Car Battery + Second Battery + Battery Charger

Discussion in 'Automotive Electronics' started by jjmalove, Sep 20, 2016.

  1. jjmalove

    Thread Starter Member

    Jul 6, 2016
    39
    1
    Hello,

    This is a small portion of a larger system I'm trying to tweak. I have attached a rough picture of this part of the circuit.

    Basically, we have a system that ties into a car battery when we want, but also has its own battery. The system is wiring to our battery by default. We can parallel our battery with the cars battery/alternator at will from other methods at our own discretion. We do this to charge off the alternator but only when the car is running. We never tie our system in during the crank period.

    Additionally we can use an AC Input to charge our battery with an off-the-shelf battery charger.

    The scenario I am trying to solve is what happens when the car is on, our batteries are tied together, and it is also being charged by an AC Input. While electrically there is no reason to do this, I am trying to safeguard against the users being stupid.

    Would the simple solution of adding a diode (large one that can handle the current running through it!) as shown in the picture work? Sometimes easy solutions make me think I'm missing something?

    Edit: I forgot something else important. We want protection in the event that the car battery gets damaged, so that it doesn't take our system with it. So lets say everything is tied together willy nilly then all of a sudden the car's battery shorts to itself or goes crazy, we want to isolate that from our battery trying to compensate and pulling everything down/high amps. Like we don't want our battery to try and power the car's functionality if the car's battery breaks.

    Thanks
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,041
    1,670
    Having the AC based charger on while the engine is running will do nothing. It's not an issue.

    If the vehicles main battery suddenly shorts to itself and it not through a fused circuit then you will likely have a fire and burn everything down. Not a realistic issue either.
     
  3. bwilliams60

    Active Member

    Nov 18, 2012
    725
    90
    When you say an AC input, are you referring to a battery charger that one would plug into a wall and connect to the battery? If so, then thr output would be DC. Can you clarify for us?
     
  4. jjmalove

    Thread Starter Member

    Jul 6, 2016
    39
    1
    Hey thanks. You have helped me in the past with other questions appreciate it. Let me elaborate a bit.

    The nature of the project is somewhat off the table, but a very realistic scenario the project is used in is after a vehicle collision. If a car crash happened we very much would want our system to keep working, even if the car was on fire and people running away from it. That is actually an important and good thing for this project. Obviously a crash could break our system, not the main battery, which we have no protection for which is fine and an acceptable risk.

    With that bit of information, since you mentioned there isn't much issue of having the charger on and the alternator on, the only other issues is keeping our battery/system safe if the main battery goes bad and not letting the car's battery drain our battery, only vice versa. Meaning we are ok with killing the car battery to keep our system running. We are not ok with the car battery killing our system to keep the car running.

    For the second part am I correct that a big diode as shown would solve that? We can pull current from the car battery but the car battery cannot pull current from us? Is there anything I could do with diode(s) to help with the first part?

    Thanks again for the info on the charger. I kind of thought it shouldn't matter but good to hear it from someone else.
     
  5. jjmalove

    Thread Starter Member

    Jul 6, 2016
    39
    1
    Yes so the system is normally charged by the alternator. Alternatively we have a COTS battery charger than plugs into the wall on one side and spits out a 12V DC charge setup for our battery. Its this guy:

    https://www.amazon.com/ERAYAK-Maint...1474409880&sr=8-2&keywords=battery+charger+6a

    Our battery is a 12V AGM battery.
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,041
    1,670
    Yes the diode would work just fine for your application. Preferably you would want one with a low forward drop like a large power Germanium type that only had a .3 - .4 volt forward drop at its maximum amperage you would run through it that way when the engine is running and the vehicle's DC voltage is 14 - 14.5 volts your battery is only a fraction of a volt behind it and still well within its normal charging range.
     
  7. jjmalove

    Thread Starter Member

    Jul 6, 2016
    39
    1
    Awesome, thank you very much!
     
  8. MrSoftware

    Active Member

    Oct 29, 2013
    504
    124
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