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Description: This is the Sunny Buddy, a maximum power point tracking (MPPT) solar charger for single-cell LiPo batteries. This MPPT solar charger provide you with the ability to get the most possible power out of your solar panel or other photovoltaic device and into a rechargable LiPo battery. Set-up is easy as well, just plug your solar panel into one side of the Sunny Buddy and your battery into the other and you are good to start charging!

The output of the Sunny Buddy is intended to charge a single polymer lithium ion cell. The load should be connected in parallel with the battery. By default, the Sunny Buddy comes set to a maximum charge current of 450mA with a maximum recommended input of 20V (minimum 6V). It’s recommended that batteries not be charged at greater than their capacity rating; thus, the smallest battery that should be charged with the Sunny Buddy is 450mAh.

Each Sunny Buddy comes equipped with a LT3652 power tracking 2A battery charging circuit and pre-installed barrel jack and 2-pin JST connectors with unpopulated areas to install your own personal 3.5mm screw terminals for added input/output options. This revision also adds a potentiometer to the input to set the holding voltage for MPPT and we’ve also tweeked the feedback resistors on the output to change the float voltage.

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Customer Comments

  • Hi, I’ve been running a couple of the v1.0 versions and the update adds some useful features. Just one query on the v1.0, however, I’d like to add a charging indicator (LED or even something I can measure) to the circuit. I can’t quite work out how the CHG and FAULT connections operate (I’ve had a look at the LT3652) but I’m struggling. Thanks

    • CHG and FAULT are both open-drain active low devices, so they can’t drive an LED high. You’ll need to connect your LED anode to a supply voltage (say, VOUT), and then tie the cathode to the CHG or FAULT pin (through a resistor).

  • Why did you decide not to connect the NTC pin to some header? There is plenty of free space on the board…

  • Guys, great product! I have 4 of them working perfectly! Just a question, what can I do in order to set the charging voltage from 4V to 4.2V? Which resistor in the eagle file should I change?

    Tnx a lot!

    • R2, R6, and R7 all need to be adjusted, and the directions for setting those values is in the datasheet.

  • hi there, in the data sheet, it says this can be used to charge a 2s liFePo4 and has a circuit diagram with a solar panel inputting the voltage. I apologise I only have basic knowledge with electronics, so is that diagram showing what resistors, capacitors etc which would be needed to make this work with a 2 cell battery? Cheers, Leon.

    • Yes. Take a look at our schematic and change out the components to match what’s in the datasheet and you’ll have it.

      I’m not a big fan of that sort of charging, though, because the cells can become unbalanced, resulting in overcharging of some cells and undercharging of others.

      • Brilliant, thanks for your quick reply, ideally I would like to charge a 2 cell lipo battery, but there is no diagram for that modification. Would that be possible? I would of thought the cells just need a higher charging voltage supplied. This setup would only be charging the battery occasionally, it would be balanced charged the majority of the time. Thanks, Leon.

        • There’s a diagram for a 2-cell LiPo from 12V wall adapter that will work; the only difference is the set voltage of the input side divider. That you’ll need to figure out based on the solar cells you’re using.

          Raising the voltage is one part of the equation; however, for best results you want to balance the cells continuously. Charging is really a current-mode operation, rather than voltage mode: you push a certain current into the cells until the voltage of the cells reaches a defined set point.

          Say the set point you want is 4.0V per cell (a decent target; below 100% but it’ll give you a bit more life). That’s 8.0V when you have two cells. Without some extra circuitry, you have no way of knowing whether that 8.0V is 4.0V and 4.0V or 3.95V and 4.05V or whatever.

          It’s a guarantee that one of the cells will charge more slowly (or less efficiently, if you prefer) than the other. Even a small difference will add up over time, and eventually, you’ll have one cell well ahead of the other. At some point, one of them will be at the maximum healthy level for a LiPo (4.2V) while the other is still low enough that the sum will be less than the set point of 8.0V we chose earlier (i.e., 3.8V or less). The charger will still be pushing the current in, trying to get the sum voltage up to the set point, and then you can have trouble. Either the safety cut-off trips and you get a partial charge, or the “good” cell (the more efficient one) pops. Either way, it’s not what you’re after.

          • Nice, I am going to go with the 2 cell lifepo4 setup. I have been looking over the diagram for that and I am a little confused, I apologise I am a aero engineering student and my electronics skills aren’t great. The diagram for the 2 cell lifepo4 shows the resistors and diodes etc, do they need to be on the circuit board in place of some existing original components? Or are they connected outside of the board. I’m confused if the pin diagram represents the chip on the board or the board itself. I cannot see the wiring diagram for the original board on the data sheet. Also I’m finding it quite difficult to find some of the really specific resisters like 542k 459k and 0.05k. Thanks Leon.

  • Hi there, I am partaking in a solar powered model aircraft project for my dissertation. Basically I am using a 3s lipo, but i do not need to charge it as my motors draw will be more than the power produced by the solar array. Therefore I need to draw all the current from the solar array and some from the battery. Does this device allow to do this if I had a 1s lipo? What could I do to allow this configuration on a 3s lipo?. Many thanks, Sacha.

    • You could use three chargers and three solar cells, one across each cell. It’s possible to redesign the circuit on the SunnyBuddy to charge up to 3 cells in series but that would require changing a number of components. See the datasheet for more details on that option.

  • Is the V_set adjustment to do with the Vin Regulation and why are we setting it to 2.8V? Do I need to adjust this to 2.8V first and then adjust pot to get maximum current at JP3 current monitor? Thanks!

    • The SunnyBuddy will servo the output current in an attempt to keep the input current at a level where the value at that pin is 2.8V. It will never, however, source more than about 400mA to the load.

      Your best bet is to put a depleted LiPo on the SunnyBuddy, to max the charge current, then go out in full sun and tweak the pot until the sense voltage is at 2.8V.

  • I have a 11.1 v lipo battery that I need to connect to the mppt but the battery has a 4 pin lead instead of a two pin one so I can’t connect it. Is there a way I can make this happen?

    • Probably not. The SunnyBuddy only puts out up to 4.0V, so it can only charge a single LiPo cell. What you have is a pack of three cells in series.

  • Hi there, is it possible to charge a 1s lipo battery with this whilst discharging it? If so how would it be connected? Many thanks, Sacha.

  • Does anyone have any idea why the float voltage for the board is set to 4.0V instead of 4.2V?? With this setting you can never fully charge a single cell lithium ion or polymer battery…

  • I would like to use the sunny buddy with a 5v Nano. Is it possible to solar charge two Polymer Lithium Ion Batteries - 3.7v 2000mAh in series, so that I can get the output voltage I need for the Nano and the 5v sensors and accessories I will be using? This will be a remote setup and unfortunately I have several accessories that require a mandatory 5v. Thanks

    • Not easily. If you use two solar panels and two Sunny Buddies and connect them only at the batteries, you can charge the two batteries independently of one another.

      Another possibility you may want to consider is using something like the LiPower boost converter to achieve your 5V. It’ll sting you a bit on efficiency but if you’re careful you may well be able to handle it.

  • I’ve built a power bank (solar buddy + battery + usb power boost) but I can’t take much power out of it because of the small battery I have. Is it worth putting a “Polymer Lithium Ion Battery - 3.7v 4000mAh” on the solar buddy? If so is there a single cell battery with even bigger capacity?

    • Yes and no. We have a 6Ah battery pack which has the cells in parallel, so they can be used with the Sunny Buddy.

      Bear in mind, however, that under ideal circumstances, the Sunny Buddy only pushes out about half an amp. You can boost that a bit with a new current sense resistor but you’re unlikely to be able to fully charge the 6Ah battery in one day, unless you’re in the extreme north or south during summer.

      • Thanks for the quick answer. Of course I was not expecting to charge a 6Ah battery in one day. But at least it would “use” as much energy as possible through 1 / 2 days and I would be able to charge more power hungry devices with it, right?

  • Does the sunny buddy normally make a high pitched whine while charging?

    I just hooked mine up to a 2000mah battery, a 2w panel (~6v, 333mah) with a spark core as the load and the SB board is pretty loud.

    I have tried different, smaller panels and the whine is about the same.

    • We’ve noticed this; it’s harmonics from the switching regulator causing either the coil in the inductor to vibrate or the ceramic element in one of the capacitors to act as a piezo element.

      It’s annoying but harmless. You could try dampening the vibrations of either element with a little epoxy.

      • Thanks for the reply, it is a bit annoying, but glad to hear it’s not anything bad happening. I tuned the voltage set potentiometer a bit and it immediately went away.

  • After chatting with “Allison” (who was very helpful) I should make a point on this item: Yes, you can use an 18650 BUT It will discharge into oblivion if you don’t have a protection circuit for the cell! And there is the potential for over-discharge-current (over 1C)! This is intended to be used with a cell that HAS PROTECTION CIRCUITRY ALREADY ON-BOARD, or you have to provide it yourself!
    What I would like to see is a version of this board that has a cell holder for an 18650 and that has the protection circuitry built into it for that kind of cell (not to replace this item, but as an alternative).

  • A few questions:

    1) What are the specs of supported solar panels? The hookup guide says the SF small panel will work but that’s listed as only having a 4.5V / 100mA output. How will that work if the minimum voltage rating for this board is 6V?

    2) Since the maximum charge current is 450mA, does that mean that only panels supplying less than 450mA of power should be used?

    3) Is it possible to use this as a general LiPo charging circuit, if the solar input is switched out for a wal-wart supplying >6V? If this would work, what would be the maximum current that could be supplied?

      1. The specs of the supported solar panels are 6-20V. You can stack the small Solar Cells (make sure to reconfigure the jumpers as directed), and be good to go.

      2. You can use panels that have a supply greater than 450mA, but the Sunny Buddy will only pull 450mA. As long as you keep it under 20V it should be good to go.

      3. Yup, if you plug a wall-wart into the sunny buddy, it will charge your battery. Maximum current should be kept under 20V. I can confirm this, because I have a 9V wallwart plugged into my Sunnybuddy charging the battery on my desk at this moment. :)

  • I am using the sunny buddy regulate the output voltage of two 12W 5V panels in parallel to charge a 3000 mAH powerbank. Does it make sense to replace the Rsense value with 0.22 ohm so that I get a output charge current of 0.9 amp?

    • You may find that at 900mA your power inductor starts to saturate. That’s dangerously close to the 1.05A spec limit of the part. Also, 5V is too low a voltage; the SunnyBuddy won’t start up below ~6V.

      Put the panels in series, and you’ll probably be okay; in a step-down system, the inductor current shouldn’t be too much higher (if at all) than the output current.

  • Hi! I want to charge a 6V Lead acid battery using these, I was thinking on changing the voltage divider values to get the one required. Do you think I can desolder this resistors and put instead a trim pot with a higher resistance value?

    • It’s not quite that simple. Please refer to the datasheet on circuitry changes needed to adopt this to charge a lead-acid battery. I think it can be done but you may need to change more than just the feedback resistors.

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