Power-Battery switch for TP4056 CC CV board

Discussion in 'The Projects Forum' started by deepak4you, Mar 26, 2015.

  1. deepak4you

    Thread Starter New Member

    May 1, 2010

    Sometime back, we had a very simple and wonderful LED product in the market. It was a small LED lamp which had a small Lithium battery in it. When the lamp was on, it would automatically switched between AC source and battery when the power went off (which happens very often in India).

    This was an excellent product for the rural market due to power problems there. Unfortunately, the company stopped that product for some reason.

    I am trying build something similar for some of my friends in the villages, but want to make it as simple as it could get. I decided to use a TP4056 based CC CV battery charger board.

    A review of the TP4056 is here:
    http://lygte-info.dk/review/Review Charger TP4056 UK.html

    It seems to be fairly reliable and offer upto 850mA of current. I know this board is for battery charging, but the size and features made me decide on this one.

    So on the board I have:

    1. Input (5V from any USB charger)
    2. Output (4.2v i think?)
    3. Red LED indicating "charging"
    4. Green LED indicating "charged" (almost)
    5. Battery (external) connected to the output of the board

    I could use input/output to power few 0.5W LEDs (eliminates use of heatsinks that are needed foe high power LEDs) depending on whether AC power is available or not.

    Of what I could think of, i could use two methods:

    1. Make a small transistor/mosfet based switch to switch between input/output for powering the LEDs based. I cant seem to find a simple circuit to achieve this. I land up using multiple discrete elements, which i am trying to avoid.

    2. Possibly connect the LEDs directly to the output in parallel with the battery and let the 4056 board do the rest. Possibly use the voltage acros the RED/GREEN indicator LEDs to switch between input/output source?

    If you gurus could help me build something relatively simplistic, some simple people will surely send some blessings in return :)

  2. deepak4you

    Thread Starter New Member

    May 1, 2010
    Ok, looks like my description was not clear enough. Attached is a rough sketch of what I had in mind (please excuse the crudeness of the diagram). The output states I want are:

    1 X 1 (Drawing power from IN)
    0 1 1 (Drawing power from Battery)

    The attached schematic is not the final one. The resistors etc are not considered. Its just a crude schematic representation of what I had in mind.

    I added the diodes to ensure the voltages from input and output dont get mixed up (since both will be at different levels).

    Also please excuse my naivity with the design since its been a long time since I got this low with electronics (since college i.e.). If there are any flaws or any better designs that I could use, please do share.

  3. ebeowulf17

    Well-Known Member

    Aug 12, 2014
    Won't that circuit function exactly the same without the MOSFETs? One of the diodes prevents USB power from backfeeding into the battery side of the charging circuit when powered. The other diode prevents battery power from circling around to the charger input when the power source is off and you're running on battery power.

    Of course, as you said, there are other factors yet to be worked out, like current limiting resistors and charge status indicator lights, but as far as switching between USB and battery power, I think just those two diodes get it done.

    Having said all that, you may want to consider bigger changes, like using an LED driver or diy current source so you're wasting less energy and generating less heat than you will with simple current limiting resistors.

    The subtleties of how to do this sort of thing well are beyond me, so I think I'll stop talking now and wait for the experts...
  4. deepak4you

    Thread Starter New Member

    May 1, 2010
    Thanks for your reply ebeouwulf17. For a moment, even i got thinking over what you said. But unfortunately, your configuration wontbe good for the circuit becaause altgoughttye diodes will prevent "cross talk" of two voltage sources, but it will create an imbalance onthe load sides due to different voltage levels.

    The idea was to ensure that the LEDs are fed power from the IN side when AC power is ON, and from the battery/OUT side when AC power is off (power to the IN side will come from a simple 1A USB charger used formobile phones).

    Using the diode also helps bring down the voltage level across the LED to about 3.5V which is just about ok for my LEDs.

    I wish to put only as many LEDs as about 30℅-40% of the output current. This way, therr will be more current available for the battery to be charged. When the battery is charged and LEDs are on, there will still be current drawnfromthe charge controller enough to run theLEDs.

    Like I said. This is not the perfect and efficient circuit, but good enough to make the above work fairly well using Li batteries instead od the chinese ones that use lead acid batteries and dont run for more that 2-3 months.
  5. deepak4you

    Thread Starter New Member

    May 1, 2010
    Would be good if someone could look atthe ckt an tellme if the ckt will function like a switch as i want and if there areany modifications that might be needed to make it work.
  6. deepak4you

    Thread Starter New Member

    May 1, 2010
    Sad not to have a reply to my query. Might be my query is just too beginner like or silly/stupid. Will post my query to another forum to see if I get a reply there. Thanks.