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High current battery switching without putting the load through the sw

  1. Jun 25, 2013 #1
    Hey guys, I am new to the forum and came to here to ask for help on a project that I am trying to construct.

    I have 3 lithium polymer batteries, one 7.4v, 11.1v, and 14.8v. The current draw will be around 150a for an extremely short period of time (a couple milliseconds) and then will settle to around 30a continuous.

    I want to use a 1 pole, 4 position (1 for off) rotary switch, but none of them can sustain very high currents. So that is where I need your help.

    I made the following circuit for it: (sorry for low quality)


    but I quickly realized that when the switch is turned to any of the positions, then the higher voltage battery would just dump current into the other two, so that would be very, very bad.

    I was advised to use SSR's, but then, after looking up the prices on them, decided that it wouldn't really be feasible due to my low budget ($75) Maybe there is a way to imitate the function of an SSR with just MOSFETs? I don't need it to function with AC nor do I need complete isolation so that is why I am trying to favor MOSFETs.

    Please correct anything that is wrong in this post and please help me with this project.

    All comments are welcome and I thank you guys in advance!
    Last edited: Jun 25, 2013
  2. jcsd
  3. Jun 26, 2013 #2
    While it seems you want to use the 4P Switch to turn on the whole circuit, but why you need to switch the controll from battery to battery power but not the power legs? - I have no idea what or why you are tying to connect these three batteries - they really are not compatible.

    As you describe the Higher voltage battery will dominate, but I would not say "dump current" - it will try to charge the lower voltage batteries and you will have multiple serious failures.

    So what are you trying to accomplish?
  4. Jun 26, 2013 #3
    Thank you for your response. I just read my OP again and did realize that I didn't state the purpose.

    I want to be able to switch between these power sources by turning the 4P switch. They will all be feeding into the same output.

  5. Jun 26, 2013 #4
    If you use IGBT in each leg - they do not have body diode property, if selected without the Anti-Parallel integrated diode - so they will block each battery from feeding each other. A conservative estimate of 1.4V Vf x 30A ... 50 W of losses - may be more than you are looking for.

    So I do now know what but I do not know why you want to switch the different batteries - to get different voltages, or some other purpose, to run them all down as far as possible?
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