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Sunny Buddy - MPPT Solar Charger

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.

Sunny Buddy - MPPT Solar Charger Product Help and Resources

Weather Station Wirelessly Connected to Wunderground

April 11, 2014

Build your own open-source, official Wunderground weather station that connects over WiFi via an Electric Imp.

Core Skill: Electrical Prototyping

If it requires power, you need to know how much, what all the pins do, and how to hook it up. You may need to reference datasheets, schematics, and know the ins and outs of electronics.

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Skill Level: Competent - You will be required to reference a datasheet or schematic to know how to use a component. Your knowledge of a datasheet will only require basic features like power requirements, pinouts, or communications type. Also, you may need a power supply that?s greater than 12V or more than 1A worth of current.
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  • You think Fluke was bad with the black & yellow case, wait til Microsoft hears about a maximum Power Point charger.

  • Hi Sparkfun,

    I think you may have an issue with your circuit - This IC isn't really a true MPPT IC, it just "guesses" the MPP based on an assumed maximum power point voltage. You have to put a voltage divider on the VIN_REG pin such that the converter regulates the input voltage to the known maximum power voltage of the panel. This voltage actually does vary, but it does so with temperature, not sunlight. so based on your current schematic, with the Vin_Reg pin tied directly to the vin node, you will only be optimized for panels with a lower voltage (looks like around 4V based on your graph). You really need to have a potentiometer on this node, which users would adjust to find the MPPT with a new panel. I hope this helps, and do not mean to come across as critical.

    • Yeah, I agree, as configured it doesn't do any power tracking at all. If I understand it correctly, since VIN_REG is hooked to VIN, it will only reduce charge current if the input voltage drops below 2.7V. Since 2.7V is below the minimum operational voltage of the charger, I interpret this as it always drawing the maximum charging current (assuming the charger isn't in constant-voltage mode, of course).

      It seems a bit like false advertising to claim that this will "provide you with the ability to get the most possible power out of your solar panel" when the relevant circuitry is disabled...

    • I'm looking at this to see if I can make this better. The problem lies in making something that will work for a lot of people; without knowing what they're all doing, you kind of have to make a best-case assumption.

      • Perhaps you could copy what the Linear Tech evaluation board did, and add a jumper that engages a voltage divider on the Vin_Reg pin, see JP4. Without this, the LTC3652 doesn't do its version of MPPT, it is just a nice high voltage switching battery charger.

  • Would this work with the small solar panels here: https://www.sparkfun.com/products/7845

    Those panels say they put out 4.5V and you say the minimum input is 6V. I am fairly new to electronics. I am planning to use two of those small solar panels, could I connect them in series and get 9V as input to this charger?

    I am trying to set up an emergency power source for an Arduino-Xbee-GPS system, that probably uses around 300mA at 3.3V. Even if I cannot run the system continuously that's okay.

    • To make this work you would have to enable to MPPT function by adding a resistor divider on the Vin_Reg pin, or by greatly increasing R1 to limit your charge current. The solar panel you mentioned is just a little under powered for what this chip wants to do. If Sparkfun make a breakout board for the LTC3331 that would be much better.

  • It says "by default... maximum charge current of 450mA". Is it possible to increase this by changing resistors or similar? What is the maximum possible?

    • The charge current is set by resistor R1 (0.22Ω). Normally max charge current is Ichg = 0.1/Rsense. So perhaps you could change (reduce) it, but I'm not sure what saturation current the inductor has. You need to make sure you stay below that with some room to spare. The LTC demo board uses a 0.05Ω resistor and an inductor with a 7A saturation rating.

    • According to the data sheet, the charge current is set by the sense resistor (up to 2A), so you'd have to de-solder and replace it.

  • What's the float voltage of this thing? They say every 0.1v reduction doubles lifespan in li ion.

  • Can I use a USB Type A Female Breakout connected to the Load and power an arduino or charge an iPod directly?

    • Maybe and no. The output is for a single-cell LiPo only, and won't be high enough to charge anything that normally charges from USB.

      A regular Arduino likes to have 5V to power it. It might work at the ~4.2V this will provide, it might not. That depends on things like the phase of the moon, distance from the ocean, recent tectonic activity, and degree of uncorrected astigmatism in observers in the room. Which is to say, you can't really predict whether it will work, and it may work fine in a room but fail on a warm day outside.

      A 3.3V Arduino, like the Pro or Pro Mini, would work better, but using a 3.3V device has its own set of difficulties.

  • Why does this board say it has a "maximum recommended input of 20V"? The ASIC itself can take up to 40V on the input. Is this due to a limitation of the input capacitor's working voltage? The schematic does not list the V rating of the input cap C1.

    • That's exactly what it is. It was a tradeoff between cap size, voltage rating and, frankly, the cost of speccing, identifying and stocking a new capacitor value just for this one product.

      I considered the solar panels we carry and the likely use cases and decided to use a cap we already carry even though it limited the input voltage of the system. Replace C1 with a more robust capacitor and you can safely increase the input voltage.

  • It doesn't seem to be possible with this board, but I'd really love a board that in addition to charging, it lets one read over I2C the current charge of the battery. A Coulomb counter system is probably the best. STC3100 looks like a pretty good chip to build on top of.

  • When will this board re-stocked?

  • Hi there...so when my LiPo (6000mAh) goes flat, can I unplug the solar cell and plug in a 9V 500mA wall plug to charge it ? It doesn't seem to be charging the LiPo..the downstream device is working OK but the LiPo voltage is hovering at around 3.4V ?

  • Taking a long bike trip. I'll have a bike hub generator (6VAC, 3W max) and a small solar panel (12VDC, 3W max). I have a rectifier circuit already for the generator that supposedly will output 6VDC. Bad idea to hook the generator output up as a second input? (People do this kind of thing with combined wind/solar, but some controllers handle it well and some don't.)

    Second, my loads would be all 5V - nano, charging for iPod, some tablet, NiMh batteries. I don't see any 5V LiPo batteries anywhere. The datasheet for the chip says it will charge other batteries. Sounds like it's set in the design for the LiPo batteries though. If I used this, I'd want to go with the LiPo batteries and use a step-up converter to power all of my 5v stuff, right?

    • You can use something like the [Minty Boost(https://www.sparkfun.com/products/10094)] to step it up; personally, I like using the [Power Cell(https://www.sparkfun.com/products/11231)] for that sort of application, because you can use it to recharge the LiPo cells from wall power when it's available, and also use it to step up the LiPo to 5V for charging your loads. You'll need to add a [USB-A port(https://www.sparkfun.com/products/12700)] to give yourself something to plug into, of course.

      I wouldn't try using the bike up generator with this charger; I'm not sure what would happen. In fact, I think 6V isn't even high enough to enable the charger, but I could be wrong. I do know there's a low-voltage cut-out, and I think that cut-out point is above 6V. Without knowing more about the generator I hesitate to suggest a solution, other than to say that Google probably knows somebody that solved that problem already.

      • Oh yeah, that power cell is perfect! Sweet! Thanks! I'll keep checking back for the update on the SunnyBuddy. As soon as you guys can update it I'll snatch one.

  • Beware that as currently configured, this is set up for a battery float voltage of 4.4V! (It has a 100k/300k voltage divider on the VFB pin.) I think this is a dangerously high voltage for a 1-cell Li-ion battery. The batteries sold by Sparkfun recommend a charge voltage of 4.2V and have a overvoltage protection circuit that kicks in at 4.3V. If batteries without such protection is used, 4.4V might do bad things to your battery. I think it would be prudent to change the resistors to lower the float voltage to more like 4.2V.

    • A revision is pending to fix this; it's currently being held up by the massive number of MicroViews we are building.

      It'll also add a couple of more tap-points, an option to change the sense resistor, and a pot to change the set point for peak-power. Look for it soon!

  • @SFUptownMaker - I'm using the SunnyBuddy Eagle files for a derivative project (a solar charger-light for the developing world) and modified it to use through-hole components. It's working intermittently, but often the float voltage is way off (1.5~2.2V for a 6.5V input, or 9V+ for a 20V input). I'm not sure if it's a bad hand-soldering job of the IC... or some other layout / component selection issue.

    I know this is way out the range of support you'd normally provide, but I was wondering if you'd be interested/willing to take a quick look to see if anything obvious stands out. Any tips on how I might go about debugging this would also be appreciated!

    My modified Eagle files are on GitHub (BOM is linked from the readme there): https://github.com/ryochiji/BootstrapSolar/tree/master/Chiqoo2

    • I took a quick look, but nothing jumps out at me. I didn't do a solid analysis of the switching portion's current loop. You might want to look at that as a source of problems; if you're pulling references off the wrong portions of the loop, you may be screwing up the servoing.

      Past that, it seems like a pretty gnarly problem. My guess is it's a current flow issue, though.

  • This board could be improved by adding solder jumpers for the following reconfigurations:

    • make it possible to disconnect the charge/fault outputs from the provided pull-up resistors for e.g. connecting to an XBee with built-in ones.
    • make it possible to bypass the timer capacitor to ground to switch the IC to C/10 termination mode instead of timer mode.
    • make it possible to change the battery float voltage by connecting a custom voltage regulator to VFB.
    • make it possible to disconnect VIN_REG from VIN so it can be connected to a voltage divider to enable the power tracking capability.

    It seems there's plenty of space on the board, and since it's a pretty pricy board it would be nice to make it more flexible. (1), (2), and (3) can be accomplished by de-soldering/replacing the smt components on the board, but it would be nice to be able to do it without permanently altering the board. (4) is impossible because the two pins are connected right to each other on the IC.

  • The schematic shows a tab on the line to the "NTC" pin, but I don't see it connected to anything, and I can't see a header for hooking up an NTC anywhere on the board. Did this go missing somewhere?

  • I am looking to build a MPPT charge controller for 4-15 watt panels, to charge 4-14.8 V, 6600 mAh Li-Ion batteries. Would it be possible to modify this schematic in order to make it work? If so, what changes would need to be made to it?

    • I haven't tried it, but there's no reason you shouldn't be able to connect the batteries in series, put one SunnyBuddy across each one, and hook one panel to each Sunny Buddy. Make sure the solar cells aren't connected to one another; they need to float relative to the overall stack.

      Modifying this board to be a 4-cell charger is probably not possible, since you'd have to change some of the parts to different packages.

  • I added this to my cart via the wireless IMP weather station tutorial along with the other components. The description of the Sunny Buddy says it's only for single cell LiPo batteries, but the battery that gets put into your cart is the 6Ah, 3 cell (PRT-08484). Is this correct?

    • The 6Ah job is a pseudo-single cell battery. Since the cells are in parallel rather than in series, you don't need to worry so much about cell balancing, and the charge voltage levels are lower.

  • I have been hoping someone would build a breakout/module for this chip - been keen to try it for trying to eek out that last bit of power from small solar/SLA setups. Not the freindliest chip for breadboarding! :P I think its time to order a Sunny Buddy and change a few values and use this as a "breakout" so I can test my setup!

  • It looks like you should NOT have JP1 and JP2 BOTH with a solder glob on them, else you'll short out one of your source Jacks. If you have JP1 shorted, the barrel jack already populated is good to go, but the other connection doesn't go anywhere. If you have JP2 shorted, you are essentially using the two inputs in SERIES. Is this correct based on the schematic?

    • That's true, and it's covered (in bold text) in the hook-up guide.

      • Sorry, but you're both using terms that semi-noobs like me won't understand. The hook-up guide uses the term "clear solder", and razor_sparks says "solder glob". Looking up "clear solder" on Google, both as "define "clear solder"" and just "clear solder" gives lots of hits for removing solder. If it's a term of art, it doesn't look like most people know what it means. Doesn't seem to be in the tutorials either. I'm not even going to look up "solder glob".

        Correct me if I'm wrong. If I want to use one panel, I can either use the barrel connector, or the "Solar In" pair of solder holes right next to the barrel connector. Or I could remove the solder ("clear solder") from J1, solder ("connect solder" or "solder glob") the two sides of J2, and use the other "Solar In" set of holes. (The schematic is a little confusing since it calls that J3, but there's no J3 labeled on the board.)

        If I want to use two panels, I should remove the solder at J1, solder the two sides of J2, and use the "Solar In" set of holes farthest from the barrel connector. And razor_sparks is right, that puts your two panels in series. I think all of this is confused some because there are three footprints for inputs on the board, it comes with a barrel jack on the first one, and the other two aren't labeled separately, and the schematic only shows two footprints, the two for the barrel connectors.

  • they're testing the terminology... Sunny Buddy listening? 1 is in my cart... I wonder how to do the same thing to a supercap? any use trying to use this? or would I get tripped up on the charging rate or apparent resistance as the regulator of the charge? ...I know... try it and capture the results... I've got a couple of 300F 3V caps... and wear safety glasses (caps can explode).

    • Hm. I hadn't considered using this with a super cap. It might work, but I'd add a small series resistor. If I give it a try, I'll post results, although it'll never be nearly 300F.

  • Can this charge controller operate with lithium battery packs or series of single cells? Even though the description specifically states the controller is "intended" for use with single-cell lithium batteries, is this due to concerns about unbalanced charging of cells within a battery pack? The datasheet for the LT3652 describes the use of a resistor divider to control output voltage, so it's not clear (to me, at least) why this couldn't be used with a variety of LiPo configurations. Any input would be appreciated.

    • Cell balance will be important for lifetime. I've built MPPT a li ion charger based on the LTC4000-1 and charged an 8 cell battery but the cells were matched at the factory so they do not need active balancing.Regular commercial batteries would quickly come out of balance and cell overcharge would occur. The charge process ignoring the balance part only involves adjusting the float voltage programed in the MPPT to be appropriate for the number of cells being charged and the appropriate charge current.

    • It can, but you'd need to make some serious changes to the values of various components in the circuit. I don't recall whether the topology is the same for multiple cells.

      One thing you may consider is putting one of these across each cell in the stack. So long as the solar cells are isolated from one another, you should be fine.

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