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Batteries & Chargers

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by AtomSoft, Aug 4, 2008.

  1. AtomSoft

    AtomSoft Well-Known Member

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    Hello all,
    Im trying to figure out or determine what batteries to use for a project and how to charge them.

    I need these batteries to supply enough voltage and amperage for:

    GLCD
    TOUCHSCREEN
    PIC
    LEDs
    External SD

    Now i dont know but to me that too much and will die on a 9v battery to fast. so i was thinking instead of using a 7805 i would just use like 3-4 rechargeable 1.2v - 1.5v batteries and make a desk charger for it. Now the thing is i dont know squat about batteries and how to charge them and how to calculate ho long they would last on a single full charge.

    That is why i am asking you people lol (guys/gals (if any(no offense)))
    Should i use small AA type? C,D type? How do i know which are best.

    Please if you recommend a battery... explain why so at least i can learn this stuff lol.

    Thanks again.
     
  2. 3v0

    3v0 Coop Build Coordinator Forum Supporter

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    The more regulation (regulator input-outputV) you use the more power you will be wasting. Unless you use a switching regulator.

    LDO (Low dropout regulators) require less voltage difference between the input and regulated output. If you need regulated 5V, 4 AA's with a 5V LDO regulator is where I would start.

    How much current you will need depends on the devices. The big one is how many LEDs you will be using and how much you pump through each.

    I do not know squat about the touchscreen or SD memory. The slower you can run the PIC the less power it will use. But compared to what a lot LEDs can eat it may be at the level of noise.

    EDIT: You left out some important info. Like how long you expect the project to run on a charge.
     
    Last edited: Aug 4, 2008
  3. AtomSoft

    AtomSoft Well-Known Member

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    I wanted to know how long would it last lol. I have no expectations at this moment since i know nothing about this subject. There will be most likely 2 LEDs. 1 For power and one as a low power warning.

    Do you have any links as to learn about LDO Regulator before i google it?

    Is there some type of formula i can use to calculate how much current the whole project will use by like adding all current requirements of each component. Like how much current does a LED pull? typical? like 20mA ? if so then 2 would eat 40mA if on at same time right?

    If i knew like how to calculate this. Or is that how its done? by adding the current usage? If so then i can simply make like a flow chart for power useage to determin what would be the max current used at any given moment.
     
  4. dave

    Dave New Member

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  5. Hero999

    Hero999 Banned

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    Can't you just use four AA cells?

    Most PICs will be alright to 6V for short periaods of time and if you're worried about that then use a diode to drop it down to 5.4V and give reverse polarity protection.
     
  6. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi atom,
    While it is possible calculate the total current in the circuit, its sometimes more feasible to construct the circuit
    [ on a project board if needed] and measure the current taken by the circuit.

    In this way you can determine the lowest voltage that gives sure operation and allow +10%
    ,as you know the local mains supply isnt always whats says on the box.!

    The same applies to operating mains transformers at different mains frequencies.

    Get the 'worst case' condition for the current and voltage, by using a variable bench psu and add 10% to 15%
     
    Last edited: Aug 4, 2008
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Ni-MH batteries are made in AA size with a 2500mAh rating. They can supply 40mA for the LEDs plus 10mA for the circuit for 50 hours. They need a charger circuit that provides enough voltage with the current regulated then the circuit detects when they are fully charged and turns off the charger.
     
  8. Hero999

    Hero999 Banned

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    A simple charger can be built with a resistor, diode and wallwart to provide the battery with a charge current of 125mA but it'll take a day to fully charge the batteries.
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Energizer and Panasonic say to reduce the trickle-charge current to only C/40 which is 62.5mA.
     
  10. Hero999

    Hero999 Banned

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    That's totally ridiculas, at 125mA the batteries will only be dissipating 200mW (charged to 1.6V per cell) so I don't think anything bad will happen, especially if you don't leave them connected for days on end.
     
  11. AtomSoft

    AtomSoft Well-Known Member

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    so the formula would be mAh / mA usage = hours?

    Like if my circuit has 2 leds @ 20mA ea. thats 40mA
    Then the uC:
    Code (text):

    LEDs:
    2 @ 20mA = 40mA
    uC:
    Maximum current sunk by all ports (combined)...200 mA
    Maximum current sourced by all ports (combined) ...200 mA
    Maximum output current sunk by any I/O pin...25 mA

    GLCD:
    Power Supply Current 15.0 mA
    LED Power Supply Current 470 to 560 mA

     
    so thats a whopping 815mA max current:
    then

    2500 / 815 = 3 hours of operation time? (Using max values)

    OR

    2500 / 525 = 4-5 hours. (typical values that will be used)

    Would this be correct?
    Would i use that from the uC datasheet?

    Um also i own 2 multimeter and some books on how to measure
    current but need look up the info on how to measure current
    so just bare with me :D
     
  12. AtomSoft

    AtomSoft Well-Known Member

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    i have about 5 - NH15 1.2v 2500mAh Energizer batteries. Can someone make a simplified drawing on how to make this charger? Or is it just resistor & diode in series? (would still like drawing lol) i dont want to explode these good(expensive) batteries :D
     
  13. Hero999

    Hero999 Banned

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    Pins can't both source and sink current at the same time and you're not going to be using all the I/O pins at the same time so you can ignore it, it's an absolute maximum rating for the :mu:C not what it uses. In reality the power consumption of the :mu:C will depend on the clock frequency and what you're doing but it won't be much, let's just assume 10mA.

    That just leaves 40mA for the LEDs but I'd run them at 10mA each so the last longer giving just 20mA.

    What do you mean 15mA for the power supply, is this the quiescent current for the linear regulator?

    This leaves a maximum of 605mA.
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your micro-controller is driving only two 20mA LEDs, not 10 of them.
    The touch screen probably has many LEDs as a back-light so its current is pretty high and the AA cells won't last long for each charge.

    Be careful about C and D size Ni-MH cells from Energizer. They have a little AA cell inside.
     
  15. AtomSoft

    AtomSoft Well-Known Member

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    Audioguru that sux lol

    hero:
    I didnt calc for both just showing the info i calc'd for 200mA for max since glcd uses like 11 pins thats 25mA * 11 (or max) but i am most likely wrong since i am indeed a noob to this lol

    So what your telling me is that a components like LEDs for example can source a max of 20mA. It depends on what i give it? so if i want i can send it only 5-10mA and brightness with vary i assume but is that how it works. lol (i dont know how i got through all this without knowing this either lol.)

    For the "What do you mean 15mA" look at attachment. I will try measuring how much actual current the glcd draws at this moment.
     

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  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi atom,
    This is the point I was making about an empirical approach to measuring the current, you will spend ages trying to calculate it precisely and at the end of the day you will still have to measure it to be sure..:)
     
    Last edited: Aug 4, 2008
  17. AtomSoft

    AtomSoft Well-Known Member

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    ok now im lost with measuring lol How would i measure a glcd? what points? I made a simple schem of a LED and a resistor. Would i measure at tp1 & tp2 ?
    [​IMG]
    if so from a actual circuit i got (using a 1.2k ohm) 2.71mA
     

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  18. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi atom,
    If you are asking me, I would set my test meter to current and measure the current in the +V lead to the GLCD.
     

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  19. AtomSoft

    AtomSoft Well-Known Member

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    yeah i was looking at my breadboard wrong lol

    but ohms law says i = v / r which would be
    5.18v / 1.2K (1200) = 0.0041mA

    Which should be 4mA why do i read 2.7mA ?


    EDIT:
    I see without the led its 4.3mA. How would i account for led?

    And when i add led why is it showing less current?
     
    Last edited: Aug 4, 2008
  20. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    If you want a precise measurement, as I dont know the value of your meters internal shunt resistor, at the same you measure the current, measure the voltage on the circuit side of the ammeter.

    OK.

    Measuring on either side of the ammeter 'A' and then 'B', will give the value of the ammeter internal resistance... [ for future calculations]

    EDIT: I would always, where possible., measure in the circuits +V input lead, not on the low side as you have shown..
     

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  21. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Its drawing less current with the LED, because of the forward voltage drop across the LED.!
     

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