Auto battery switching - feasibility

Discussion in 'The Projects Forum' started by vsaero, Jun 20, 2011.

  1. vsaero

    Thread Starter New Member

    Jun 17, 2011
    1
    0
    Hi,

    I found some posts related to this but not exactly what I am looking for.

    I have a brushless dc motor which draws close to 150 Watts in continuous operation. There is however, need for up to 700 watts at certain times. I want to use two batteries and switch between them. One primary battery that is low capacity high ampere hours for normal continuous operation and the other with high capacity less ampere hours for high power requirement.

    I am operating at 18 V, so am drawing at peak power close to 39 Amps and normal operation close to 8 Amps.

    I don't want to power off my motor and then restart while switching.

    My first question: is this feasible?
    If yes, your suggestions? Or shall I try something before giving up the idea?

    Regards,
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,763
    You are asking for two opposing things from the same battery. High surge capability, low capacity means lots of plate area but not much plate area. Low surge capability with high capacity means having a small plate area with a large plate area.

    Quit before you get a headache.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    #12, you missed that he has 2 batteries.

    Just thinking out loud here: Detect current flow, using a low ohms shunt (0.1Ω or less) or a hall effect sensor, or just a drop in voltage of the battery, and then switch in the high current battery using a relay triggered by that measurement. So when high current is needed, both batteries are in parallel. The hi-current battery could kick out when current drops. (You could disconnect the low current battery when hi current is needed, but why?) I think you'd need some sort of method to prevent oscillation or relay chatter.
     
  4. #12

    Expert

    Nov 30, 2010
    16,257
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    There are current operated relays that will keep a "start" supply connected until the motor gets some speed and stops requiring so much current. Ancient technology, really. Old refrigerator compressor starters would be similar, but not for 39 amps. Certain motors I've seen in air conditioning use a current relay to hold the start capacitor in circuit. That's more like the size needed. With enough research one could be matched to the DC requirements here, but who would allow somebody to come to their store and run DC through relays until they find the right one?

    Better to stick with high tech electronics.

    By the way, what are these high surge-low amp hour batteries called? They would be perfect for my new electric start lawn mower and I'm sure the battery provided by the MFG will not last long.
     
  5. #12

    Expert

    Nov 30, 2010
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    That's what I thought.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    I don't know what the OP is up to but I assumed it was something like what boaters do; use a "starting" battery for high-current starting needs, but switch to a "deep cycle" battery when the engine is off, for powering lighting and radio. This keeps the starting battery fresh and allows a smaller, deep-cycle battery than would be needed if you used a single battery for both tasks. As explained, the OP's needs didn't make much sense to me except it appeared he wanted to automatically switch between batteries.
     
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