • Solar panel rating: 20W (12V) or 10W (6V)
    • Output voltage range: 5 to 14V (adjustable) (may be reduced further by shorting R2)
    • Max power dissipation: 10W (includes power dissipation of D1)
    • Typical dropout voltage: 2 to 2.75V (depending upon load current)
    • Maximum current: 1.5A (internally limits at about 2.2A)
    • Voltage regulation: ±100mV (due to regulation of series rectifier)
    • Battery discharge: 0mA (this control will not discharge the battery when the sun doesn’t shine)

    Solar battery charger schematic

    solar battery charger circuit schematic

    6V Applicaton

    • Output Voltage: Set for 7V
    • Input voltage:
      • Battery discharged (6V): 8.75V Min @ 1.5A (this is a little high for panels that are characterized for 6V applications)
      • Battery charged (7V): 9V Min @ 10mA (e.g.)

    12V Application

    • Output Voltage: Set for 14V
    • Input voltage:
      • Battery discharged (12V): 14.75V Min @ 1.5A (Available from solar panel characterized for 12V operation)
      • Battery charged: (14V): 16V Min

    Minimum Head Voltage

    This is also referred to “drop-out voltage.” The input voltage must exceed the output voltage by about 2.75V @ 1.5A. Fortunately, when the battery discharged, the output voltage is lower so the solar panel voltage will also be lower.

    When fully charged, the battery voltage will be high, but the current is very low—at this point, the drop-out voltage reduces to about 2V and the open circuit solar panel voltage also comes into play. The schottky rectifier was selected to reduce this head voltage requirement—the voltage drop of the schottky is about 0.5V @ 1.5A or about half that of a typical silicon rectifier.

    More advanced controls have a much lower head voltage requirement and will function better under marginal conditions.

    Maximum Power Dissipation

    In this solar battery charger project the power is limited by the thermal resistances of both the LM317T and the heat sink. To keep the junction temperature below the 125°C Max, the power must be limited to about 10W. If a smaller or less effective heat sink is used, the maximum power dissipation must be de-rated. Fortunately, the LM317 has internal temperature limiting so that if it gets too hot, it shuts down thus protecting itself from damage. Max power comes into effect when charging a 12V battery @ 1.5A: e.g. battery voltage = 12V, solar panel = 18V. P = (18V – 12V) * 1.5A = 9W. So thermally, it is carefully matched to the current rating.

    If a solar panel that is characterized for 12V is applied with a 6V battery, the maximum current must be reduced to about 0.7A: e.g. battery voltage = 6V, solar panel voltage = 18V. P = (18V – 6V) * 0.7A = 9.6W. In this case, the solar panel power may not exceed 10W.

    When charging, the heat sink normally runs warm. When beginning to “top off” or completing the charge at maximum voltage, the heat sink runs hot. When fully charged, the heat sink runs cool. This heat is not exactly wasted power—it is excess power that is unneeded in the process of charging a battery.

    Current Limiting

    Current limiting is provided by the solar panel—it is not a commonly understood fact that the solar panel tends to be a constant current device. For this reason, a solar panel can withstand a short circuit.

    Therefore, the control does not need current limiting.

    Float Charge of Lead-Acid Batteries

    This control charges the battery at a constant voltage and also maintains a charged battery (float charge). The float charge voltage specification is a little lower, so to accommodate both charge and float charge voltage, a compromise is reached by simply reducing the voltage slightly—that is how ALL automotive systems operate. To obtain maximum charge in a 12V battery, set the control to 14.6V. Automotive systems further reduce voltage to 13 to 13.5V in order to accommodate high temperature operation as the battery is usually located in the hot engine compartment—battery has a negative thermal coefficient of voltage.

    solar battery charger project

    Application with other types of batteries

    It is difficult to determine how to set the voltage in this case. The easiest way to do this is to charge the battery fully using other means and then transfer the charged battery to the control and connect an ammeter in series. Increase the voltage setting until there is significant current and then back off the potentiometer until the current drops to perhaps 10mA or so. Some types of batteries such as lithium ion types must be disconnected after charging to prevent degradation.

    Solar Battery Charger Protection

    C1 provides substantial protection against static discharge.
    There is no protection against reverse polarity or mis-wiring. It is protected if solar panel is connected reverse without battery connected, or if battery is connected reverse without solar panel connected. However, if the battery or solar panel is misconnected simultaneously, who knows? anyone care to experiment?

    
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    85 Responses to "Solar Battery Charger Circuit"

    1. Great circuit for its simplicity and effectiveness and the minimum use of components. the use of the wiper resistor at R2 is something commonly not used but a sheer necessity for regulation.

      I prefer the diode in all solar circuits to be immediately after the Solar panel. To protect damage to the panel from the reverse flow from the battery after dusk, That is when the battery voltage is higher than the Solar panel voltage.

      Normally Solar panel nowadays do come with a built in Schkotty diode.

      • Jim Keith Jim Keith says: on August 6, 2012 at 3:22 am

        Point well taken–should potentiometer R3 open (common failure mode) the output voltage goes to maximum–that is the effect of putting a potentiometer in the feedback loop. For reliablility, only a high quality cermet trimpot is recommended–not “any” as indicated in the bill or material.

      • Great Ckt, Could you please help me for CKT recommendation for 24V, 20 Amps. I have installed 250W/24V panel. Bat : 100AH -2Nos.

      • The power dissipated by the linear regulator is unmanageable without paralleling series pass transistors–not recommended.

        What the commercial guys do is to simply use a bang-bang regulator with a fixed off time. It works by closing a high current relay contact so that the battery charges–when the battery reaches the maximum voltage, the contact opens for a fixed off-time of perhaps 10sec. and then the cycle repeats. When the battery is fully charged, the contact remains closed only for an instant before it re-opens. This would make a good project–will think about it.

    2. I like this ….

    3. Allwin M.A. says: on August 13, 2012 at 2:05 pm

      Hi,
      How can i calculate the charging current of this circuit ?

      • Jim Keith Jim Keith says: on August 13, 2012 at 3:12 pm

        Max charging current = solar panel watts ÷ nominal output voltage (usually 12V).

        The charging current is limited to the capacity of the solar panel. If the solar panel output exceeds approx 2.2A, the internal current limit of the 7805 regulator takes over. Beyond that, if the 7805 runs too hot, its internal current limit further reduces the current to a safe level.

      • Jim Keith Jim Keith says: on August 15, 2012 at 2:09 pm

        woops–meant LM317, not 7805, but you get the idea

    4. Allwin M.A. says: on August 15, 2012 at 7:30 am

      Thankz…One more question…

      How to redesign the circuit for charging 12v battery with 50w solar panel so that the charging current will be 1A ?

      • The solar panel WOULD provide you 3 AMPS ( tYPICAL RATING OF current for a 50W panel) a typical Solar panel where Vcc would be 17 to 21V) The circuit would be OHMS law W=V*I

        A Few changes
        1. The Schoktty Diode D1 would have to be above 10A/50V would have to be higher rated than 3A a HEAVIER HEAT SINK FOR THE 317 RECOMMENDED )

        The battery would have to be approx 50Ah NOT OVER ENGINEERING @70% DOD (depth of discharge to provide more cycles SO MORE LIFE FOR THE BATTERY ) Assuming you are charging it at 4Amps(WE have assumed a higher Current. Solar energy would vary depending on sunlight, Position of the solar panel and the hours of sunlight.It would receive.) in 8 hours it would charge approx safely 30 to 35Amps.

        Your load would have to be about 35A. if you are using an inverter, which we dissuade as the would be power losses and efficiency would drop to 70%. Prefer use of 12V lamps available from any Auto store. Use of LED lights recommended Lumen to Lumen they provide more light and consume less power

        If you are using an Inverter 600W recommended. Simple thumb rule for load W=V*I In case of an inverter being used 400W is all that is available for use (12V*35A) Inverter of about 600W recommended.

        Hope this is helpful or read our Understanding Solar Energy on our website at http://www.care-india.com/solar-energy.html

        Using 12V lamps would take it further and you could reduce the dependence of the inverter. Now 12V fans are also available.We recommend that instead of spending on an inverter invest the same on DC devices

        The figures provided are recommended putting a higher capacity battery would not help as it would not match the Solar panel .

        In use of Solar power use of 12V DC lamps. Do not do not use this power for running of devices that draw heavily..like motorized items look at the ratings.

      • Jim Keith Jim Keith says: on August 15, 2012 at 2:07 pm

        I plan to publish a 4A solar charger within the next few weeks–keep watching.

    5. What is the alternative for schoktty? Thanks!

      • Jim Keith Jim Keith says: on September 2, 2012 at 2:37 pm

        Actually, any 3A diode (1N5401, 1N5402, 1N5403, etc.) can be used, but the conduction drop doubles from about 0.5V to 1.0V. If your solar panel puts out sufficient voltage, then it makes no difference.

        The reason a schottky was recommended was to help offset the relatively high drop-out voltage of the LM317.

      • Im living here in philippines i cant find that kind of diode here.. Thanks sir..i let you know if i done this simple controller..thanks again! god bless!

      • Jim Keith Jim Keith says: on September 3, 2012 at 3:18 am

        For all practical purposes, the current rating of D1 needs only to equal or exceed the solar panel current. So if the solar panel is rated at 1A, a 1 or 1.5A diode may be used.

    6. naba kr mitra says: on September 11, 2012 at 2:34 pm

      I have a solar panel of max power of 8.7 volt/.57amp/5wattis it possible to charge 7.4 volt/1500mah

      battery

      • The drop-out voltage of this control (2 to 2.75V) is too high for your application. Will post within a week a 6V LDO Solar Charge Control that has a drop-out voltage of less than 1V. I already have the schematic worked out. It is similar to the 12V LDO Solar Charge Control. This will suit your application well.

    7. Experience: MIN Voltage drop = 1.6Volt.
      (I had used a SB540 schottky diode. 5A 40Volt)

    8. i like this very good circuit

    9. I would like to talk to you more about this circuit. Could you send me an email? Thank you

    10. Hello Dears,

      Please Suggest me a circuit, with 6V Solar panel, 6V SLA Battery, 1W High power LED.
      With Battery full and drop Indicator.

      Thank You.

      • Jim Keith Jim Keith says: on October 19, 2012 at 1:13 pm

        Take a look at this circuit:
        electroschematics.com/6916/6v-ldo-solar-charge-control-circuit/

        You can then add your high power LED and series limiter resistor.

    11. Hello Dears,
      Please Suggest me a circuit, with 6V Solar panel, 6V SLA Battery, 1W High power LED.
      With Battery full and drop Indicator.
      Thank You.

    12. Pravin Chavre says: on December 13, 2012 at 8:07 am

      I want to buy a solar panel to charge my 12V 7Ah Dry Battery.

      Please tell me whether I can use the circuit given above without making any changes. Also tell whether 12 Volt 20 Watt Solar panel will be sufficient to charge the the Dry battery of 12 Volt 7Ah.

      Have a nice day.

      • Pravin Chavre says: on December 13, 2012 at 8:47 am

        The sunlight is available for about 6 to 8 hours.

      • What? The Dry Battery?
        No, you should not use that.
        This is an answer!!
        “Sealed (MF) Rechargeable Battery”
        The letters are written on the battery surface.
        :D

      • Pravin Chavre says: on December 13, 2012 at 1:30 pm

        Hello Yon,

        I know that it is written on it as rechargeable. ;-)

        But I want to know which option will work better? Lead-acid or the dry battery. Also I need the exact specification of solar panel which will work efficiently.

        Have a nice day.

      • No problem!
        Go a head !

    13. Is it okay to use this circuit with my solar panel 1.2amps @20v? thank you for the response

      • Yes –good choice

      • how much current will be drop out by this circuit? few watts??

      • how much power will be drop out by this circuit? few watts??rather

      • Assuming your solar panel is a perfect constant current device, total power:
        P = 1.2A * 20V = 24W

        Amount of power actually going into battery:
        P = 1.2A * 14.5V = 17.4W

        Remainder of power goes into regulator:
        P = 24W – 17.4W = 6.6W

        Since the solar panel is not a perfect constant current source, voltage tends to drop at maximum load. As a result, the power that goes into the regulator is roughly half or 3.3W. The regulator picks up the “slack” or excess power.

        Another more expensive and complicated control The MPPT (Maximum Power Point Transfer)style control actually turns some of this excess voltage into an increased charging current via switching regulator techniques. e.g. in your case, charging current could increase to perhaps 1.5A.

        For more info:
        http://en.wikipedia.org/wiki/Maximum_power_point_tracking

      • haha ,nice information, im looking for some circuit you call mppt, buti have lack of time as of now, trying to finish my project this coming 2weeks maybe next tym i will go to that, for now gotta used this circuit,by the way im a student here at philippines. thank you again-

    14. abbas babannan says: on January 19, 2013 at 11:23 pm

      this is an excellent job, but please can you help me with the circuit that will charge 2v 600a batteries by 12 Pisces. thanks

      • You need a 24V solar charger?
        That is possible with two 12V solar panels in series.
        I do not understand what you mean by “600a batteries”

        600A in an automotive battery refers to cold crank amps.
        600AH refers to a VERY LARGE capacity battery such as is used in an electric fork lift truck.

        A solar charger can charge any size battery, but the charge time will be long for a large battery–simply calculate the AH (ampere hours) and remember that solar does not generate without direct sunlight.

      • The 2 volt MF battery had a very demanding conditions.

        The charging conditions is that every terminal shoud meet 2.30 volt/cell and under charging conditions temperature of 25 degrees Celsius or less only.

        If you break any one of these two conditions, the life will be shortened dramatically.

        So now introduce the 2 volt battery, the solar charging circuit is not appropriate.

        The 12 Volt MF Battery is recommended.

    15. am i have to worry if my lead acid batt. is full, can it cause to damage my batt??

    16. Hi Jim,

      I have a question regarding this circuit. I have analyzed several circuits like this but in each have encountered problems due to oversights. I am currently a student studying electrical engineering, but as I’m just starting out I don’t have a ton of knowledge yet.

      My goal is to use this circuit to charge a 12V 18Ah battery using a 20W 12V solar panel. I was wondering if this circuit would work for that purpose.

      • If your solar panel output voltage = 18 to 20V at no-load, it should work fine.

        The maximum solar panel current = 20W /12V = 1.67A. This is a little above the LM317T recommended current rating, but is well within its typical current limit rating. In practical applications, solar panel output current tends to be lower than specified rating due to atmospheric conditions, and/or less than optimum panel orientation.

        Then, there is the specsmanship issue in which the advertised power rating specification is “enhanced” as much as possible so that it competes favorably with competitors units. One way of doing this is to rate the power rating at maximum battery voltage e.g. 14V or so rather than 12V. The bottom line is that the maximum current is generally less than what is expected.

      • Jim,

        I think the capacitor is a .01 microFarad capacitor; would that be correct?

      • Sorry, I meant that to be a .1 microFarad.

      • If not otherwise stated, capacitance is generally indicated in microfarads.

    17. Jim, I have a couple of questions: What power/wattage resistor should I use for R1, R2, R3. I assume C1 is ceramic 50v 0.1uF capacitor, right? And since you mentioned drop out voltage is 2-2.75V (though someone above mentioned practical drop out was lower), can I still use this circuit to charge a 6v4.5aH battery with a solar panel (Rated V=8.7V, Open circuit V=10.7 and short circuit current 0.38A)? If it is possible, it should still take 2 days to fully charge the battery, isn’t it? Thanks, in advance, for your reply.

    18. Jim,

      Thanks for posting these details. I am using a LM317 for the charger for some time now. I also used a 7812 with a few diodes to get 13.8V output and it works fine as well.

      -Kiran M R

    19. edalcor says: on June 6, 2013 at 7:15 am

      hi good day. i have 2 solar panel each rated at 20watts/ 17.5 volts output can i connect it to parallel to make it 40 watts/17.5volts/2.28amp. output and can i hook it up into this circuit to charge my 40AH battery for how long? thanks.

    20. parmvir singh says: on September 11, 2013 at 7:07 am

      i want to make a solar charger to charge my li-ion bateries (batteries are connected in parellel to make capacity of 3.5 Ah).would this circuit do the job? or please refer me to a better battery charging circuit

    21. Quick question.
      Currently I’m doing an assignment for one of my teachers in school, to make a “Solar USB Charger” as an example for her Alternative Energy Students, and since she knew that I have done something similar in my Electronic’s class for a project that I chose to do last year using two of your schematic’s (in which I gave full credit to the designer of the circuits). I was wondering if you’d think it would be possible to do so, using the circuit above if I set the output voltage of the battery to around 5V and attached a USB port to the output-terminals, or would there have to be an additional circuit or “driver” since it would be used to charge a cell phone or iPod.

    22. This is a good idea and should work well–if you check the schematic, you will see that the min adjustment is approximately 5V. If it will not adjust this low, reduce R2 to 470Ω. Good luck!

      • Don’t worry, I have my circuit from last year to reference if needed. All that I’ll probly add to it is the USB port with a 5V Voltage regulator tapping into the battery, and maybe an additional small circuit that lights an LED when the battery reaches full capacity, but only when the switch for it is set in the On-Position (so that the LED won’t drain the battery once it’s charged as an option.) I was also considering on adding one of those “Self-Reseting Fuses” to the power pin on the USB configuration, just incase if there was a short or fault, it wouldn’t wreck the fancy electronic’s being charged, or the circuit it’s -self. Only problem is that I’m a tad unfamiliar to which fuse would be appropriate for the situation, since fuses were hardly talked about in my Electronic’s Class.

        I do appreciate you answering my question in such a short notice, and for that I shall say Thank You Sir.

    23. hi, does the solar panel have to be 9 to 20v? what if i use a 5V solar panel ?

    24. sally koundoul says: on November 26, 2013 at 4:10 am

      hi thanks for sharing your circuit with us. I was wondering how can i add some LEDs to indicate the charging process of the battery. I’m using a 20W solar panel with a 12V-18A/H battery do i need any modification for the given circuit ? Once again thanks a lot please help me Sir.

    25. sally koundoul says: on November 26, 2013 at 4:20 am

      One more question sir, if i understand the circuit well once you adjust the voltage to the desired output and when the charge in the battery get to that point it wont charge anymore ?

    26. sally koundoul says: on December 2, 2013 at 1:01 am

      Hi i built the same circuit and it is working perfectly but i would lie to know can i add LEDs to the circuit to know if the battery is charging ? Or how can i measure the current flowing into the battery while charging ? Please help me Sir

    27. Hi Jim, I have 5W solar panel with working current of 2.86mA, if i use the circuit above to charge a 12V 12Ah lead acid battery, does that means that i will need longer time charging the battery or the circuit is not suitable for my solar panel?
      Thank you

      • Your charging current should be around 400mA in full sunlight. 2.86mA is way too anemic to charge anything–perhaps your solar panel is defective or your measurement is in error. Measure your short circuit current. Open circuit voltage should be around 18V.

        12AH /0.4A = 30hours charging time for a fully discharged battery.

    28. I love this circuit.I have 200watts solar panel, How do I go About upgrading this circuit so that I can obtain a 10amp 1evolts output? Thanks in anticipation

    29. Sir, we plan to make a solar inverter and we already have a 20w solar panel and a 100w power inverter.now, what kind of battery that shall we use?. thank you. we will wait for your reply sir.

      • Get a deep cycle battery –also known as RV (recreational vehicle) battery –as large as you can afford. They cost about $100 here. You could possibly get one that has about half the capacity of the RV battery, but there may be insignificant cost savings due to lack of sales volume.

    30. thank you sir! can you give me a circuit of solar charger that fits to my project sir?

      • Hello! Found this page very useful. Thank you for the detailed information. I am actually looking to build a solar charger to charge my NiMH rechargeable batteries which will in turn run a dc stepper motor. My NiMh batteries are 12V 2700mAh with a peak voltage of 14.6V(10 AA batteries). The solar panels that I am using are two 6V panels with a power of 1.5W each. The DC motor’s working voltage is 12V and current is 0.33A. Will this circuit schematic work for the solar panel and the rechargeable batteries? How will I determine the resistor values? I will be very obliged if you can help me out.

    31. Hello! Found this page very useful. Thank you for the detailed information. I am actually looking to build a solar charger to charge my NiMH rechargeable batteries which will in turn run a dc stepper motor. My NiMh batteries are 12V 2700mAh with a peak voltage of 14.6V(10 AA batteries). The solar panels that I am using are two 6V panels with a power of 1.5W each. The DC motor’s working voltage is 12V and current is 0.33A. Will this circuit schematic work for the solar panel and the rechargeable batteries? How will I determine the resistor values? I will be very obliged if you can help me out.

      • It should work OK as is, but you will have to experiment with the charging voltage. One means of determining the maximum charging voltage is to charge the batteries fully using a commercial charger –then transfer the batteries to the solar charger and adjust the voltage to obtain a low charge current (approx 10mA). This will be close to optimum and will prevent overcharging.

      • Thank you for your prompt and detailed response,Sir. I really appreciate it. I had another query. I was looking at another schematic that had been posted pertaining a solar rechargeable circuit
        http://1.bp.blogspot.com/-SKZ-kdlhiAQ/TvbsdTDNtSI/AAAAAAAAAig/4xDUDPEiYB4/s1600/solar+regulator.png
        The author has used components that you have not used. I was wondering if you could shed light on whether I would need the additional components (transistor etc) for my purpose. Also, would I need to make addition to your circuit schematic if I want to prevent overcharging of my batteries? And would I need to measure the voltage across the battery terminals or the current in series in order to test if the circuit is working?

        Sorry to bombard you with a plethora of new questions but I will greatly appreciate any help from your side. Thanks!

    32. What changes is to be made to charge the Lead acid battery – 12 V, 7.5Ah?
      What should be the specifications of the solar panel to charge the above mentioned battery?

      Awaiting your reply

    33. Use a 12V, 20W solar panel. Open circuit voltage is 18 to 20V and rated current is about 1.5A.

      • Can I use the same circuit to charge the Lead acid battery with specifications – 12 V, 7.5Ah ?

      • And will it be enough to use 12V solar panel to charge 12V lead acid battery ? I mean arent we supposed to use >20V soalr panel to charge 12V lead Acid battery ?

        Is there any formula for this ? Kindly do share.

        Awaiting your reply and thanks for your previous response.

      • The standard 12V (nominal) solar panel puts out approx 18 to 20V in full sunlight. It will charge a battery of any capacity –large batteries simply take longer –the only math is AH (ampere hours) –with that you can determine both approximate charge and discharge time depending upon charging current and discharge current respectively.

    34. Dear Jim,
      1) Would like to know whether I should use quarter watt/ half watt resistor?
      >>>>>> Batter Specifications : Lead Acid Rechargeable battery-12V, 7.5Ah
      Constant Voltage Charge – Max. Charging Current: 2.25A.

      2) And also would like to know whether the capacitor used is 0.1F or 0.1uF ?

      3) I have a 12V, 20W solar panel. How long will it take to charge the battery completely once?

      Kindly answer to all these doubts separately. Awaiting your response

    35. Hello Jim !

      I am using a 17.2 V optimal voltage panel, 0.3 amp and 5W max power. Combined with a 12V sealed battery. Is this circuit suitable? also any recommendation for the battery Amp/h values? thanks

    36. Steve Trout says: on July 2, 2014 at 9:03 pm

      This is a wonderful project.
      Very easy yet very useful!
      Thank you!

    37. William says: on August 3, 2014 at 2:07 pm

      James
      Thank you for your web-site.

      My panel is rated at 700 milliamps

      Is the circuit O.K.to maintain a motorcycle battery.
      I draw, intermittently, 4 amps for a about 30 minutes per day from the battery.

      • This should work fine for your application. Even when you draw the 4A load, the solar panel current cannot exceed its 700mA rating due to its current limiting properties –the voltage will sag during this period and there should be no issues with the regulator.

    38. William says: on August 3, 2014 at 3:35 pm

      Wow, a reply already this is amazing!

      Thank you James

    39. I am VERY new to circuit building. I like this circuit, very simple. What I’m needing to do is to charge a 9v 170mAh NiMH battery for a usb charger I have built. What do I need to change on this schematics to achieve this? Thanks!

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