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charger for Cordless phone battery.


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hi all
iam working on a school LED project.where we need to use a 3.6v 300ma battery(one used in cardless phones).u know cordless phone chargers are dumb type.can u suggest me some good charger for this battery pack.
i don't want  hi-fi type of charger.just simple one which makes the battery last for long times without destroying it.and if it got some indictor it would be great.
thanks in advance

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i dono much abt these abt these rechargbles.but i think ni-cd shld be chrged at 1.5+pack voltage and c/10 of its current rating it.
if it is true then i need 5.1v @ 30ma for 14Hr .

I don't know where you learned the wrong voltage formula.
Each cell will charge to 1.4V to 1.5V. Therefore the 3-cell battery will be 4.2V to 4.5V when fully charged.
The LM3117 needs at least 2V across it and the current-sensing resistor has a 1.25V drop, so the minimum input voltage to the circuit is 7.75V.
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is current calculation is right(c/10 for 14Hrs)?

Yes, if the battery is rated for 300mA/hrs.

if yes then the final result will be 7-8v input to LM317 with R1 82Ohms resistor.(as ante said) can charge a 3.6v battery pack safely in 14Hrs

No. 7V is too low. "The  minimum input voltage to the circuit is 7.75V."
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got it.
thanks.

is there any particular formula for this resistor value.
imean in furture if i want to charge packs of 700ma or 800ma etc i need to change resistors too right.

and gyus to charge AA or AAA "single cell" what modification shld be done in  lm317 design.

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The voltage drop across R1 is always 1.2Volts (controlled by the regulator), using (I = U / R )you get the charge current.
Example; 1.2 V / 82 Ohm =  0.0146 A that means about 15 mA!
If you increase the charge current to 800mA you need a heatsink for the LM317!
Got it? ;D

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Hi fever,

That’s correct for C/10 but I suggested 15mA so that you could leave the pack unattended for an “unlimited” period of time. If you like you can add another 82 Ohm resistor in parallel to have C/10 – 14 h charge. I made a drawing for you, if you leave the switch open its C/20 if closed it’s C/10.
If you don’t need the C/20 setting just omit the switch. The reason for two resistors is that it’s not easy to find a 40 Ohm resistor but two 82 in parallel is 41 Ohm and that is just fine. 8)

post-929-14279142731984_thumb.png

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cool.
thanks a lot ante.
tomorrow i'll get some resistors of different values.so that i can make it variable.
imean 15ma-c/20
          30ma-c/10 for 14Hr
          60ma-c/5  for 7Hrs (ofcourse not preferable)
thanks once again.for helping me
u ppl are just really amazing .  :-*

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