Solar Multi Charger - tips needed

Discussion in 'The Projects Forum' started by kaning, Jun 5, 2014.

  1. kaning

    Thread Starter Member

    Jun 2, 2014
    43
    0
    Hi, this is my final project for my bachelor. hope you guys can help in some matters. :)

    My project consists in a 6v 3,7w solar panel to charge different types of batteries/devices, wich are: lipo battery, usb devices and 4 AA nimh abtteries.

    Also i'll be using a pic16f648a to control and monitor each circuit (voltage/current), turn on and off each circuit and display info on LCD. i'll be using 4 AA 2300mha batteries to power this last part.

    I have several questions/doubts regarding all this. Also try to keep it simple, im not such an expert :p Any tip/criticis welcome! So:

    Nimh charger: Im using a current limitter circuit (500ma +/-) with lm317.

    Q: First thing i would like to know is if lm7805 has less voltage drop than lm317 because i.e. for Nimh charger i will need 5,6v output (to charge nimh to 1,4v each) and with lm317 im getting 5,52v. i need at least 5,6v so then microcontroller could shut off the circuit.

    USB charger: For this one im using voltage limitter (5.15v +/-) again with lm317 and current will depend on the load.

    Q:but for i.e. Iphone charges by usb at 500ma but because usb's (laptops etc) are limitted to this current, while charging from "wall" it charges up to 1A. so in my circuit iphone will draw as much current as it can get. Am i right? therefore the voltage output from solar panel will be lower wich will make lm317 Vout also lower since im already on thresold input voltage (6v) for lm317 to give me 5.15v. im missing something here. any tips?

    Now comes the second part of the problem :D
    as i said im using pic16f to switch on and of each circuit. i've been reading a lot about what switches to use and i cant decide. im between transistor and relay combination, transistor (2n222) to power transistor (tip32c/tip117) or only adequated power transistor (read a lot of datasheets). Main thing here, the less voltage drop, the better!! i bought finder D06 5v relay,measured the resistence in the "switch" itself and has 0.3Ω
    wich gives me for the max current panel can draw a 0.18V max drop. i think its quite good, anyway i wonder if there is any transistor that is a better solution i came across 2n222 transistor but Ic (collector current) is 0.6A max so in a very shiny day solar panel will draw more than that. Any tips??

    Last thing, and as curiosity. 16f648 has inbuilt 4Mhz clock is this good enough for this monitoring i desire? (i think it is for its simplicity) i wanna hear your opinion about this. Also because i never know when to use higher one and what value should be good for any given project.

    Many thanks!!! if you need any extra info, data etc. just tell me what :)
     
  2. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    I see several problems.

    Either of the mentioned voltage regulators needs at least 2V above the output voltage, so you are not going to get anything like 5.6V or even 5V out of your 6V solar panel. Also the 6V rating is probably the open circuit voltage, so under load it will be less. Have you determined it actual characteristics under load? You will at least want a low-dropout regulator, and may even need a buck-boost converter. I would go for a higher voltage panel myself for this project, 8V at 500mA would give you the headroom you need.

    How are you expecting detect end of charge on the NiMH batteries? This is actually quite tricky, and at 500mA, if you don't do it right, the batteries are likely to overhead and vent.

    As for the switching element, use MOSFETs. They can have on resistance in the low milliohm region.

    Bob
     
  3. kaning

    Thread Starter Member

    Jun 2, 2014
    43
    0
    Hi Bob, thanks for the reply

    About solar panel, in open circuit i get around 7v.
    the only circuit i tested sucessfully so far is the lipo charger where i got battery near to 4,2v (4,13v) then it got very cloudy and i stoped the testing. Now the Nimh circuit is the most problematic, reading the terminals of Nimh circuit below with 6v supply i get 5,52v(no load). will this value go down when i plug the 4 Nimh batteries to charge?i raised a bit to 6,4v on the supply and i got the 5,6v, and thats why i limited the current to 500ma, for the solar panel dont go below the 6v (P=VI, with 3.7W / 0.5A= 7,4 V so, it means solar panel can give this current without voltage going down or to much down lets say) am i thinking right?

    About decteting end of charge i was thinking about either controlling it with microcontroller (main goal) where i would use voltage divider (where R1 and R2 the same value to cut voltage in half so microcontroller can read values on analog input without burning it) or with voltage comparator (lm393).
     
    Last edited: Jun 5, 2014
  4. kaning

    Thread Starter Member

    Jun 2, 2014
    43
    0
  5. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    You are missing the fact that even if your solar panel is at 6V and 500mA, you will get only about 4 volts out of your regulator. The regulator drops 2V at that current.

    Your panel is right on the edge of being able to supply enough power for what you are doing. To make the most of it, you should use either a switching regulator (which can have 0 dropout) or at least a low dropout regulator.

    Also, charging both NiMH and LiPO batteries is very tricky. You should go to this site and read about it lest you end up destroying your batteries at best and having them catch fire at worst.

    http://batteryuniversity.com/
     
  6. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    duplicate....
     
  7. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    No, that MOSFET needs 10V on the gate, which you cannot directly drive from your micro.

    Look for "logic level" MOSFETs. These typically require 4.5V or less and can be directly driven by a PIC pin.

    Bob
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You are over-thinking it! The 3.7W solar panel will be struggling to make >500MA anyway, so you don't need a 500mA current limiter!

    An LM317 is a poor choice to give 5v from a 6v solar panel (as BobTPH has said).

    Just use any off-the shelf 3-pin 5v LDO regulator! They generally accept up to 16v Vin, and will make exactly 5v out. And dropout voltage (headroom) can be as low os 0.1v, so that is 5.1v in and 5.0v out.

    Some brands of LDO regulator have a "shutdown" pin, making them ideal for your PIC to switch the regulator on and off.

    Other hint; if you get a 1.8v LDO regulator, you can use it just like a LM317 with two resistors to set the output voltage to 5v or 5.6v as needed. But it will still have the LDO feature. :)
     
    kaning likes this.
  9. kaning

    Thread Starter Member

    Jun 2, 2014
    43
    0
    Thanks for the help Bob. im now looking for logic level MOSFETS. i've seen some with very low Rds and thats good, now i just to pick the best for me.

    Now,it might look stupid but its bugging me. remember you told me how to detect end of charge on the NiMH batteries?
    Q:Is it possible charge the batteries while they feed microcontroller, so it can control voltage and shutdown circuit ?? otherwise i need to do full analog circuit for Nimh batteries with LM393 and simple analog switch/button to turn it on.something like this

    http://imgur.com/OohJzkN

    Q: The digital output voltage of microcontroller is the same as the one that feeds it? i.e. the batteries will eventually drop to max4v when they will be cut off andstart charging= digital output 4v?

    Thanks for the reply. i started to realize that. and im also checking some

    Q: So with a Vout=1,8v regulator and using "2 resistor voltage divider" i can get either the 5 or 5,6v?:confused:
     
    Last edited: Jun 5, 2014
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,235
    Do you have any restrictions on the time to charge them?
    Do they need to be 100% charged or might 85 or 90% be okay?
     
    kaning likes this.
  11. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    The battery university link I gave you tells you how to detect end of charge, but 500mA is a very bad charging current to use for a 2000maH battery. It is too low for the end of charge detection (should be C/2 or 1A) and too high to be safe and not require end of charge detection (i.e. trickle charge).

    As for your questions about voltage regulators for charging, this is a bad idea. You should be controlling current, not voltage when charging batteries. The ideal way to do it would be to make a buck converter controlled by your micro with a current sense to control it. This would give you any voltage up to the output of the solar panel - the loss in the switch to maintain the charge current.

    Lithium ion batteries require a constant current followed by a constant voltage phase, so you must be able to do this as well.

    Bob
     
  12. kaning

    Thread Starter Member

    Jun 2, 2014
    43
    0
    Hi Ron,
    if you mean about Nimh batteries, they could be like 90%. even because microncontroller only can get 5,5v max unless i use one of those low drop regulators they could be 100% charged
     
  13. kaning

    Thread Starter Member

    Jun 2, 2014
    43
    0
    About first paragraph i already realized i dont have enough power to fully charge them because they might get up to 1,5v each cell. thats why i was stablishing a limit of 1,4v each cell therefore voltage comparator would be enough although they wouldnt get fully charged. :/

    About Lipo, the schematic im using follows those instructions. the charge starts with constant current at max 500ma (mine is 1000 mAh) and it has voltage limitter at 4,2v. once that voltage is acomplished the current will start droping where i will cut the circuit at about 20mA.
     
  14. kaning

    Thread Starter Member

    Jun 2, 2014
    43
    0
    So, after some research regarding regulators i think i found some good options and maybe you guys can help me choosing between theese 2. they have enable pin wich makes it perfect for this project since i wouldn't be needing mosfest to turn on/off the circuit, i could do it directly with microcontroller. am i right?

    LIPO / USB charger options:

    NCP3335A - http://pdf1.alldatasheet.com/datasheet-pdf/view/178178/ONSEMI/NCP3335A.html

    NCV5500 - http://pdf1.alldatasheet.com/datasheet-pdf/view/166587/ONSEMI/NCV5500.html

    Nimh charger:

    NCP3335A - http://pdf1.alldatasheet.com/datasheet-pdf/view/178178/ONSEMI/NCP3335A.html

    would this do to regulate voltage at 5,6v? i read it can be adjusted up to 10v,if i read it right dropout voltage 340mV.

    Nimh Charger Sugestion - could i charge directly through solar panel with capacitor on Vin and then change my efforts into a smart cut off system? (temperature or voltage related)
    P.s. they dont need to be perfectly 100% charged.

    Many thanks for the tips, they already changed my overview concept of this project. and its nice making the same with less (power).
     
  15. kaning

    Thread Starter Member

    Jun 2, 2014
    43
    0
    i've decided to change the 2300 mAh for 800mAh/1000mAh since both microcontroller and lcd together need a very low current supply. like this will meet the requirements mentioned above, also it will be a lot faster to charge them.
     
Loading...