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As battery nears charging voltage

  1. Jul 7, 2012 #1
    Dear Experts

    As a rechargeable battery , say a li-po battery, is being charged to near charging voltage, there is less potential difference which in this sense means it takes longer and longer to reach remaining few 0.x volts.

    Could you be so kind as to explain a bit how a typical charger works to overcome this situation such that the charging rate is not as slow towards the end where battery voltage nears the charging voltage?

    Thanks for reading.
    Have a nice day.

    Best regards
  2. jcsd
  3. Jul 8, 2012 #2


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    Science Advisor

    I have seen fairly expensive battery chargers that work like that.

    Even worse are the ones that just continue to give a charge even when the battery is fully charged.

    One way of making a better charger is to have a timer which cuts off the charge after some fixed time.
    Unfortunately, this assumes you know how charged the battery was to start with. So, the battery may be overcharged if it already had a charge in it.

    Another way is to charge the battery and remove the charging supply as soon as the battery voltage was equal to or greater than some decided voltage. This is done with a comparator.
    This has the problem that battery voltage depends on temperature, so it may give over charging or undercharging depending on the temperature.

    I made a charger for NiMH batteries that stopped charging once a minute, waited 5 seconds then took a voltage reading. This was compared with the previous voltage reading (one minute earlier) and if there were 20 identical voltage readings (over 20 minutes) then the battery was considered to be fully charged and charging was ceased.

    This is very easy to do with a micro and I have been using the same batch of NiMH batteries I got 4 years ago, so it seems to keep the batteries safe as well.
  4. Jul 8, 2012 #3
    Lipos are usually charged with constant current until the voltage reaches 4.2V, so it's actually the other way around for lipos; the voltage increases sharply towards the end of the constant current charge phase. To charge your lipo cell to full capacity you need a second and final charge phase with constant voltage. In this phase, the voltage is regulated until the charge current decreases to say 10% and the charging is completed.

    This didn't answer your question, but I'm mentioning it in case you are in fact using lipo cells.
  5. Jul 9, 2012 #4
    Hello Vk6kro

    Thanks very much for sharing with us your experience.
    I am thinking of using a comparator. But not sure how accurate it can be considering the sensitivity of the li-ion.

    How about using a 4.2v zener diode to cut-off the current going into the battery when it reaches 4.2v? Maybe there isn't such a zener?

    Hello Gnurf

    Sorry, I am using li-ion to be exact. But I read somewhere they are very similar if not fundamentally the same thing. Correct me if I am wrong.

    I was thinking of increasing the current while maintaining constant voltage to compensate for the potential difference towards the end.

    But I have got it wrong since the voltage charging rate increases towards the end for Li-Po.

    Thanks to both Experts here for sharing.

    Best regards
  6. Jul 9, 2012 #5
    If it was me I would charge the cells as recommended by the manufacturer (i.e with a constant-current constant voltage regime) and use a ready-made charger IC. Why reinvent the wheel?
  7. Jul 12, 2012 #6
    Dear Gnurf
    Glad to know charger ICs are available.
    Yes, i would not re-invent the wheel.
    May i know which IC do you recommend?

    Thanks for your tips.

    Have a great day :)

  8. Jul 12, 2012 #7

    jim hardy

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    Gold Member

  9. Jul 13, 2012 #8
    Do a parametric search on your favorite IC manufacturer's website (TI, Linear, National, etc) or a distributor such as Digikey. As Jim points out, there are lots of them. Pick one that suits your needs and the rest is easy--you might have to set the charge current and/or time with a couple of resistors, or something like that.

    See what you find and rapport back?
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