guide on using watt meter to estimate range

[EstebanUno]

Why go to all the hassle of sending it back if the amp-hours reading is correct? You can calculate watt-hours in your head if you have the amp-hours.

The beauty of watt-hours from the range perspective is that it adjusts to the declining voltage. 1Ah with a fully charged battery yields more watt-hours or measure of work than 1ah with a battery nearing its min voltage. So the watt-hours calculation is always changing based on changing voltage.

Put another way, if your capacity is 10ah, then the first 5 will yield more watt-hours than the last 5. Something to keep in mind when estimating watt-hours from amp-hours.

I have the same watt meter and its watt-hour calculation is wildly inaccurate and inconsistent. Amp hours and voltage are consistent and yield about what I'd expect. Been calculating watt hours, but it's a hassle compared to just reading the meter. Don't like the odds of getting a new one that doesn't have the same issue. Has this same watt hour inaccuracy been reported for the Turnigy version?

 

[wesnewell]

You don't need a meter to estimate range. All you need to know is the wh capacity of your battery and divide the wh used per mile into it. For example, I have a 888wh capacity battery. Typical wh usage for 20mph is about 21wh per mile, so 888/21=~42 miles. At 30mph, ~22 miles. and at 40mph ~10 miles.
AH is about useless unless compared to same voltage battery. A 10ah 36V battery has 360wh capacity, while a 10ah 48V battery has 480wh, and so forth. Nominal voltage times AH =WH.

 

[gwhy!1]

You don't need a meter to estimate range. All you need to know is the wh capacity of your battery and divide the wh used per mile into it. For example, I have a 888wh capacity battery. Typical wh usage for 20mph is about 21wh per mile, so 888/21=~42 miles. At 30mph, ~22 miles. and at 40mph ~10 miles.
AH is about useless unless compared to same voltage battery. A 10ah 36V battery has 360wh capacity, while a 10ah 48V battery has 480wh, and so forth. Nominal voltage times AH =WH.

Wh's and Ah's are both useless for predicting/estimating range ,, but Wh's is even less useless as there just are to many variables with riding environment and how you ride.

 

[999zip999]

Where did you get this 25r battery ?

 

[EstebanUno]

You don't need a meter to estimate range. All you need to know is the wh capacity of your battery and divide the wh used per mile into it. For example, I have a 888wh capacity battery. Typical wh usage for 20mph is about 21wh per mile, so 888/21=~42 miles. At 30mph, ~22 miles. and at 40mph ~10 miles. ...

Nothing like a watt meter to track that "wh used per mile." Sure there's typical wh usage, but there's atypical as well. Those pesky variables of wind, route, elevation gain, pedaling effort, etc. Simple miles traveled can't match a meter.

 

[EstebanUno]

Wh's and Ah's are both useless for predicting/estimating range ,, but Wh's is even less useless as there just are to many variables with riding environment and how you ride.

Wh per mile will vary. But wh used is just what it is. Knowing your wh use enables you to estimate what's left and at what rate you're using up your battery. Whatever the variable, it makes a good fuel guage.

 

[gwhy!1]

Wh's and Ah's are both useless for predicting/estimating range ,, but Wh's is even less useless as there just are to many variables with riding environment and how you ride.

Wh per mile will vary. But wh used is just what it is. Knowing your wh use enables you to estimate what's left and at what rate you're using up your battery. Whatever the variable, it makes a good fuel guage.

This is assuming you took to time to know your wh of your batterys., so unless you know the wh of your battery then its no difference to using ah , except ahs is more logical and can also be used as a fuel guage.

Edit: which ever way you want to do it if you want to make it more accurate then you also need to take the c rating into account. I personally dont use either and find thT just a volt meter is a much better fuel gauge for my use.

 

[EstebanUno]

This is assuming you took to time to know your wh of your batterys., so unless you know the wh of your battery then its no difference to using ah , except ahs is more logical and can also be used as a fuel guage.

Well no need to bother with a meter of any kind if you're not going to learn the empty value. But there is a difference between using ah and wh. Sure they are both measures of capacity use, wh being ah * voltage. They both display equivalent empty values. But wh compensates for voltage drops during usage, making it more accurate estimating percent of miles left. If the ah measurement is adjusted by voltage then there is not difference.

 

[gwhy!1]

This is assuming you took to time to know your wh of your batterys., so unless you know the wh of your battery then its no difference to using ah , except ahs is more logical and can also be used as a fuel guage.

Well no need to bother with a meter of any kind if you're not going to learn the empty value. But there is a difference between using ah and wh. Sure they are both measures of capacity use, wh being ah * voltage. They both display equivalent empty values. But wh compensates for voltage drops during usage, making it more accurate estimating percent of miles left. If the ah measurement is adjusted by voltage then there is not difference.

I know what the empty value is volts which have always worked well for me.. and depending on what Im doing on my bikes ah's or wh's will not give me any indication of estimated distance I can cover as it also depends on how much current I am pulling on my bikes (C rating of the batterys). The higher the C rating of the battery's and if you are never exceeding or its a real advertise value of C rating then you are in with a better chance of guestimating the distance using wh's and/or ah's.

a extreem case would be sla's , a 10ah 12v battery will be around 0.1C and this will give you the full 10ah (120wh) but if you exceed the C rating i.e 1C then the capacity will approx halve 5ah (60wh) . I know most ebike batterys have a bms that should be matched to the pack and the C rating should never be exceeded so this will make things more accurate but the only real constant when discharging would be the voltage value at different SOC. wh's only work if its a constant load as do ah's but we all know that a ebike is far from a constant load.

But what it really boils down to , no matter what method you use is you get to know your battery and get a feel for how the bike is performing.

 

[d8veh]

Why go to all the hassle of sending it back if the amp-hours reading is correct? You can calculate watt-hours in your head if you have the amp-hours.

The beauty of watt-hours from the range perspective is that it adjusts to the declining voltage. 1Ah with a fully charged battery yields more watt-hours or measure of work than 1ah with a battery nearing its min voltage. So the watt-hours calculation is always changing based on changing voltage.

Put another way, if your capacity is 10ah, then the first 5 will yield more watt-hours than the last 5. Something to keep in mind when estimating watt-hours from amp-hours.

I have the same watt meter and its watt-hour calculation is wildly inaccurate and inconsistent. Amp hours and voltage are consistent and yield about what I'd expect. Been calculating watt hours, but it's a hassle compared to just reading the meter. Don't like the odds of getting a new one that doesn't have the same issue. Has this same watt hour inaccuracy been reported for the Turnigy version?

You're all guilty of intellectual masturbation. The whole thing is only an estimate because of so many variable factors. Any schoolkid should be able to estimate his remaining range if he knows how many amp-hours he started with and how many he's used. We all know that the final amp-hours are a bit smaller than the first ones. If you want to be clever, you can bear that in mind, though I wouldn't bother because it would be balanced out by other factors, like going slower and becoming more economical as you panic whether you will get home. The first wattmeters that I used for years, didn't show watt-hours. I never had any trouble figuring out how much further I could go. It's not exactly difficult. If some professor asked me how many watt-hours I used on my last ride, i would say that I used 5ah, about half my battery, for 20 miles, so the average voltage would be about 38v, which is 190wh or 9.5wh/m.

 

[EstebanUno]

Only had the meter for a few rides. 20 dollars ... Gone.

Today US ebay seller emilyandlily refunded full price after 6 weeks for watt-hour issue without contest. Listing advertises 2 month warranty. So if you want to try one these, the risk is low. http://www.ebay.com/itm/G-T-Power-L...855?pt=LH_DefaultDomain_0&hash=item485d4abe97

 

[striider]

EstebanUno - what was wrong with yours?

 

[EstebanUno]

EstebanUno - what was wrong with yours?

Watt hours reported was grossly inaccurate and inconsistent. Deriving the WH from AH yields a V of close to 80 for my 48v system for full cycle. Also 3.25 AH = 104 WH for 1 trip, and 240 for another. This is the same issue reported earlier on the thread.

 

[opperpanter]

Is there a good (non CA) alternative to these turning style Watt meters? Mine died after about 400km.

 

[striider]

I met a guy on the trail Friday that has been using a WattsUp branded one for years with no issues. Maybe they are better?

 

[dogman dan]

I'm sorry , but I think you guys are just wrong. One watt hour is one watt hour. Period. It doesn't vary any, ever. Use watt hours to get the very most accurate estimate of your range.

But yes, as your battery voltage drops, you cannot calculate wh using the voltage you had at full charge. Calculated wh of a battey is always a bit of a guess. It's an estimate, not a real world measurement.

But I'm saying real watt hours, actually measured with an accurately calibrated watt meter is always exact, and the best way to estimate your range. It starts with knowing what your real world wh capacity for a given battery is, AT a given rate of discharge.

You of course, cannot compare your capacity at a 300w rate of discharge, with your capacity at a 1000w rate of discharge. They may produce very different capacities, especially with an older battery.

Ok, so now lets say your watt meter is not very good, not guaranteed accurate. Well, if you know your real world measured capacity with that watt meter, you can still do some simple math, and get your range, based that particular watt meter. It won't compare with others, but you will still know what your capacity is, even if measured with a flawed tool.

 

[EstebanUno]

Ok, so now lets say your watt meter is not very good, not guaranteed accurate. Well, if you know your real world measured capacity with that watt meter, you can still do some simple math, and get your range, based that particular watt meter. It won't compare with others, but you will still know what your capacity is, even if measured with a flawed tool.

I agree with the principle. But the measurement taken with the flawed tool must be consistent to be useful. The G.T. Power meter I have is consistent reporting volts and AH, but it calculates wh by some whim I am not able to understand. Example: first trip: 3.26 ah, 104 watt hours from full charge, average resting V = 51. Next trip 3.29 ah = 240 watt hours, avg resting V = 47.5. Same route, similar effort. Dividing wh/ah to derrive V, Trip 1 calculates 32V, and trip 2 calcs 75V.

Come to think of it, the watts display jumps around more than I would think it should. Maybe that's the underlining problem. In any case, if you want to use this cheapy watt meter, better double check the watt hour display with some simple math before relying on it.

 

[Skrzypas]

What about such comparisons:

Wh measuring - equivalent to measuring liters, or galons used by a car, with a flowmeter
Ah measuring - eqv. to observing km or miles driven since last refill
V@rest - eqv. to watching old school fuel gauge, you need to know the car very well to estimate range from it.

Wh is the best only in case of a top quality Wh-meter. Unfortunately, what I read here is that cheap Wh-meters have rather 50% accuracy than 5%. And that makes them suck a lot. Ah measurement is a little easier

The cheapest is to measure distance with standard bicycle cheap computer, that you already have. You determine the range from time to time and know you bike's limit.