DC Motor / Battery calculations and how many working hours.. How?

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Okay, unfortunately, im not that good at math because simply i haven't seen and got enough of it and im not an electrical engineer or mechanical engineer. 

 

I have 2 questions.

 

If i have for example a x watt DC motor.

The first question is: How many mAh batteries would i need? 
The second question: If now i know how much batteries i need or already have. How to calculate running time and distance from a vehicle (knowing the weight of the thing plus driver)

How is such thing measured? I kinda feel ashamed even asking this kind of question..

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If i have for example a x watt DC motor.

The first question is: How many mAh batteries would i need? 

Say you have a 50W motor that is actually running at 10V then by P=IV you know the current must be 5A. So if you then also have a 2500mAh battery than that is a battery that can supply 2.5A for an hour. So if it is supplying 5A then it's going to last for 30 minutes.

zerco wrote:
The second question: If now i know how much batteries i need or already have.

Well say we are talking about 10V (actually an odd kind of number!) but say you can get 2500mAh batteries that supply 1.25V per cell then to supply 10V you clearly need 8 cells.

 

So the key unknown you have not given us here is the voltage.

 

Oh and I said that 2500mAh batteries might supply 5A. That is effectively asking them to deliver twice their nominal current. This is often called 2C. If you were asking for them to deliver 20A then that would be 8C and so on. But many batteries have an internal resistance that precludes them from delivering current at such high rates. So the "C rating" of the cells may be important too.

 

Also note that if a battery operates at 1C then it will deliver the stored charge in 1 hour. At 2C it will deliver the charge in 1h / 2 and at 8C it would deliver it in 1/8th of an hour. So if you really did pull 20A from 2500mAh cells it would only run for 7.5 minutes.

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Well, your question is both easy and hard.

 

Your first question about mAh rating of batteries is simple - "It all depends". It depends on the desired running time. There is, however, a possible hidden aspect to this question. IF you use rechargeable batteries, most such small (especially lithium-ion) batteries have a maximum discharge rate. If it is a 100mAh battery, the maximum discharge current is usually 100mA (in other words, the current that would discharge the battery in 1 hour). Lead-acid batteries, especially for vehicles, have a "cold cranking" current rating of 100 Amps or such. Those batteries will deliver that current for a short time, as when you start an engine. Current is Watts/volts.

 

You second question is nearly impossible to answer without more details, lots more details. We can talk about motor efficiency and power train architecture (motor per wheel vs single motor with differential), speed, rolling friction, aerodynamic drag, controller efficiency, and on and on. You need to be far more details than that.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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clawson wrote:
if a battery operates at 1C then it will deliver the stored charge in 1 hour. At 2C it will deliver the charge in 1h / 2 and at 8C it would deliver it in 1/8th of an hour.

Well, that would be true of a perfect battery.

 

In practice, the higher the discharge rate, the greater the losses - so you won't actually get the full capacity at the higher rates.

 

Some types of batteries are better at high discharge rates than others - so you need to choose your battery accordingly, and read its specifications carefully...

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awneil wrote:
In practice, the higher the discharge rate, the greater the losses - so you won't actually get the full capacity at the higher rates.
Maybe true. The specific example I was thinking of was LiPo cells flying high performance model aircraft. It's not unusual to push the batteries into the 10C to 20C range (6 minutes down to 3 minutes of flight time) and even at those rates the batteries do deliver pretty much all their stated capacity. However these are cells that have been bulk produced for this very application and engineered to perform well in the 10..20C range.

 

Of course at the other end of the scale you have something like the battery in your mobile phone. It may be 2500mAh but it's hoped it will deliver for 24 hour+ so by implication that suggests the optimal drain is down in the 0.1C or even lower range so it may be delivering 250mA or less. I imagine those kind of cells are engineered to have best performance in that kind of targeted range.

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Caution - the OP wrote

 

How to calculate running time and distance from a vehicle (knowing the weight of the thing plus driver)

To me, this implies a passenger vehicle. That is a whole different ball game compared to models. Sure, the energy relationships hold,  but, to me, it looks like the OP is asking about battery sizing vs vehicle range. Too many variables with too little information. For example, the simple yes/no regenerative braking?

 

Jim 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I think the OP might need to do a somewhat more extensive introduction round.

"playing around" with high voltages and high currents is not something you want to do as that can be or become very hazardous very quickly when doing something small wrong.

 

the most important thing to remember is that you need to keep track of the current. when running with a very light load the current drawn at a certain speed will be less than the current drawn if the vehicle is heavy.

then again when keeping an eye on the current you can keep track of how much of the available power you have used.

In addition you can keep an eye on the batteries voltages to see what the actual condition of the individual batteries is and compensate when batteries (or even cells)  are showing decreased performance

 

the regenerative breaking can then be just part of that measurement system. to take it very safe with every amp you put in with the braking count half an amp as actual power being added,

then again I think when doing normal driving you will not gain much distance with the braking as that is very short compared to the driving time or current needed to speed up the vehicle again, but I could be wrong there and have no experience with that.

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It is a complex question you ask. As the others have noted, you've not given enough information. The question is complex as there are a number of factors involved. Sometimes you have to rely on empirical evidence to come up with a figure and work from there.

 

It takes around 15horsepower to propel a 550kg vehicle at 100km/h. All very approximate. There is wind resistance, rolling resistance, drivetrain losses etc. The solar electric vehicles used in distance challenges are carefully engineered to minimise these losses, so they need much less power.

So 15 horsepower is around 20kW. Watts is Volts times Amps. Work that back into your battery calcs. This is what is called a 'back of a fag packet calc' - start with some rough figures to get an idea.

Think of the poor scientists designing the first nuclear bomb - how much of this stuff do we need? What, if we get it wrong it will vapourise us and make a crater 1km wide?

Or sending a man to the moon - what? we need a rocket that big? That will make guy fawkes night look like a sparkler!

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zerco wrote:
im not an electrical engineer or mechanical engineer.

Kartman wrote:
It is a complex question you ask.

Which is why Electrical & Mechanical Engineers spend years studying this stuff, and then need further years of experience, to tackle these questions

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awneil wrote:

zerco wrote:
im not an electrical engineer or mechanical engineer.

Kartman wrote:
It is a complex question you ask.

Which is why Electrical & Mechanical Engineers spend years studying this stuff, and then need further years of experience, to tackle these questions

... and a janitor to clean up their mistakes.

Ross McKenzie ValuSoft Melbourne Australia

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For a DC motor (ideally) Torque is proportional to current (amps), Speed (RPM) is proportional to Voltage (after motor IR losses taken into account)...so a "100Watt" motor might well draw a much different amount, depending on what it is driving, how fast, etc.

When in the dark remember-the future looks brighter than ever.