Turnigy power meter accuracy
- Thread starter Pedalo
- Start date
I understand all of what you and NRG are saying but for this one point.WH is just a measurement of energy.
When you buy a battery, it has a WH rating which shows how much energy it can store. For example my battery has a WH rating of about 90WH.
This means it can deliver 90W continuously for 1 hour or 45W for 2 hours or 180W for half an hour etc. When I attach my Wattmeter, use my bike for a while and see a reading of 45WH it means I've used half the energy in the battery. It might have been that I've used the battery at 45W for an hour, or 90W for half an hour. In actual fact when I ride my bike, the power usage continuously varies depending on how steep a hill is and how much I use the throttle. All that is important to me is if I see a reading of 45WH I know that my battery is now only half full.
Run your bike at 45watts for 1hr = 45wh on which we all including the meter agree.[Even I can see the watt-hour connection]
But run your bike at full power until your meter displays 45wh and the running time is far less than 1 hour, so why display it as Watt-Hour when there is no hour involved? Total watts would seem a better description.
Watts is an instantaneous measure of voltage x current and tells you how much power the battery is delivering at a specific instance. It needs the time element to provide a measure of how long it's been delivering that power (or equivalent).I understand all of what you and NRG are saying but for this one point.
Run your bike at 45watts for 1hr = 45wh on which we all including the meter agree.[Even I can see the watt-hour connection]
But run your bike at full power until your meter displays 45wh and the running time is far less than 1 hour, so why display it as Watt-Hour when there is no hour involved? Total watts would seem a better description.
It's just the same for your household electricity usage which is typically measured in units of kilowatt hours.
Not sure if I'm right here but the error could be due to how well the electronics tracks the changing currents. So with a constant current the meter will be accurate but with a current that keeps varying (ebike) then it is approximate as eg the current rises the meter tries to keep up.
I found exactly this problem with my GPS (simple distance measuring only) as compared to my magnet bike speedo.
I found exactly this problem with my GPS (simple distance measuring only) as compared to my magnet bike speedo.
I just boiled my kettle while monitoring it on a watt meter, slightly varying constant reading of 2253w[per hour] and a reading when boiled of 0.11kwh.Watts is an instantaneous measure of voltage x current and tells you how much power the battery is delivering at a specific instance. It needs the time element to provide a measure of how long it's been delivering that power (or equivalent).
It's just the same for your household electricity usage which is typically measured in units of kilowatt hours.
Again I can't see the point of the "h" reference when 0.11kw would have done.
It is simply the case that:I just boiled my kettle while monitoring it on a watt meter, slightly varying constant reading of 2253w[per hour] and a reading when boiled of 0.11kwh.
Again I can't see the point of the "h" reference when 0.11kw would have done.
Power is measured in Watts
Energy is measured in Watt hours.
So quoting energy in watts is just wrong.
It's not a great analogy, but it's a bit like saying I travelled in today at a speed of 15 miles per hour. Why do we bother with the 'per hour' bit?
Yes I wondered the same thing. I also heard that the measurements for low current are not that accurate. Given the meter can take up to 160A I guess even 1A might be considered to be a low current.Not sure if I'm right here but the error could be due to how well the electronics tracks the changing currents. So with a constant current the meter will be accurate but with a current that keeps varying (ebike) then it is approximate as eg the current rises the meter tries to keep up.
I found exactly this problem with my GPS (simple distance measuring only) as compared to my magnet bike speedo.
It does seem pretty consistent though - so I think it will still be quite useful to determine how my battery is doing, say in 6months or a years time compared to the readings I get now. I just wont trust it for absolute measurements.
!5 miles per hour give's us two piece's of information miles covered and 1 hour. From that we can work out how long 60 mile's would take or how far you'd cover in 15 minute's.It is simply the case that:
Power is measured in Watts
Energy is measured in Watt hours.
So quoting energy in watts is just wrong.
It's not a great analogy, but it's a bit like saying I travelled in today at a speed of 15 miles per hour. Why do we bother with the 'per hour' bit?
In the case of my boiling kettle 0.11kwh, how much information can you derive from that and how do you do it?
It tells me your kettle has consumed the equivalent amount of energy as 0.11W for 1 hour. (In actual fact it was 2253W for just a few minutes but the energy used works out the same).!5 miles per hour give's us two piece's of information miles covered and 1 hour. From that we can work out how long 60 mile's would take or how far you'd cover in 15 minute's.
In the case of my boiling kettle 0.11kwh, how much information can you derive from that and how do you do it?
Let's say a unit of energy is 1kwh and your electricity company charges you 10p per unit.
You measured 0.11kwh so boiling your kettle cost you 1.1p
It doesn't tell me whether you prefer tea or coffee or decaff
Nice try, but I already gave you the load[2253watts] earlier.It tells me your kettle has consumed the equivalent amount of energy as 0.11W for 1 hour. (In actual fact it was 2253W for just a few minutes but the energy used works out the same).
Let's say a unit of energy is 1kwh and your electricity company charges you 10p per unit.
You measured 0.11kwh so boiling your kettle cost you 1.1p
It doesn't tell me whether you prefer tea or coffee or decaff
I wouldn't have been able to tell you how much it cost to make your cup of tea with just the 2253 figure.Nice try, but I already gave you the load[2253watts] earlier.
I think we're going round in circles. Shall we just agree to disagree?
Please don't think I'm disagreeing with either you or NRG, I'm not.I wouldn't have been able to tell you how much it cost to make your cup of tea with just the 2253 figure.
I think we're going round in circles. Shall we just agree to disagree?
I'm simply trying to understand the logic behind Watts and Watt hours.
It's become a bit of a quest for me as I don't like things getting the better of me.
So far my reading and experimenting has led to this.
1 watt = 1 volt x 1 amp.
1 watthour = 1 watt x 1 hour[0.5watts x 2hrs or 2watts x 0.5hrs etc]
Charge your battery at 40v 1a for 1 hour and power in equals 40wh.
40wh will power a 40w bulb for 1 hour. The same amount of energy will power a 50watt bulb for 0.8 hours. The load on the battery[50watt bulb] is equal to 50watts per hour[40v x 1.25a x 1hr = 50wh] for 48mins[0.8hrs]
This is how I understand it and I can't see where I'm going wrong.
The meter discrepancy is bizarre being exactly 10wh. I'd expect an inaccuracy to be more evident on the power in process being the longer of the two processes being that time is part of the equation. I did wonder if stop's during your journey or times where your beyond the assistance level may effect the meter readings.
Sorry for any misunderstanding, I'm not arguing, merely trying to understand.
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Understood......however, this maybe where you are going wrong, the load on the battery is 50w in your example not 50w per hour. Power is measured at a point in time, energy (the amount of 'work' a system can do) is measured over a period of time. So the 50w load will drain the battery in 0.8 or 48 mins, the energy used is therefore is 50w x 0.8hr or 40wh
I don't know how to explain this any other way.... there are loads of links on Google about the difference between energy and power like this one:
Energy and Power Definitions
I don't know how to explain this any other way.... there are loads of links on Google about the difference between energy and power like this one:
Energy and Power Definitions
Thanks for that. Maybe you can advise me with another query. I've converted a cheap mtb with a cheap 36v 350w kit. My bms is rated at 27amp max. Will it be up to the job? Battery is 37v 24ah Li-ion rated at 1c.Understood......however, this maybe where you are going wrong, the load on the battery is 50w in your example not 50w per hour. Power is measured at a point in time, energy (the amount of 'work' a system can do) is measured over a period of time. So the 50w load will drain the battery in 0.8 or 48 mins, the energy used is therefore is 50w x 0.8hr or 40wh
I don't know how to explain this any other way.... there are loads of links on Google about the difference between energy and power like this one:
Energy and Power Definitions
I'm assuming bms will suit battery but battery may not suit 350watt bike.
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What is the controller rated for?
Its the same controller as supplied with this kit. I think its 350w although the listing doesn't actually say. The wheel/motor is the same as the one shown on the red/silver bike shown down the page. Although not shown with this kit, I got the pedelec sensor kit as well.
36v 350w Rear Electric Ebike conversion Retrofit Kit on eBay (end time 30-Oct-10 12:11:14 BST)
I'll give it a go and get back to you. I ran the motor with the wheel off the ground and it was so fast it appeared to be going backwards like an airplane propeller.That's a good price, the listing picture looks like a BPM motor but the red bike looks like a direct drive. The e-crazyman 350w is rated at 25amp, if this is the same you should be OK, can you check with the seller?
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