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Author Topic: 775 motor amp draw  (Read 370 times)

ChrisH1981

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775 motor amp draw
« on: January 11, 2022, 02:15:37 pm »

Good afternoon all, I know this is a a difficult question to ask but Iím hoping some knowledgeable people will be able to advise.


I have recently bought a second hand model with a twin motor setup. The motors in question are believed to be 775 type motors (not confirmed because they have no markings on) but going by can length and diameter they fit the bill.


Second hand nimh batteries (6v 4300mah) were given me with the boat, wired in series to give 12v 4300mah.


On testing the boat first time performance was scale, but very quickly performance dropped off untill eventuallly after 10 mins or so the boat struggled to make it back to shore.


On charging the batteries at home using a nimh charger set at 300 mah, one battery took an overnight charge, the other refused to charge on the nimh charger. After a few attempts of trying the nimh charger i switched to my lipo charger ( on nimh setting - set to 1amp charge) and successfully charged the other battery, or so I thought.


Second sailing of the model was exceptionally poor. Barely a minute or two before the model showed signs of slowing and eventually cutting out in the middle of the lake!


So to actually get to the point of my post, can anyone please give me some idea of how many amps these motors could be pulling in the water driving 35mm props?


Could one or both of the nimh packs be faulty?


What kind of performance should I expect?


Does anyone have experience using these motors and if so what setups do you have?


Regards Chris

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Tug Fanatic

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Re: 775 motor amp draw
« Reply #1 on: January 11, 2022, 02:28:12 pm »

There are many different types of 775 motor and the results for one would be very different to others. The only way to really know is to connect an ammeter + voltmeter and get factual information.

If the motors are overloaded they will get hot!


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ChrisH1981

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Re: 775 motor amp draw
« Reply #2 on: January 11, 2022, 02:40:06 pm »

Thanks Tug Fanatic! Motors are cool to touch, if anything itís the batteries that warm up when running! I know what your saying regarding the different versions of 775. Going by other models I have these motors put out about 5000 rpm at 12v  :-))
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Netleyned

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Re: 775 motor amp draw
« Reply #3 on: January 11, 2022, 03:17:20 pm »

Sounds like you may have a few dud cells.
You basically have 10 x1.2v cells in series.
2x5 x1.2v. 
Smart chargers will sometimes push the other cells
higher and think the voltage is right
for the charge.
As soon as they come off charge, the
voltage drops and as soon as a load is applied
plummets even further.
After charging, note the voltage
and put the battery back on charge
after an hour and note the voltage at the
start of charge. If its close to the original
voltage all is ok, but if the voltage has dropped
noticeably, there are dead cells.
Ned.
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ChrisH1981

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Re: 775 motor amp draw
« Reply #4 on: January 11, 2022, 03:37:33 pm »

Thanks Ned, I will give this a try  :-)
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Tug Fanatic

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Re: 775 motor amp draw
« Reply #5 on: January 11, 2022, 04:01:32 pm »


....................Going by other models I have these motors put out about 5000 rpm at 12v  :-))

I have seen examples with up to 15000rpm free running on 12v. I am afraid that there is no substitute for measuring.

If the motors stay cool and the batteries get warm first guess would, I agree, be a battery problem.

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ChrisH1981

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Re: 775 motor amp draw
« Reply #6 on: January 11, 2022, 04:57:29 pm »

That is very true Tug Fanatic :)
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Fastelectrics

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Re: 775 motor amp draw
« Reply #7 on: January 12, 2022, 10:57:03 pm »

I would check the voltage of each cell after discharge. It might be worth making a mix & match pack of the best individual cells to make up a single decent pack.

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RST

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Re: 775 motor amp draw
« Reply #8 on: January 13, 2022, 05:13:46 am »

It does sound like a battery is at fault but when you say "scale performance" nobody knows what that means?  What model do you have, how big? 35mm 2 blade propellers, 3 blades or more? Pics help also please.
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ChrisH1981

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Re: 775 motor amp draw
« Reply #9 on: January 18, 2022, 11:19:56 am »

Many thanks for all comments! A bad cell was indeed the fault!
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