Help - Engineers (real ones)/ Physics people

Peter Diffey

Peter Diffey

Peter Diffey

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28 Feb 2007
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Hey, this may be a very simple question, as it should be, but here goes.

So i have a hedge cutter with a motor of stated power consumption of 350W, I presume that in normal use the motor would not pull the full 350W constantly?

Therefore......how would you calculate the required battery capacity for a 24v battery?

P=VI
V=24v
I=?
P= < 350W

So if the motor was pulling 350W, the current drawn would be:

I=P/V
I = 14.6A

How can that be right? surely the battery wouldnt deliver that many amps?
also for the battery to last one hour, it would have to be 14.6Ah, which would be massive..........
So basically, is my physics correct and if so what would be a reasonable average amount of power drawn by a small hedge trimmer?
Thanks
DIFFEY
 
Calculations look right to me although as you said the 350W will most likely be the peak power, a lot of the time the motor will be idling or cutting lighter branches which will use considerably less power.
 
14.6A is very high indeed. The battery for my cordless drill is at 12V with 2.0Ah. I would imagine 350W is its maximum operating power. It probably doesn't draw that much current under "normal" use.

If you post up the model number, there maybe a specification about with this information in it. Sorry for not being a real engineer or physicist , just thought I'd help get the ball rolling ;) Your calculations are correct by the way.

If you're feeling brave, you could just get a multimeter on it :)
 
Thanks for confirming.
Its actually for some final year degree work :p
I just wanted to get a figure of power consumption of a hedge trimmer, because I have seen these with batteries as small as 1.5Ah, so the power draw must be very small in order to have a decent battery life.
I guess I will continue googling and work backwards in order to find average power consumptions of products already on the market
 
but battery powered hedge trimmers aren't designed to be used continuously for 1 hour.

your 1 hour of use might only have about 20 minutes of actual runtime, just a factor :)

otherwise, your calculations are correct.

<edit>
Actually, you've missed a factor: batteries being drawn at max output suffer lower efficiency.

say, a cell can discharge at a max of 5A, and has a capacity of 5Ah.
at 1A it will last for 5 hours, but at 5A it will only last, say, 40 minutes.
 
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You can get cells around the 2Ah area that can do continous discharge of 20C, so that means they can discharge at 40amp. Though won't last long at all. Really you need to decide what length of time you want them to last for, then you can decide what cells you need. Lipo's are good capacity for the size of them, plus each cell is 3.7v.
 
saitrix said:
You can get cells around the 2Ah area that can do continous discharge of 20C, so that means they can discharge at 40amp. Though won't last long at all. Really you need to decide what length of time you want them to last for, then you can decide what cells you need. Lipo's are good capacity for the size of them, plus each cell is 3.7v.
Click to expand...

but you can't hard-discharge a Li-Po or it'll explode/swell/die.
 
saitrix said:
You can get cells around the 2Ah area that can do continous discharge of 20C, so that means they can discharge at 40amp. Though won't last long at all. Really you need to decide what length of time you want them to last for, then you can decide what cells you need. Lipo's are good capacity for the size of them, plus each cell is 3.7v.
Click to expand...

Yeh I have been considering Lithium Polymer for the design, I have noticed that all the current products on the market tend to use big old heavy NICD.
Owning one of these battery hedge trimmers i have noticed that they last at least half and hour continuous use, with a 2Ah battery, this means that it is drawing 4 amps average current. 4 x 24v (IIRC) = < 100W
So would you think its reasonable to assume a power draw of 100W?
 
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Peter Diffey said:
Yeh I have been considering Lithium Polymer for the design, I have noticed that all the current products on the market tend to use big old heavy NICD.
Owning one of these battery hedge trimmers i have noticed that they last at least half and hour continuous use, with a 2Ah battery, this means that it is drawing 4 amps average current. 4 x 24v (IIRC) = < 100W
So would you think its reasonable to assume a power draw of 100W?
Click to expand...

Yeah I would say so, it will only draw 350W at around 0rpm, as this is where the maximum amps are drawn. So you just need a battery that can cope with ~40A burst and at least 15A continous.
 
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saitrix said:
Yes you can, just look into the whole remote control planes area, 20C safe discharge is just about the normal now.

http://www.allerc.com/product_info.php?cPath=3_4_70&products_id=3343

For example there, 25C max continuous, 50C burst. Best I know about is 30C continuous right now.
Click to expand...

Wow, prices have come down a lot since I was looking at LIPO setup for my mini-T brushless RC car back in the day :p

That is an amazing bit of kit right there
Thanks for your help too
 
One big plus is they are built so you can directly charge each cell individually. You want to do this when designing a battery setup cause if you are using lipo's you will need about 6 of them! So they all need to be charged separetly to make sure the cells remain balanced, as that is what causes them to fail.

These are very good for the money.

http://www.maxamps.com/Lipo-2100-222-Pack.htm
 
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