Charging batteries.

Discussion in 'Power Electronics' started by markdem, Jul 4, 2016.

  1. markdem

    Thread Starter Member

    Jul 31, 2013
    Hi All.

    I have a requirement to charge 2 x 12v truck batteries. No problem here, I have a dual output charger (2 x 12v outputs).
    Problem is, the thing I am charging needs to be able to put the batteries in series or parallel depending on what it is doing.
    It will not need to be in series when charging, nor is there a load on the device while charging.

    So, I came up with this;

    So the logic goes as follows;

    If SW 1 in in the up position, put batteries into parallel (output 12v) and open RL2.
    If SW1 in in the down position, put batteries into series (output 24v) and open RL2.
    If there is 240v on RL3, force the batteries into parallel and close RL2.

    RL2 is there as the switch will have 3 positions - 12 - off - 24. I just can't find a model.
    The charger will be connected to the same 240 input as the primary side of RL3.

    2 questions;
    1. Will this work, or have I done something stupid.
    2. Why is proteus showing +4v on the output when RL2 is in the off position, as in the attached file?

  2. wayneh


    Sep 9, 2010
    I haven't traced out the logic of your schematic, but the meter issue is likely due to neither lead being connected to anything when the output is "off".

    The only other suggestion I have is to consider Murphy's law. Figure out your worst case scenarios, especially if someone else tries to use your device. The worst case scenario eventually becomes reality. Consider fuses, circuit breakers and so on. Indicator lights. Think about whether the relays switch quickly enough to avoid any bad combinations.
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
    If the charger has two outputs that are isolated from each other then the is no point in making things any more difficult than necessary which again they are isolated just hook one output to each battery and be done with it.
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    Your schematic shows that the two charger outputs are not isolated and thus it is impossible to charge the batteries when they are in series. When the batteries are in series one output of your charter will see 24 volts (not good).

    Using two isolated chargers would greatly simplify and improve the design.
  5. markdem

    Thread Starter Member

    Jul 31, 2013
    Thanks for the input guys.

    I think I did not do a good job at explaining what I am doing here.

    I have 2 12v batteries in a box. Sometimes the box needs to output 12v (batteries in parallel) and sometimes needs to output 24v (batteries in series) depending on the position of SW1. While it is outputting the charger will not be plugged in. (physically impossible).

    While the charger is plugged in, Relay 3 will force the batteries into parallel for charging. At no point will the batteries be in series during charging, nor will there be a load on the batteries while the charger is plugged in. (As relay 2 will disconnect the output.)

    The simulation works, I just wanted to run it past another set of eyeballs to make sure I have not missed anything silly.

    So, is there anything I am not thinking of?

    Re the meter, I would think it should read 0v with nothing connected. Indeed, if I disconnect the relay, it does. Not too concerned with this as it looks like a issue with the sim, I was just wondering what the hell is going on.

  6. markdem

    Thread Starter Member

    Jul 31, 2013
    After having a fresh look at this, I found a issue myself. It looks like the two positives from the batteries where connecting in charging mode.
    I have attached all 3 modes this time.

    batt.jpg batt12.jpg battcharge.jpg

    I have made all the high current wires thick and the control wires thin to make it easier to see.

    I would be forever grateful if someone casts a second set of eyeballs over it.

    Last edited: Jul 5, 2016