Battery pack charger remake from 18V to 6V (or other)

Discussion in 'General Electronics Chat' started by eimix, Oct 2, 2008.

  1. eimix

    Thread Starter Active Member

    Sep 2, 2008

    I have broken electric drill with battery pack and charger. Drill it self is broken, but batteries are still very good (15pcs ~2-2,5Ah, 1,2V),
    so i decided to use batteries and charger for my RC model (car).

    first attempt was to make 5cells in series and such packs in parallel - so i got 6V 6-7,5Ah battery pack. In such case cells are discharged equally and i can to connect those 3x5cell packs in series and plug into original charger.
    But it weights too much :)

    I wish to use those 5cell packs separately (less weight), but in this case packs will be discharged not equally, and using original charger is not possible.

    So is it possible to remake original charger from 18V (15cell pack charger) to 5cell charger? If yes - i will draw it's circuit and post it here soon (it looks quite simple for me - transformer: 220VAC->~20VAC, ~5 diodes, few resistors, 2x capacitor, 1 relay, probably for starting and auto stopping charging).

    p.s. Original charger is very good(fast) with 2,5A charging current, and such fast charger cost a lot, so i do not want to bye new one.
  2. eimix

    Thread Starter Active Member

    Sep 2, 2008
    I tried to think my self, but not sure if i'm in right direction:

    if i have power supply of 20VDC would it be correct to wire some diodes (10pcs or more, 0.6V drop voltage) in serries with battery?
    or zener diode - with voltage Vzener = 20V - BatteryChargeVoltage?
  3. eimix

    Thread Starter Active Member

    Sep 2, 2008
    I did it on my own :)

    i put voltage regulators, but now i am not sure if i chose right values :(

    for 8x1.2 700mAh (transmitter battery pack) i used 78L12 (12V, 100mA) voltage regulator - battery charges up to 11.5-11.8V and stops, regulator cools down and it seems that current stops flowing.
    It seems to be good - something like tricle charging, but i have not measured current in that case. Is my thinking right?

    for 6x1.2 3000mAh (motor battery pack) i used 2 of L7809 (9V, 1.5A) in parallel and looking for heatsink right now :D
    this battery pack is with heat sensor (termo protector) that sensor in original battery was used so i used in this one, hopefully it will work. right now voltage regulators provide low current and battery back does not get hot :)
  4. eimix

    Thread Starter Active Member

    Sep 2, 2008
    Echs, nobody is helping me :(
    are my questions too dummy? or my enlgish is not so good? ;)

    anw, probably last question on this issue:
    constant current phase works fine, constant voltage at the end of charging also works fine. (current decreases to 1-2mA)

    I need some indication or circuit cut off if low current is.
    Is it possible some how? i tried googling, but nothing worth i found, probably i do not know how to express what i need.:(


    Some sort of current treshold - on if current > 10mA and off if current < 10mA.

    Please help
    Last edited: Oct 8, 2008
  5. Audioguru


    Dec 20, 2007
    The 100mA 78L12 probably overheats and shuts down when it has an 18V input and nothing limits the current. Its max current is about 100mA and a discharged battery is about 8V so the regulator is trying to dissipate 10V x 100mA= 1.0W.
    Its max allowed internal temperature is 150 degrees C and will try to be 165 degrees C when it dissipates 1.0W with an ambient temperature of 25 degrees C.
    The thermal oscillation of it heating, shutting down and cooling, then heating again over and over will stress it and break it.

    Voltage regulator ICs do not work properly when they are in parallel.

    Use battery charger ICs. They properly sense when the battery is fully charged then turn off.
  6. eimix

    Thread Starter Active Member

    Sep 2, 2008
    I know that i should bought new charger instead :) but it is this way and i still like it ;)

    I overcome this regulator overheating problem (even it is said in datasheet - indestructible) - i put L7815 (1.5A, 15V regulator) with heat sink in series before 78L12, so 8L12 has less voltage drop to make. Heatsink touch 78L12 too, and now i can touch it and feel warm.

    Problem remains low current indication - i got tip in other forum - use Ameter :D nice, but i cannot cutoff circuit with it :/
  7. Andrew Leigh

    Well-Known Member

    Sep 8, 2008

    at 40mA effectively you have cutoff the charge. From your posts on ther other forum you are charging at 2,5A. I believe that this small residual current is part of the design and the current is used to to replace the natural internal discharge of the battery pack when the drill is plugged into the holder but not used.

    I would not change a thing.

  8. eimix

    Thread Starter Active Member

    Sep 2, 2008
    I have two battery packs - 8cell 700mAh and charging it with 0.1A (with 78L12) and 6cell 3000mAH charging with 2-3A (with L7809+heatsink).
    Audioguru was commenting 0.1A issue.

    You probably are right - leave some small current to flow is not very bad. It keeps betteries fully charged.

    Anw, some led indication instead of Ameter would be smaller and more clear.