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question about charging a battery

Discussion in 'Power Electronics' started by mikgol, Jul 22, 2013.

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  1. mikgol


    Jul 6, 2013
    Hey there electronics guru's. I was wondering if someone here can help me with ...

    I'm building a gate opener using a normal cordless drill as the engine.
    The drill will be unmodified - it's battery will power the opening/closing of the gate - but I want to wire it into the house so I can charge the battery (eventually, I'll put in a solar panel to charge it).

    Normally, to charge the battery, I would disconnect the battery from the drill and plug it into the charger.

    Is it OK to just wire the battery to the charger, while still being wired to the drill, so it stays charged but still powers the drill? Or do I need to separate the battery from the drill while charging (using a relay?)

    Thank you for your help :)
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    A lot depends on the charger.

    Can the battery be left permanently attached to the charger? Read the instructions -- Do they tell you not to leave it connected for more than a particular period?

    The second problem is more tricky. What if it is connected when the device is in use? Most chargers are going to limit current, and this should be OK, however a smart charger might interpret this as battery damage, or restart the charge cycle with bad results.

    Try it (carefully) before you attach it to the gate and see what happens.
  3. mikgol


    Jul 6, 2013
    Thanks for the quick reply steve.

    Good point about checking the instructions to see if it can be permanently attached.
    My guess is no, since you can't charge the battery (normally) without detaching it from the drill.

    It's a pretty cheap drill too, so I imagine that it's electronics isn't of the highest quality, so by the sounds of it I shouldn't be doing it this way.

    My plan in that case is to have the battery attached to the drill, as well as the charger, and use a relay so that when the juice is flowing from the charger it disconnects the drill. I wont be able to open the gate while charging, but that's ok - I'll just charge at night time.

    I've very new to electronics but I'm imagining that I can do this with a single relay, with the input connected to the charging wire, and the output connecting the drill wire, something like this ...


    ... will something like that work?
    Last edited: Jul 22, 2013
  4. BobK


    Jan 5, 2010
    If it is a cheap drill, the charger is likely a trickle charger that have not much more than a voltage source of a couple of volts above the battery voltage + a resistor to limit current. If that is the case, you could probably safely leave it connected all the time and it would not interfere with operating the gate since the charger could not supply the current and the battery would have to when the motor was operating. This kind of charger would take probably 8 hours to fully charge a battery. That would be a clue.

    On the other hand, if the charger is a smart one, it might cause problems. Take the charger apart a post a picture and we can probably tell more.

  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    ...or simply tell us how long it takes to charge a battery from flat.

    Generally the instructions for this type of charger will say something like "Before the first use, leave the battery connected to the charger for xx hours" (where xx might be 10, or 14, or some largeish number.

    your diagram above doesn't make a lot of sense because a relay has different types of connections and you don't specify which is which.

    Nevertheless, I can't see a combination of connections that would work the way you have it connected.

    The two options are:

    1) the relay is activated by the charger, disconnecting the motor (this means that while the charger is running the door cannot be operated).

    2) The relay is connected with its coil in parallel with the motor and the contacts disconnecting the charger when the motor operates.

    The first option won't work well for a whole host of reasons. I would not recommend it.

    The second option is better. However it is possible that the charger might be affected if the battery is effectively removed from the charger during mid charge. I would not expect it would be a major effect.

    Even better is a modification of this second method where the relay is used to power the motor, whilst simultaneously disconnecting the charger. (so it toggles the connection tot he +ve side of the battery to the charger (NC contact) or the motor (NO contact). This has the advantage of minimising the length of high current power cables required, and your gate switch need only switch the current needed for the relay, not the motor.
  6. mikgol


    Jul 6, 2013
    Thanks very much for your help guys. I'll get some more info on the charger and will post back.

    Steve, option 2 (motor disconnecting the charger) sounds like the way to go.

    I'm going to use a DTDP switch to turn the motor clockwise/off/anticlockwise, but eventually I'll add remote control as well (by ripping apart a cheap RC car and replacing it's motor with relays). Will this be a problem with option 2? (I'm guessing that option 2 will only work with current in one direction?)

    I'm very new to electronics, but i'm keen to learn. Could you please explain why option 1 is a bad idea, just for my own learning purposes :)

    Thanks again guys, this is a great forum!
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