25.2v battery running a 12v motor w/ pwm controller

Boats, Jet Skis, Kayaks etc., including hovercrafts
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forke   1 µW

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25.2v battery running a 12v motor w/ pwm controller

Post by forke » Apr 05 2019 11:05am

I am currently planning to put a trolling motor on my canoe, with the main goal of having maximum efficiency at a given, reasonable speed. Probably around 3.5mph.

I will be using 6S LiPo batteries (that I use for ebikes) that will range roughly from 20v to 25.2v. I have a DC to DC converter that will step down to 12v, but I would prefer not to use it, as it decreases to efficiency.

As I understand, the PWM controller will limit the voltage applied from 0v up to the input voltage, as you turn the pot. I dont know if the throttle/pot is linear, but assuming it is, approx 1/2 throttle would be near the 12v mark.

My current question is whether or not I can safely exceed this voltage, as long as max power is observed? I will be using a Minn Kota C2 30, which most tests I have observed draw around 28A at full throttle (@12v). So about 350w. Could I safely run it at 25v x 14A ?

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amberwolf   100 GW

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by amberwolf » Apr 05 2019 11:37pm

If it's a brushed motor, then the brushes will heat up more at higher voltages, from greater arcing, and wear faster.

So even with the PWM trimming the *average* voltage to half, the actual applied voltage is still higher, so the motor may run hotter than it should, even at the same current limit and total average power.

But otherwise, as long as you're running it at the same intended power, it should run approximately like it would at the original voltage.

forke   1 µW

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by forke » Apr 08 2019 4:06pm

tmp_22139-20190408_140731_resized_16789313562896752445.jpg
I added the dc to dc step down converter for peace of mind. I'd prefer to be able to "crank" the throttle when needed and not worry about frying the motor. Looking forward to water testing, but that will be about a month away at my latitude.

I added a lot of xt60 connectors to make each part of the system easily removable.

I plan to add a prop spinner, at the very least. Most likely an RC prop as well, somewhere from 10x3 to 10x6 pitch. I always like keep-it-simple-stupid, so if I get reasonable efficiency with the PWM controller and a spinner to reduce turbulence drag I would rather keep the stock prop on.

That being said, I hear a faint rubbing of the stock prop on the motor housing that is bothering me and adding wasted friction, so I'll likely take it off.

Anyone have experience "tuning" these motors by rotating them, or are they pretty good from the factory?

johnnyfoos   10 mW

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by johnnyfoos » Apr 11 2019 5:59am

I've done what you are doing but not with a stepped down dc to dc.
just PWM 24v to the 12v trolling motor, didn't seem to get hot being in water and didn't wear out.
on a smaller craft like you are using should be great.
Also the Model airplane props are a good thing, but a bit larger than you listed like 11-6, 11-7 or 12-6.
just look for ones that can be fitted to your shaft size without boring out to much and leaving little to hold.
Make sure you carry a few extras bored out ready to mount with extra mounting hardware!
Floating tools are great in boats, and a fishing magnet can be helpful.
My latest is a little 4 stroke weed eater using these props and WOW!
a couple of these and a flying car is looking easy.
lol

PS: an old jumper cable clamp on a rope can be VERY helpful boating a LOT of times.

just
say'n

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by johnnyfoos » Apr 11 2019 6:12am

Also-

" the PWM controller will limit the voltage applied from 0v up to the input voltage,"

No, not how they work-
it will be giving it the full input voltage, just
it will be turning it on and off a lot
like lets say it is @ 5,000 cycles per sec.
so 50% is only on 2,500 of those cycles
And this is so much better than the stock system that
at 50% it is using resistors to lower your 12V to 6V and wasting the other 6V as heat
Anyway your run times will be MUCH better.
My 17.5' Glastron SKI n FISH was so much better using PWM over the junk the stock trolling motors had.

Just
sayn

forke   1 µW

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by forke » Apr 11 2019 10:00am

I did some testing with adjusting the timing of the motor. Now, these tests were NOT in the water, so I do not know if the results will transfer while under load. If someone has an easily reliable way of measuring rpms while in the water I may attempt that at some point. I was using an android app that uses audio to determine rpm, so it might be possible to make the phone contact the shaft and hear the vibrations while submerged.

tmp_13751-unloaded_timing results7443927236114212164.png
tmp_13751-unloaded_timing results7443927236114212164.png (9.27 KiB) Viewed 112 times
tmp_13751-20190410_220609_resized1577497146758218200.jpg
Retarding the timing slightly, reduced the current draw by a material amount, while RPMs didn't materially change. Advancing the timing was not advantageous from an efficiency standpoint.

This may be useless data, as the results may vary under load.

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by dustNbone » Apr 11 2019 10:37am

Water is very good at transmitting sound, so your audio based tachometer should work fine just by putting the microphone against the outside of the container you're using.

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fechter   100 GW

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by fechter » Apr 11 2019 1:05pm

Normally you would get minimum current at neutral timing. Advancing or retarding from neutral will increase the current. Ideally you'd want to test it with the actual running conditions to get the highest efficiency.

A good PWM controller should work directly off the battery with no problem. I wouldn't use a converter, one more thing to waste power or fail. You can place a fixed resistor in series with the throttle pot to limit the maximum. I'm sure the motor can handle a little more than 12v too.
"One test is worth a thousand opinions"

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by amberwolf » Apr 11 2019 4:01pm

Most likely the timing was already advanced, in the stock configuration. So retarding it may have actually made it neutral (or closer to).

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Re: 25.2v battery running a 12v motor w/ pwm controller

Post by forke » Apr 14 2019 9:18am

An update with a little more data...

I was not able to determine RPM in the water. The audio-based rpm app could not get a consistent reading. But here are the other readings I acquired. After I dumped all the water out I wished that I had adjusted the timing and tested for amp draw at various voltages, but I was in a rush. Another time.

I maxed out at 8v, as I was making a MESS of the basement at that point.

If, as these results suggest, I can achieve 3.5mph using only 115W, with a more efficient RC prop, I will be very happy. Of course this is without pushing a boat, but I will be using a lightweight canoe that has a fairly low hull resistance. So much speculation, until the ice melts off the lakes :)
power testing under load.jpg
power testing under load.jpg (81.62 KiB) Viewed 77 times
loaded_power results.png
loaded_power results.png (22.36 KiB) Viewed 77 times

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