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Voltage ratings of Li-Ion batteries

  1. Nov 10, 2012 #1
    Dear Experts

    I bought a lithium-ion battery rated at 3.7v.

    I charge it with my existing phone charger rated for 3.7v batteries and charging at 4.2V.

    i like to know why are batteries are rated like this. That is, a 3.7V rated battery is actually charged to 4.1 or 4.2 when its considered 100% fully charged. So, why is it not rated at 4.2v?

    Why the misrepresentation? Of course, I am assuming there is misrepresentation.

    And yes, I understand that you need a higher voltage to charge to a voltage that is near but lower than that voltage. So my question is on the choice of the rating and not the reason for using higher voltage to charge a battery rated lower.

    Thank you for reading.

    Best regards
  2. jcsd
  3. Nov 10, 2012 #2
    Constant current discharge curve for a Li-Ion cell:

  4. Nov 10, 2012 #3
    It's understood within the industry that all battery chemistries have their own discharge curve. They also have idiosyncrasies regarding their needs during charging and discharging.
    With the exception of reference cells, batteries are not intended to hold a constant voltage - their intended to maximize their performance at supply energy or power.
  5. Nov 11, 2012 #4

    Thank you gnurf for letting me know about the curve.

    @Mike In Plano
    Thank you as well.

    My apologies. My phone charger is rated at output voltage 5V , 1 amp.
    I measured the charger without load, the output voltage was actually 4.85v.

    I took out my phone and with this 4.85V, I directly connect to the phone's terminals of the li-ion battery compartment.

    The phone does not start without the battery. I checked again, the charger cable is supplying 4.85v. So, I wonder :

    1. Is it that the phone did not start up when connected like that because the voltage was too high? That is , the standard phone battery is rated at 3.7v to be charged by 4.2v.

    2. Given the phone li-ion battery is rated at 3.7v requiring a charging voltage of 4.2v, why is the phone charger rated at output 5v? Is it to compensate for the loss due to battery internal resistance?

    Thanks for reading.

    Best regards
    Ramone (Pascal is my nickname :) )
  6. Nov 12, 2012 #5
    Also - from a ratings perspective, we use batteries for energy, not just supply a voltage, so the Nominal Voltage combined with the Rated mAh = total energy that the battery is rated for.

    As for the charger - the phone or battery module itself, may have a V and I regulator integrated - and is rated for 5V input ( notice this is also the USB voltage - so most cell phones are prepared for 5V - and regulate themselves) . Overcharging these is a problem ( hazard) and they put some effort into this aspect of the design.
    For a cell phone - more value is placed in the longevity of the battery - for a household cordless phone -- cost - cost - cost- so they may just provide a cheaper charger.
    So many factors in product design.....
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