Best way to convert a 12V battery mantaner solar panel to 5V - Page 1
Forum section: electronics / beginners

author: poot36
author_rank: Frequent Contributor
timestamp: November 26, 2015, 03:52:19 am
content: I have a 1.6W to 1.8W 18V solar panel made for maintaining 12V car battery's and and wondering the best way to convert the output of it to 5V to charge some video game controllers (Nintendo Wii U gamepad and perhaps my 3DS). I have found that the gamepad will charge with ~30ma of current at minimum but normally charges with 700ma of current. I tested that with a 33 ohm resistor in series with the original power supply. I have tried a few 12V car cellphone chargers based on the mc34063a but they are not current limited and cause the output to drop below 5V and then not charge the gamepad. Would I be better off getting a specific DC to DC converter from eBay that would allow me to limit the current or can I modify the car cellphone chargers to work? I can provide a reverse engineered schematic of the cell phone chargers if required.

author: AustinEngy
author_rank: Regular Contributor
timestamp: November 26, 2015, 04:39:35 am
content: I'm not sure if this will help you, but for my circuit project I want to have a 12V input to the board, but the ICs need 5V. I'm thinking of using this:http://www.recom-power.com/pdf/Innoline/R-78Bxx-1.0_L.pdfit takes a sizable range of input voltages and produces a steady 5V output. It's like a $10 part, but would probably make your project a lot simpler!Good luck

author: poot36
author_rank: Frequent Contributor
timestamp: November 26, 2015, 05:13:47 am
content: That regulator replacement looks really cool. I will look into it. Keep in mind though I only have 30ma to 50ma from the solar panel to use though.

author: jt
author_rank: Regular Contributor
timestamp: November 26, 2015, 05:28:54 am
content: I think the gamepad doesn't necessarily have a minimum current limit as much as it has a minimum voltage limit. When you add the 33 ohm resistor, that means the 30mA you measured is causing the voltage to drop by about 1V across the resistor such that 4V is seen by the gamepad. The Li-ion charge controller inside the gamepad is probably designed to expect a constant 5V input and fails to operate under 4V. This makes sense, because the charge controller is likely a buck regulator which requires input voltage to be higher than output voltage; in this case the output voltage is the voltage of the Li-ion battery with a range of ~3.3 - 4.2V. If the charge controller has a "undervoltage lockout" feature, it won't let any current flow below a certain voltage even if input V > output V if the delta isn't great enough.But you are still correct in that you need a way to limit the current. Solar panels are high-impedance power sources, meaning the output voltage will drop as you suck more current out of it. In your tests with the cellphone chargers, the solar panel voltage probably drops below 5V rendering the cellphone charger nonfunctional (you could confirm by measuring the voltage across the panel in this setup). Note that there may be an oscillatory behavior in this state: the voltage may drop as the load increases, until the load is cut off due to under voltage, then the panel voltage shoots back up and the process starts over. Depending on how fast this oscillation is you may get misleading results looking at a multi-meter (an o-scope would be the best tool).So, as to limiting the current, the quick and dirty method would be to add back your 33 ohm resistor between the cellphone charger and the gamepad. You may need to play with the value a bit to find the perfect compromise between satisfying the gamepad minimum input voltage level and the cellphone charger minimum input voltage level while maximizing the power you can get out of your solar panel.

author: Seekonk
author_rank: Super Contributor
timestamp: November 26, 2015, 05:42:57 am
content: I have used those LM2596 buck converters on solar panels that cost less than $2. f you have the skills they will work even better if the panel is kept at the power point of about 16-17V. Pin #5 can be lifted, the on/off pin, and controll the input voltage with a transistor and a few other components. Without this the panel voltage will be dragged down in low light conditions but still better than a resistor.

author: Ian.M
author_rank: Super Contributor
timestamp: November 26, 2015, 05:53:21 am
content: The devices with 5V USB charging expect at the very least several hundred mA to be available at 5V. If you don't supply 5V or if you restrict the current, they are unlikely to fully charge. As such, the only valid strategy is to have enough energy storage to accept the full output of the panel for several minutes, then enable the DC-DC converter to provide the 5V charging supply till the stored energy is used up. As the devices may take several seconds to detect the 5V supply and enter charging mode, you need to have about 200 Joules stored to make it worthwhile (500 mA @ 5 V for 1 minute is 150 J). You may have noticed that commercially available solar powerbanks tend to have their own internal batteries - this is why.Adding resistors etc. just throws away power. In direct noon sunlight, your panel may deliver as much as 100mA @ 14V. The simplest way of getting that down to 5V with reasonable efficiency would be a SLA battery or a stack of ten 1000mAH NiMH cells, a 12V cellphone charger and two voltage sensitive switch circuits, one to disconnect the charger if the battery drops too low and the other to disconnect the panel if it rises too high.

author: jt
author_rank: Regular Contributor
timestamp: November 26, 2015, 06:27:17 am
content: QuoteHe said it was accepting 30mA at what calculates out to be 4V - so it appears to still function (agreed not nominal conditions). As the battery charges and its cell voltage rises, the current will reduce (less delta V over constant resistance) and it should eventually top off. This wouldn't really be much different than the constant-voltage stage of a Li-ion charge controller.QuoteI certainly agree the resistor idea throws away power, but because this is the beginners forum I offered it up as the simplest solution. If you want simple and efficient - wouldn't just using a better solar panel be vastly simpler and cheaper than adding a separate switched battery bank? He stated 700mA @ 5V as max input; shouldn't be hard to find a >3.5W solar panel.

author: Ian.M
author_rank: Super Contributor
timestamp: November 26, 2015, 06:44:35 am
content: Solar panel ratings are almost invariably an exercise in specsmanship. To get the rated output would usually involve noon sun (approx 1KW/sq m irradiance) normal to the panel surface while the panel is being actively cooled. To have a reasonable expectation of averaging 3.5W out on an ordinary sunny day in temperate latitudes, you'd need at least a 10W panel. That pushes the cost up considerably.OTOH, any smaller used SLA that still takes a charge and holds it for a few days would be suitable. If you know an alarm installer, ask them for SLA batteries that have been replaced as preventative maintenance, and if they service emergency lighting they may have some NiMH packs as well. Shouldn't cost you more than beer or doughnut money as otherwise they have to pay for hazardous waste disposal or even to recycle them.

author: poot36
author_rank: Frequent Contributor
timestamp: November 26, 2015, 06:55:19 am
content: Here is what I have tried so far.33 ohm resistor was used in series with the original charger when the game pad battery was totally dead. The charging light did light up so I assume that it was charging. Right after this measurement I tested the game pad without the resistor and it started drawing the 700ma and was decreasing in value.I have looked at the output of the cellphone charger on a scope and I can see it dropping out and turning back on every second or so (I have a analog scope so it is hard to see) I can also hear the charging circuit in the gamepad clicking and if I hook it up to the 3DS I can see the charging light flashing on and off.I have tried the 33 ohm resistor in series with the cellphone charger output and all it did was slightly change the frequency of the clicking or flashing light. (I also tried a variable pot and I could change the frequency as well but no continues charging).I have also tried just loading down the output of the cellphone charger with the variable pot and was able to get around 50ma at 4.7V before the load became to large and the input voltage dropped to around the output voltage. If I remember it was only 30% or so efficient, I had 16.5V or so in at 36 or so ma from the solar panel. Would getting a adjustable constant current converter work or would the control loop of this not be able to react quick enough to prevent the voltage from dropping?I have thought about using some old laptop li-ion batterys and charging them from the solar panel and then charging the gamepad but the real goal of this application is to just prevent the self discharge/standby consumption of the gamepad battery when I am not using it. It goes flat around the 1 month time frame and I have heard storys of it the battery gets too low it may not be able to be recharged! Right now it is at 3V. I can provide the datasheet for the charging controler in the gamepad if that would be useful. I also have some 12V 7Ah SLA batterys as well if this would be better but I just want to offset the standby current draw.I do have a 15W 12V solar panel available but no way of getting its power to where the gamepad is. Currently the small panel is sandwiched between my window and the mesh screen at a 90º angle and is not covered in snow right now and I can get its power to the gamepad. I do have a second of these panels that I could try to see if it would fit in the window alongside the other one but I don't have high hopes for that. Plan B is to get one of those 6V 300ma polycrystline solar panels off eBay with the USB lead but I do not know how well they would work in low light or what ever weather that my window experiences. The current panel is amorphous and weather resistant.

author: Ian.M
author_rank: Super Contributor
timestamp: November 26, 2015, 07:37:27 am
content: Your panel is intended to provide a trickle charge current into a lead acid battery (nominal float voltage 13.8V) as cheaply as possible. That's nearly three times more voltage than you need for USB devices. To get as most power out of it as possible without the complexities of a MPPT controller, you need to run the panel into a constant voltage load at approximately 75%-80% of its nominal O/C terminal voltage. That's a fairly good match to a SLA battery, and it sounds like there is little risk of overcharging, so you'd only need an under-voltage cutout, set fairly high so the SLA battery itself isn't killed by repeated cycling to below 50% charge.If you want to eliminate the SLA battery, that would involve hacking the phone charger DC-DC converter's feedback loop to drop the nominal 5V output voltage proportionately as the panel output drops from around 15V to 13V, and *IF* there is enough bulk capacitance on the input to the DC-DC converter to smooth out any pulsing due to the load's charging algorithm, it will then settle at a current and voltage that can be sustained. This may require a very large reservoir cap if the load has a 'smart' charge controller.If you lift the top end of the lower resistor in the MC34063A feedback potential divider and add a small N channel MOSFET with a Vgs rating >25V (or use a 2n7000 with an 15V Zener from gate to ground to protect it) between the feedback pin (D) and the lower resistor (S), with a potential divider feeding its gate from the input voltage (I suggest a 220K upper resistor and a 100K pot for the lower resistor), it should do the job. Start with the pot at max so the extra circuit has virtually no effect (MOSFET hard on), load the 5V rail with resistors till the voltage drops to about 10V, then tweak the pot to get it back up to about 13V. You can do the same thing with a bipolar transistor, but it needs to go between the lower feedback resistor and ground, and obviously it needs different resistors in the divider.

author: jt
author_rank: Regular Contributor
timestamp: November 26, 2015, 07:43:25 am
content: QuoteI'll concede you are likely correct here - I don't have much consumer solar panel experience. However, I think there should still be a way to use a low cost panel for this application w/o intermediate energy storage since products like these exist:https://goo.gl/2wzrwgQuoteI think what you are describing is essentially a solar peak power tracking circuit. You can play around with this, but I'm concerned the interplay with the charge controller in the gamepad will cause issues.QuoteThis could be a deal breaker. If you don't have good lighting conditions then you will have a very steep I/V curve on the panel and you may not have a way to eloquently drive the charge controller in the gamepad no matter what you put inbetween. You'd be able to trickle charge the battery if you were to bypass the charge controller, but I'm guessing you don't want to do that. I also wouldn't advise doing it unless you are very comfortable with Li-ion safety requirements.One (un-eloquent) thought - you say you have been able to get the charge enable light to oscillate on the gamepad. There is a chance in this condition you are still delivering enough energy to overcome the standby draw of the unit. Might need to just let it sit for a week or so and see if there is any difference (positive or negative) in the battery state of charge.Otherwise, if you can't improve your solar cell configuration I suspect that Ian.M might have the only solution with the battery bank.QuoteAs a word of caution, unless you can guarantee that both panels have similar lighting conditions, connecting solar cells in parallel can damage your array. If one cell is shaded and the other is not it will back feed current through the shaded cell which can cause damage (also it can make it glow red like an LED). You can protect against this by adding diode protection, but this will cause you to loose power in the diodes. If you are okay with the complexity you can use an ideal-diode controller circuit to mitigate this (Linear Tech has a lot of options).

author: Ian.M
author_rank: Super Contributor
timestamp: November 26, 2015, 08:12:28 am
content: At the level of complexity being worked to here, a couple of 1A Schottky diodes will be plenty good enough to combine the panel outputs. At 50mA the power lost in them will be fairly minor compared to losses in the rest of the system.

author: jt
author_rank: Regular Contributor
timestamp: November 26, 2015, 08:18:21 am
content: QuoteGood point

author: Ian.M
author_rank: Super Contributor
timestamp: November 26, 2015, 08:37:49 am
content: I get the feeling that the panel illumination is pretty marginal. Snow is mentioned, so its likely to be at a fairly high latitude, and if the panel is partially shadowed or not south facing and properly angled, its average output is likely to be low enough that any sophisticated control circuit will waste more than it can gain. Probably its best use is to simply maintain the collection of 12V 7AH SLA batteries over the winter, either swapping them weekly, or isolating each with a Schottky diode so the lowest gets the charge and the others aren't dragged down.The Gameboy self-discharge problem is probably best solved with a mains charger and a mechanical timeswitch. Give it 15 minutes a day and if its been used enough to need a full charge, simply twiddle the timeswitch override knob to turn it on till the next day's off time.

author: Seekonk
author_rank: Super Contributor
timestamp: November 26, 2015, 04:22:35 pm
content: Your in game charger is likely similar to a TP4056 which is not much more than a silicon resistor. I bought a bunch of these at a quarter each for charging my LI batteries. They come set at 1A and good luck changing the urrent set resistor. Data sheet says you can put up to 1/4 ohm in series with 5V power. I used a 4.7 ohm cause it was easy to grab and was using it on some questionable batteries. It worked fine as a method to reduce charge current.That said, I doubt that there would be much of an issue charging at lower currents periodically. You really need to use something like a 2596 switching module to boost available current three times what the panel produces. With simple modifications it can be ORed to maintain not more than 5V out or less than 17V on the input. Time to learn some electronics and build a power point controller.

author: poot36
author_rank: Frequent Contributor
timestamp: November 27, 2015, 02:34:27 am
content: Well I finally re-found the datasheet to the charge controller chip:http://www.ti.com/lit/ds/symlink/tps65070.pdf,It is the TPS65070 one. I think it is way more complicated then the TP4056. On pin 5 of the cellphone charger chip I have a 1K resistor going to ground and a 0.470K resistor and a inductor connected to the 5V output line. This charger does not break the ground line. It is a high side switching converter. I don't think I can connect the MOSFET with a voltage divider to the input with out disrupting the rest of the circuit. I am going to try directly connecting the gamepad battery and its protection circuit through a diode to the 5V output of the cellphone charger and see what happens.

author: poot36
author_rank: Frequent Contributor
timestamp: November 28, 2015, 05:03:58 am
content: Well with the current setup I got the battery voltage from 3.1V to 3.6V in one day. My window faces almost directly north. I am in Calgary and the weather was -10C in the morning warming to +2C in the afternoon. I am going to try to fully charge the battery with the current setup and then put it back into the gamepad and see what happens. The battery is 3.7V 1500mAh 5.6Wh so we will see how long it will take to charge.

author: Ian.M
author_rank: Super Contributor
timestamp: November 28, 2015, 07:15:56 am
content: Without a proper charge controller, the most likely thing to happen is that you overcharge and damage the battery. If your circuit can provide over 4.2V, and you leave it unmonitored for any significant interval, that's a near certainty.

author: jt
author_rank: Regular Contributor
timestamp: November 28, 2015, 09:50:26 pm
content: QuoteI agree with Ian.M that you should be concerned. A lot of Li-ion batteries/cells have cell protection circuitry built into the pack, which will include overvoltage protection (will go open circuit and not accept additional charge). You should go to some effort to confirm this is the case, and confirm the open circuit voltage of the solar panel will not exceed the absolute max voltage that the protection circuit can tolerate.Also note that while you have the charge controller bypassed you should never plug the unit into the standard charger.

author: nbritton
author_rank: Frequent Contributor
timestamp: November 29, 2015, 01:12:56 am
content: A buck converter is probably the most power efficient cost effective solution, you can pickup LM2596 modules for $0.75:http://www.aliexpress.com/item/Free-Shipping-1pcs-LM2596-LM2596S-DC-DC-4-5-40V-adjustable-step-down-power-Supply-module/32339997211.html

author: poot36
author_rank: Frequent Contributor
timestamp: November 29, 2015, 06:27:55 am
content: I can confirm that the battery has a protection circuit in it and that it works. The battery is a user removable one so powering the gamepad without the battery in it would not damage anything. I am hopefully going to get a TI based adjustable voltage and current dc to dc converter from eBay and try it out when it gets here (probably going to take over a month but could be here in 3 weeks). Today the battery got to 3.7 volts so if the weather cooperates it may fully charge in 4 to 5 days asuming that the charging is relatively linear.

author: poot36
author_rank: Frequent Contributor
timestamp: December 06, 2015, 06:32:42 am
content: Well at the end of today it is now at 3.8V so way slower charging then I expected but at least it is charging. I don't think it is linear at all. At least I won't have to worry about it overcharging!

author: poot36
author_rank: Frequent Contributor
timestamp: December 16, 2015, 05:54:54 am
content: It is now at 3.9V so it is getting there. I really hope the converter that I am getting will be more then 30% efficient. I also ordered a 6V 160mA polycristaline solar panel as well as a plan B.

author: poot36
author_rank: Frequent Contributor
timestamp: December 24, 2015, 03:45:09 am
content: Well 2 days ago it reached 4.0V and today it is at 4.1V so we will see what happens in the last 20% or so of battery voltage.

author: poot36
author_rank: Frequent Contributor
timestamp: December 27, 2015, 07:56:48 am
content: Yesterday the battery has reached 4.2V so if there is sun today and it doesn't increase in voltage anymore I think it will be fully charged (or very close to it). That took around 1 month of charging but it was a great experiment though.

author: poot36
author_rank: Frequent Contributor
timestamp: January 19, 2016, 03:53:46 am
content: Well I finaly got the buck converter! I had to modify it for a lower minimum current setting by changing the sense resistor from 0.24 ohms to 1.44 ohms. My new problem is that now the gamepad charging led flashes faster but it still will not stay constant on. I have tried adding 1000uF of caps to the input and output of the converter in conjunction with the converters built in 220uF caps on the input and output. This helped a bit but I am thinking that my current control loop now cant react quick enough and that the output voltage is sagging below the minimum input voltage of the gamepad. When I looked at the voltage output on my scope I could see it was sagging to about 4.6V or so and when I added the caps the frequency of this downwards spike changed (it got longer). I am guessing this is why most power supplies have very minimal output capasitance to be able to react to rapid current draws. Would it be a good idea to remove the 220uF output cap on the converter board or would this make the current control loop unstable?

author: jt
author_rank: Regular Contributor
timestamp: January 20, 2016, 05:18:56 am
content: I think you are re-experiencing some of the same issues. A standard buck converter will try to source as much power as it can from the input in order to provide a regulated voltage on the output. A regulated voltage applied to a Li-ion battery could translate to a lot of current, and more power than your input source (the solar array) can supply.So the blinking LED may be indicating an oscillation condition - basically the regulator starts to provide positive current to the battery, but the solar array input voltage is pulled too low in the process and the regulator goes into undervoltage lockout. Hysteresis will cause it to oscillate within this system. Adding your caps and manipulating the values may slow down this oscillation, but will not be a (eloquent) solution to the problem.So, you either need to do direct energy transfer (as you have been doing with the direct connection), or you need a peak-power tracking solution which will be a buck regulator with added intelligence to maximize power delivery rather than blindly trying to regulate the output voltage despite the load. It seems like this is more of an educational than practical project - so perhaps you'd be interested to dive into the intricacies of peak power tracking; google will be your friend here and after that you may look at linear tech and ST micro's options for ASIC solutions.

author: poot36
author_rank: Frequent Contributor
timestamp: January 21, 2016, 04:25:30 am
content: I will have a look into how to use the pin 5 on the converter chip (a LM2596S) to turn it into a MPPT style controller. You are right this is more of an educational project then a useful one.