need diode for battery charger

Discussion in 'The Projects Forum' started by mjdodson, Apr 6, 2014.

  1. mjdodson

    Thread Starter New Member

    Apr 6, 2014
    I currently have a battery charger for a 12 volt battery set at 10 amps and it is connected to a deep cycle battery. The battery is then connected to a 400 watt inverter to run my laptop. the problem is I can not disconnect the charger from the battery very easily so I would like to put in a diode in series with the charger to prevent discharge of the battery through the charger when the charger is off. I am not sure which diode I might need all I know is that the charger has a output of 12 to 15 volts DC at 10 amps max to charge the battery. I know the battery is discharging through the charger because after leaving the charger off for some time usually 12 to 15 hours the charger will charger the battery for at least 1 hour when it was fully charged when the charger was turned off.
    can you please suggest a diode for this application or at least tell me what I need to look for in the store when i go to purchase one.
    thank you
  2. wayneh


    Sep 9, 2010
    I think you need to look a little harder to verify the charger is really causing the battery to discharge. I'm a little surprised it would do that. It may have a "battery detect" circuit that is causing the drain. Anyway I think it would be worth your time to verify. You could use a low-ohms resistor in series and measure the voltage across it. The polarity of that voltage will tell you the direction of the current flow.

    You could find an automotive diode rated for 15A or more that will work. The downside is that it will drop ~0.7V across itself and this means the charger will keep your battery tended to 0.7V lower than it should. So it would be nice to find a more elegant solution. An auto-disconnect circuit might be required. Something like a small relay that requires power from the charger to stay on. Once the power goes off, the charger is no longer connected to the battery. Again, an automotive relay would be a good choice.
  3. #12


    Nov 30, 2010
    Here's another way:
    It's dead simple and eliminates any doubt about how much voltage the diode will use up.
    Last edited: Apr 6, 2014
    inwo likes this.
  4. wayneh


    Sep 9, 2010
    That's exactly the kind of thing I was alluding to. Thanks for drawing it up.
    Note for the OP: That requires an AC relay - not a repurposed DC automotive relay.
  5. inwo

    Well-Known Member

    Nov 7, 2013


    A diode would not work with smart chargers. ie. Those that check connections and conditions before charging. IMO