LVC for Drill lipoized

Discussion in 'The Projects Forum' started by joecool73, Sep 21, 2012.

  1. joecool73

    Thread Starter New Member

    Sep 21, 2012
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    Im sure you guys get these types of questions all the time. I have a 18V B&D drill that I converted to lipo. Being into RC I have plenty of lipos lying around a a great 540W 30A PSU with a 400W 20A balance charger. I converted the drill by removing the NiCd and replacing it with a 5S 1.8A 40C lipo. soldered on the leads and traxxas connectors to the lead and to the lipo. Works great no problems there. I also have LVC alarm that also tells me the volts of each cell as well as the overall voltage of the pack. While this works and there is nothing worng with it, I have gotten bit by the electronic bug. PCB's , ICs, resistors, etc, and some guy named OHM. I cant seem to read enough on the subject. I want to build a low voltage cutoff for my drill. Ive seen several schematics for this online but not sure if is what I need. I have learned quite a bit off this website and thought Id post here. Here are a few pics of the finished drill. I just want something more permenant than having to wait for a alarm or keep checking my voltage. I am very familiar with built in lvc's on esc's for my erevo, msr, etc. But dont have a real good understanding with making one without a esc. The simplier the better. The drill does not have the amps or watts on it. All I know is that the RPM max at 1300 and its powered by the above lipo at 18.5 V. How would I go about building a lvc based on that criteria? Thanks for any help.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Nothing I can see.

    But anyway, it would be simpler to merely indicate low battery than to actually control the drill motor. Is that an option?

    There may be more elegant ways to do it, but I'd 1. establish a reference voltage using a voltage regulator or a zener diode, 2. divide down the battery voltage with resistors, to a level near the reference 3. use a comparator to change state when the voltage drops below the reference.

    When the comparator changes state, it'd be easy to light an LED. To switch a drill motor, you'll need to add a MOSFET with a high enough current rating to (more than) handle the stall current of the motor.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    It's more complicated: the battery voltage will vary significantly based on current load.... which depends on drill motor load and RPM. The higher the load current, the more the battery voltage dips. Sensing voltage alone can't tell state of charge.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  5. joecool73

    Thread Starter New Member

    Sep 21, 2012
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    Sorry forgot to add the pics. [​IMG][​IMG]
     
  6. joecool73

    Thread Starter New Member

    Sep 21, 2012
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    Yes that above link is very good, thanks for all the input. That is true that the load on lipos do vary significantly. Even with my voltage monitor attached to the JST balance port on the lipo you can see how the battery cells get out of balance very fast under a heavy load. Im assuming this drill pulls about 50 - 100 A under max load. This is a guess based on others that have a 12V -24V drill that Ive read. For instance, my Erevo Brush-less Edition has a 2200KV motor and ESC that puts out 120A under full load. This is powered by 2 3S 5A 40C wired in series. So that is 25.2V. Ive gotten this curb rocket clocked up to 53 mph. I actually raced it against my full scale car to get my max speed. My wife drove as I controlled the RC erevo from the passenger seat. So Im figuring on trying to build a PCB that can accommodate that kind of load for my drill to be on the safe side.
     
  7. wayneh

    Expert

    Sep 9, 2010
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  8. joecool73

    Thread Starter New Member

    Sep 21, 2012
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    I didnt build that lvc that you guys see in the picture. I bought that off ebay for 7 bucks I think. I just wanted a more permanent solution. I haven't build my own circuit yet, any circuit, zip, zero,Nada. Im going to pick up a bread board at radio shack today to help my learning curve into making circuits. For an example, my EREVO ESC can plug directly into my computer via usb. From there Im able to adjust the voltage cut off to whatever I wish, right now it sits at 3.4 per cell, I can also adjust the motor timing, torque, you name it. Thats kinda what I was after. However just having the circuit cut off power to eliminate over discharge would be fine by me. Im starting to think this is more complicated and difficult than I once thought.
     
  9. joecool73

    Thread Starter New Member

    Sep 21, 2012
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    Any suggestions for a good breadboard kit? Online or in-store. Like to hook it to my 21 V lipo
     
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