Will 18 volts damage a 12 volt trolling motor?

Discussion in 'Electric Bicycles' started by bigriver, Nov 30, 2010.

  1. bigriver

    bigriver New Member

    Probably a no brainer question for somebody. Looking at using
    dewalt 18 volt lithium ion batteries to power a 12 volt trolling motor
    on a kayak. Will this damage the motor? If so what is the best method
    to step the voltage down 12 volt?


  2. loquin

    loquin Active Member

    It may well damage the motor - or more precisely, the motor controller... (the motor itself would probably handle the overvoltage just fine)

    The most efficient method would be to add a switching regulator between the batter and the motor. A switching regulator works by switching the 18 volts on and off rapidly, adjusting the on time to off time ratio so that the average output voltage would be 12 volts. This switched voltage is fed through an internal filter on the regulator output circuitry, which 'averages' the output voltage to the load.

    You could use a big resistor, but, the voltage drop across the resistor is wasted power. And, as the battery voltage drops, the motor voltage drops proportionally.

    A linear regulator doesn't have the output voltage drop issue, but, it wastes just as much power as the resistor. (at least 1/3 of the total available power is wasted in the resistor/linear regulator approach)

    However, the Dewalt tool batteries may not have the capacity to keep your trolling motor going for very long...
    Last edited: Nov 30, 2010
  3. bigriver

    bigriver New Member

    Any quick links for someone that sales switching regulator or does this have to be built?
  4. loquin

    loquin Active Member

    search for High Efficiency DC-DC converter

    or, use 12V batteries (which would certainly be simpler...)
  5. bigriver

    bigriver New Member

    thanks. Yea considered the 12 volts just already have 18 volt tools and was thinking "double duty". May just run at 18 volts and see what happens, i.e. damage, performance, etc. These may not perform well enough to justify anyway. From what I can find the DC/DC convertors are pricey not to mention I have not found one that appears to be high amp or wattage.
  6. loquin

    loquin Active Member

    I did a quick search also, and didn't see anything. I did run through TI's switching design software online and worked out an 18v to 12V switching regulator design, good for up to 21 amps... (which is the max rating of the output filter inductor)

    I've attached the design, in case you're interested.

    Attached Files:

    Last edited: Dec 1, 2010
  7. Ypedal

    Ypedal Member

    do you have a variable speed throttle for the motor ? or is it just an on/off switch ?

    If there is NO controller, ie: if the battery goes direct to the motor, inline with a switch, more voltage will simply make the motor spin faster.

    however, as said above, if there IS a controller involved ( black box that allows you to ajust the motor speed with a throttle ) then you need to investigate the controller's limits.

    got pictures ? a model number ? etc.. ?
  8. bigriver

    bigriver New Member


    5 speed/off/ 3 reverse

    I think the battery goes direct to the motor. The control wiring and main line voltage from battery all go down the shaft to the motor. Im thinking the controller does not carry any load because it is very light weight.

    Last edited: Dec 9, 2010
  9. Ypedal

    Ypedal Member


    I would not use your dewalt batt for this application.

    1 - the run time will be very short

    2 - the multi speed switch / speed coil as they call it will not like the higher voltage.. ( it will likely work, but would more than likely die an early death.. compared to staying at 12v.. )

    How big of a lead acid battery are you using now ?

    ex: your 18v pack, if it had 2ah ( 2000 mah ) of capacity, contains 36wh if the motor draws 20 amps on high , it will only last ( 18v x 20 amps = 360w ) .. 360w / 36wh = 6 minutes.

    And also.. 20 amps is too much draw for that battery... not good.
    Last edited: Dec 9, 2010
  10. bigriver

    bigriver New Member

    75 AH OPTIMA 70 LBS "Run this one upstream until dead"

    55 AH OPTIMA 40 LBS "Run this one downstream"
  11. Ypedal

    Ypedal Member

    There you go....

    130ah of lead acid provides something like 70ah ( peukert effect ) x 12v = 840 wh

    vs the 36wh of thedewalt pack ( assuming 2ah, i don't know what pack you actually have )
  12. bigriver

    bigriver New Member

    The new 18 volt ion phosphate batteries @ 2.4 amp hours & weigh 1.5 lbs. Right now I have the 18 volt XRP which I'm going to start replacing with the 18 volt ion.
    Last edited: Dec 10, 2010
  13. loquin

    loquin Active Member

    So, it's 43 watt hours.

    A 46 pound thrust minnekota motor will pull between 30 and 40 amps when full-out, so, even IF you could efficiently drop the 18V down to 12, you would have power to run the motor for about 5 minutes, maybe less. And that current draw would likely melt the innards of the battery.

    I would strongly recommend that you get a 12V deep-discharge battery for that motor.

    But, here's the other question... why are you using a 46 pound thrust trolling motor on a kayak? A 25 pound motor would work very well, as a kayak (or a canoe) has a very low drag hull and there's little wind resistance. The larger thrust motors aren't going to take any faster than a lower output motor; the motor RPM is essentially the same, the main difference in output thrust is the prop size. And, the larger motor is less efficient than a smaller one when used on a smaller boat. The larger motor would accelerate the kayak faster than a smaller motor would, but, that's about it. Under the same conditions of speed, battery, wind, and boat, a smaller motor will run longer than a larger motor.

    The controllers do pass the current; on the minnekota, they pulse the full 12V power to the motor rapidly. They vary the width of the pulse to vary the power. Since the power is either on or off, the losses are low, and the controller is lightweight.

    On a standard controller though, the power isn't pulsed, and in these cases, there's little difference in power consumption between full output and partial output - the excess power is just dumped as heat.
    Last edited: Dec 15, 2010