charging circuit with multiple batteries w different voltages

Discussion in 'General Electronics Chat' started by w00t, Jan 19, 2008.

  1. w00t

    Thread Starter New Member

    Jan 19, 2008
    Hi all,

    I have built a simple charging circuit to charge a 9.6V and 7.2V rechargeable batteries from a simple 12V DC wall wart. The 9.6V battery powers my RC tank, the 7.2V battery is for powering a micro controller that I have added to the tank.

    here is my circuit

    After building the circuit everything is not working and I would appreciate if someone could give me some insight to what is happening.

    The 12V battery is grounded through a couple of npn transistors. The first npn on the left grounds the 7.2V battery and the second npn grounds the 9.6v battery for charging. The base of the transistors come from the microprocessor being powered by the 7.2V battery.

    You can see from the diagram that the load is connected across the 7.2V and 9.6V batteries.

    The resistor values chosen were to charge the 7.2v and 9.6v batteries at approx 0.2C

    When the 12V is connected and the npn transistors are 0V, I would not expect any current to flow from the 12V charger. This is not the case I am observing a ~150mA current flowing and I do not know where it is going??? Setting the npn base voltage to 5V increases the current from the 12 V battery. btw I do have current limiting resistors into the npn base.

    I would expect the load across the 7.2V and 9.6V battery to always measure 7.2V and 9.6V across their respective loads even when the 12V battery is connected and grounded.

    When the 12V battery is not connected, I disable the transistors so the 9.6 and 7.2v negative terminals are not connected. Is this sufficient to ensure the 9.6V does not start charging the 7.2v battery?

    I was thinking of getting rid of the 7.2V battery and just going with the 9.6V battery. It looks more complicated than I first though to have two batteries on the tank. Space is at a premium so I only wanted a very simple charging circuit. The only reason i thought of adding the 7.2V battery is that I heard a high current draw on a battery (like when the rc tank starts moving) could cause a brown out to the micro and reset it.

    Would really appreciate some help on this.
  2. Audioguru


    Dec 20, 2007
    If the batteries do not have a load when being charged then their charging current is determined by the values of the current-limiting resistors at the collectors of the NPN transistors.

    Charging begins when the 12V supply has its negative connected to the emitters of the NPN transistors and the transistors are turned on.

    The 12V is too high for the 7.2V battery so it will probably become over-charged (which reduces its life) unless you have a way of detecting when it is fully charged then turning off the charger.
  3. w00t

    Thread Starter New Member

    Jan 19, 2008
    thanks for your reply audio.

    I see your point about the 7.2V possibly getting overcharged if Im not careful. I will investigate a voltage regulator or using the micro a/d to prevent an overcharge in my final design.

    After some testing this afternoon I have figured out why the 12V was delivering a large current without the transistors switched on. The micro controls the trannys. The micro is powered from the 7.2V. Because of where I am grounding the 7.2V load (right before the current limiting resistor before the collector aka - on the 7.2V negative terminal) I am not getting a common ground. The transistor base is getting a Voltage of 5V when it should be 0 because of the grounding problem.

    Im not sure how Im going to get a common ground.

    If you could suggest a way I would appreciate it.
  4. rwmoekoe

    Active Member

    Mar 1, 2007
    you are right. you should get a common ground. this can be easily done by reversing the circuit, that is, use pnp trans instead of npn's, and they either connect the batteries to the 12v's + terminal or disconnect them (through your limiting resistors). (it's better understood imagining it upside down, because you have got them in a common positive configuration right now).

    aside of that, maybe you wanna consider not using the separate batteries instead. to prevent the microcontroller (which consumes very very small current itself) from being browned out and resetting itself, use diode (1n4148 or equivalent would do) and sufficient capacitor (say a 10uF tantalum) between its vcc and the main power.

    from main power's vcc --->|----- to the micro controller's vcc

    to the micro controller's vcc
    = 10uF tantalum
  5. w00t

    Thread Starter New Member

    Jan 19, 2008
    thanks for all your help.

    I am going to redo the circuit using pnp tansistors and a single battery with a cap and diode.
  6. ginagirl

    New Member

    Feb 17, 2008
    can you give me an example circuit