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Author Topic: Brushless Motor running on SLA batteries  (Read 371 times)

chipchase

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Brushless Motor running on SLA batteries
« on: June 14, 2020, 09:45:37 AM »

Hi all when I started on my new build the Stevns Arctic I decided to buy brushless motors and ESCs, don't ask me why I came up with this idea, just thought that would be a good as I don't know the first thing about them  but I went ahead and bought them. Then after a bit more thought it suddenly hit me LOL what is the main source of ballast I use on my tugs yes you have it BATTERIES. I have even used full sized car batteries on a couple of the models that I have built over the years I find its better than carrying lead. so the question is can I use a 12v car batterie on the two brushless motors instead of the Lipo batteries I bought. Or should I just put it down to an expensive learning curve and go back to brushed motors.Any help would be appreciated.  I have included phots of the motors, ESC and Lipo batteries Brian

unbuiltnautilus

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Re: Brushless Motor running on SLA batteries
« Reply #1 on: June 14, 2020, 09:55:28 AM »

If the current drain is low enough, there is no reason not to use lead acid gel batteries. If the current drain is above the rated capacity of the cells, for example 7amps, for the most common type of 12v gel battery in use in models, the run time will start to drop off greatly as the cells will not be able to take the discharge rate for long.
The other thing to remember is to set the speed control to Nimh mode. This will prevent it tripping out early as it tries to protect the onboard Lipo battery it thinks is being used.
Tank test in order to check the current drain. Take it from there.
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JimG

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Re: Brushless Motor running on SLA batteries
« Reply #2 on: June 14, 2020, 11:57:00 AM »


The other thing to remember is to set the speed control to Nimh mode. This will prevent it tripping out early as it tries to protect the onboard Lipo battery it thinks is being used.

No need to set to NiMh mode as the cut off voltage for LiPo is less than that of a totally flat lead acid. The esc will think that a 12V gel cell is a fully charged 3S LiPo so will normally cut off at around say 10.5 volts. If your gel cell reads 10.5V then it is dead.
Jim
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chas

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Re: Brushless Motor running on SLA batteries
« Reply #3 on: June 14, 2020, 03:45:03 PM »

I can't see why the motors shouldn't be fine running from a car battery, but there might be another problem. Presumably it's a large heavy model, so will have a large 3 or 4 blade propeller. That would load up the motor a lot, and it will be wanting to turn at 7000 revs plus, hardly ideal for a tug. You don't mention using a gearbox or belt drive for reduction of the revs but It sounds like you'll need to..
  That's a nice motor, but never meant to turn a 70 to 90 mm prop directly which is the sort of size you might end up with an a model large enough to carry a car battery.
Chas


Edit.  A freshly charged car battery will be over 14 volts, won't that be too much for your 3s max speed contoler?

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Taranis

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Re: Brushless Motor running on SLA batteries
« Reply #4 on: June 14, 2020, 04:08:36 PM »

Valid point Chas 12.6 and 14.4

Tug Fanatic

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Re: Brushless Motor running on SLA batteries
« Reply #5 on: June 14, 2020, 04:12:37 PM »

I don't see any real problem with the car battery as long as each motor is only drawing at full load say 10-15amps - which is a lot for a model tug.

I note Chas comment re 7000rpm+ but as I don't know what motors you are using it is hard for me to comment except to say if 7000+rpm is the no load rpm on 12v then you have a problem. That is far too high for an effective large model tug on scale size props - you would need reduction gearing.
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Taranis

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Re: Brushless Motor running on SLA batteries
« Reply #6 on: June 14, 2020, 04:53:01 PM »

Yes but itís not a car battery being suggested but SLA

unbuiltnautilus

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Re: Brushless Motor running on SLA batteries
« Reply #7 on: June 14, 2020, 05:40:03 PM »

Using a 6v gel battery might be a better option, revs around the 3500 range sound more practical with big props, on those motors. Then you may have to tick the correct box regarding low voltage protection.
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malcolmfrary

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Re: Brushless Motor running on SLA batteries
« Reply #8 on: June 14, 2020, 06:19:21 PM »

Yes but itís not a car battery being suggested but SLA
The title is SLA, but in the first post car batteries get a mention.  A choice needs to be stated. 
A car battery will provide plenty of current and lots of ballast, but will likely vent acid fumes into the boat.  SLA is less likely to do that, but will probably not supply the required current for long.  Probably not too many times, either.
Thats quite aside from any consideration of the size of the model and the load that it will need to support the required performance.
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Colin Bishop

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Re: Brushless Motor running on SLA batteries
« Reply #9 on: June 14, 2020, 06:36:13 PM »

Just to confuse things a but more(!). Some car batteries are sealed lead acids but they are not Gel cells which can be used in any attitude.

I think the key point here, as has already been mentioned, is that the big props of a tug need to be fairly low revving for efficiency and minimum current consumption whereas brushless motors are high revving and if asked to drive a large prop at that speed will at the very least draw excessive current that a lead acid battery might find hard to deliver whilst the motor is likely to become extremely hot. (and the performance, shall we say - ' brief and interesting'. I would think that with that setup some form of gearing down, probably by using toothed belts would be essential in reducing both prop speed and the current drawn by the motor. Gearing down using brushed motors in these situations is usually in the region of 2:1 or 2.5:1 but with brushless motors you would probably looking at rather higher ratios, maybe up to 5:1 but you would need to experiment.

Of course you could have a smaller prop size but then it wouldn't look right.

Colin
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Re: Brushless Motor running on SLA batteries
« Reply #10 on: June 14, 2020, 07:06:24 PM »



....................I think the key point here, as has already been mentioned, is that the big props of a tug need to be fairly low revving for efficiency and minimum current consumption whereas brushless motors are high revving ....................and if asked to drive a large prop at that speed will at the very least draw excessive current that a lead acid battery might find hard to deliver whil


Colin

Now I am logged in I can see the picture of the motor and yes 560kv on 12v gives a speed of 6720rpm which means that on direct drive it needs a prop that will turn at around 5500rpm which as I said above is around twice what I would expect for a large tug.

Colin
The idea that all brushless motors are high revving has not been true for a number of years. The link below is a brushless motor with a 280kv which on 12v is 3360rpm with a loaded prop rpm of around 2700rpm. It would be perfectly happy on 12v. There are many others. I give this motor link as an example not a recommendation.
https://hobbyking.com/en_us/turnigy-aerodrive-sk3-5055-280kv-brushless-outrunner-motor.html?queryID=e7e62f6e20b896df93886eafa06d3771&objectID=17266&indexName=hbk_live_magento_en_us_products
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chipchase

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Re: Brushless Motor running on SLA batteries
« Reply #11 on: June 14, 2020, 07:17:56 PM »

Hi all thanks very much for your response to my question its very much appreciated and given me plenty to think about, it looks like I will be keeping the brushless motors and investing in big gel cell batteries. The props in my drives are 80mm but I have them geared, I have included a photo show drives.Thanks again for your help. :-)) 

Colin Bishop

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Re: Brushless Motor running on SLA batteries
« Reply #12 on: June 14, 2020, 07:22:19 PM »

I must admit I had assumed that brushless motors tend to run at higher revs than brushed ones, obviously not from what you say but you need to know what you are buying for a particular application. The small ones in my Fairey kits are staggeringly quick and powerful compared to their brushed equivalents sizewise.

For scale models I have always been in favour of gearing down, usually by pulley drive as I have posted previously. Generally motors will always run more efficiently at higher revs whilst high putch props are best at lower speeds so bridging the gap is a win win situation. My 48 inch Fishery Cruiser draws only 2.5 amps total at cruising speed with two low drain brushed motors and 2:1 reduction drive.

Colin
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Colin Bishop

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Re: Brushless Motor running on SLA batteries
« Reply #13 on: June 14, 2020, 07:31:53 PM »

That is pretty impressive engineering Chipchase. I think most of us assumed from your initial  post that you had a model with conventionnal drive but you have obviously incorporated the reduction gearing so that is a different ball game really. If you have already installed the drive units then you can measure current consumption in a test tank which will give you an idea of the best battery setup to meet the maximum current draw.

Colin
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chipchase

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Re: Brushless Motor running on SLA batteries
« Reply #14 on: June 14, 2020, 09:54:11 PM »

That is pretty impressive engineering Chipchase. I think most of us assumed from your initial  post that you had a model with conventionnal drive but you have obviously incorporated the reduction gearing so that is a different ball game really. If you have already installed the drive units then you can measure current consumption in a test tank which will give you an idea of the best battery setup to meet the maximum current draw.

Colin


Thanks Colin, I have not fitted them into the new hull yet (to many builds on the go  LOL )
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