Battery charger constant current stage issue

Discussion in 'General Electronics Chat' started by Rphil, Oct 12, 2012.

  1. Rphil

    Thread Starter New Member

    Oct 6, 2012
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    Hi, i'm writing because i'm building a battery charger PWM controlled by a mcu ATmega8535. Now the issue is that having done the circuit with a step down design to charge the battery, i expect to have a constant current of 600mA, but it rises soon to 200 mA and then rises slowly to 300 mA, on the other hand on the battery cell terminals i can measure 5,7 V and it is dangerous because LI-Ion maximum voltage is 4.2V. I think the software is good, because it's trying to increment the duty cycle of the step down and then the voltage on the battery until a current of 600mA flows across it, but this current threshold cannot be acheived. I post here a schematic of the circuit. Could anyone help me please?

    Thank you in advance
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,984
    3,223
    Typically a battery charger has both a voltage and current limit to avoid overstressing the battery. Also LI-ion batteries have specific charge requirements that must be followed to safely charge them. Look at the battery manufacturer specs for info on that.
     
  3. Rphil

    Thread Starter New Member

    Oct 6, 2012
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    I think this is not the problem, because this is the battery:http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&sqi=2&ved=0CCUQFjAA&url=http%3A%2F%2Fwww.tadiranbat.com%2Fpdf.php%3Fid%3DTL-5276%252FW&ei=DlR4UNCXH4eg4gSCvIDIAg&usg=AFQjCNE8NixuzbAg_IFXI0SyGF1B5Ca_tA&sig2=ewSXtcY71lGJ3220OvRI9w it is a battery of 1Ah and i'm trying to charge it with a current, which is 0.6C.
    I've tried also with another cell, this one: http://www.astc.ru/data/delivery_list/content/eemb/pdf/LIR18650%20(1800mAh).pdf In this case i can see in the datasheet that i'm not violating the charge current limits. But if in the first case i was able to see 300mA across the battery, now with this other cell no current flows. I'm really going mad.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The battery you want to charge is a tiny low power one. Its capacity is measured when its load is only 1mA (1000 hours at 1mA).

    Its maximum allowed output current is only 50mA and the maufacturer does not say the maximum allowed charging current in the datasheet but is probably much less than 50mA. The datasheet also does not say the maximum allowed charging voltage and shows a maximum voltage of 3.6V in the discharge graph.
     
  5. Rphil

    Thread Starter New Member

    Oct 6, 2012
    10
    0
    Yes but with the other one battery i've the same behaviour, and it can be charged at 1500 mA.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The datasheet for the 1800mAh battery shows it being charged at 900mA and the voltage slowly rising to 4.2V. The charger limits the voltage to 4.2V then the battery slowly reduces its charging current until almost no charging current flows when it is fully charged.
     
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