guide on using watt meter to estimate range

Hello everyone,

I just installed a watt meter and I've been watching it and trying to make sense of it.
Would somebody give me a rundown on how an experienced ebiker use such meter?

Like total consumed watt-hour/miles to get Wh/mi to check for efficiency.
What do you do with Ah? Is it a better gauge for range than Wh?

Oddly, watt hour and amp hour don't seem to match up if you do the calculation. I have exceeded my battery's reported total watt hour and the bike was still going strong. I have a 13S4P 47V 9.8Ah 25R battery and used up more than 477 watt hour today while riding around the neighborhood and the bike is still going strong. It said I only used 6Ah.

in order to use a wattmeter to measure the remaining range of the ebike you need to have measured the storage capacity of the battery first. you would have to be certain that the battery is properly balanced and all channels at full charge and then discharge until you reach the LVC to know what the capacity of the battery is.

you would measure capacity by doing a measured discharge through the wattmeter into a dummy load until the first cell reaches the 2.1V LVC for lifepo4.

then to use the wattmeter to estimate range, you would have to be sure to fully charge the battery initially and have it be balanced by the BMS before use so you would have an idea of how much charge is still remaining in the battery during use.

Using a bicycle computer to find distance traveled, take a trip and write down your distance traveled and watt hours used for that distance. After that, a little simple math is all that is needed, lets pretend you traveled 10 miles and used 200Wh over that distance. You divide 200(Wh) by 10(miles) to get 20Wh/Mi. Keep track of this sort of information regularly. So lets say you know you have a 1000Wh pack and you typically run around 20-25Wh/Mi on most of the routes you travel. You would know you have a range of around 40-50 miles by dividing your packs total watt hours by your typical average Wh/Mi figure. Simply by looking at your watt meter and seeing how many watt hours you've consumed, you'll be able to gauge fairly easily (eventually) how much range you have left.

As noted, you need to do a total capacity discharge test. The load you use should be of a similar discharge rate to the typical discharge rate you use when riding, basically what your controller draws on average through out a trip is what you want to use as a load in your discharge test.

http://imgur.com/zH9IVQA This picture should help illustrate how I've setup my discharge test. I cut the plug off of a power strip and plugged in a lamp, any lamps with plugs will do. Obviously from here I connected the watt meter, battery and power strip using anderson power pole connectors. You probably want to keep a close eye on the test and either flip the switch on the power strip to end the test or record a video focused on the watt meter. Obviously if the BMS cuts power, the watt meter screen will go dark and it's data with it. Very simple and easy to do.

you can put the wattmeter in the B- lead so it does not lose power or you can use a small 9V battery to provide power through the aux power input plug and never lose the data.

So the storage capacity of the battery is not just V x Ah, which in my case 47V x 9.8Ah = 460.6 Wh?

I guess I can ride until LVC or until BMS cuts power and determine what my total Wh. I wonder how much more than the calculated 460.

What about Ah? Can I ride until 8-9Ah?

No, it is V x Ah. But it's real world actual V. This means at the start of the ride it's more volts, and near the end it's less. So its hard to pick the right V to use for a rough estimate of your wh. Somewhere around half way between empty v and full charged v. With my batteries that charge to 58v, 53v gets me close.

A "48v 10 ah" should be close to 500wh. But might yield more riding slow enough.

Then, the peukerts effect will result in real world total wh varying, depending on your rate of draw on the battery. high rate will have less real world total wh. As the battery ages, internal resistance will get worse, exaggerating the loss at high rates. Old batteries just warm themselves more.

Lastly, your wattmeter could be off some. So you could be + or- 50wh easily, and possibly even more. No matter. All you really need to know, is where is half way, so you can turn for home in time. So half as shown on your wattmeter is still exactly half.

Find out what your real world wh is. At some point you will have to test it. Ride slow near the end, the battery won't like full blast for the last few miles. Then you know your real world wh today. Next year, it will be less. But you can use todays wh for the summer, till cold weather lowers it for the winter. Test again in the cold, to know where halfway is in winter.

With a good calibrated speedo, then you can divide wh by miles, to get wh/mi. Or using ah, you can get miles per ah. or ah per mile.

puregsr said:

What do you do with Ah? Is it a better gauge for range than Wh?

Click to expand...

puregsr said:

What about Ah? Can I ride until 8-9Ah?

Click to expand...

Absolutely. That technique will give you the best results across varying riding styles (discharge rates).
Do not use Whs if Ahs are also available.

• By definition, Amps are an exact measurement of the number of electrons (energy) that pass in or out of your battery - this is the measurement of charge that you seek.
• Watts (power) are a derived value - and again by definition - are altered by the voltage - which due to resistance will vary according to how rapidly the amperes are withdrawn. Because of this, you can get different Wh readings for the same Ah expenditure.

• Here's a post by Justin on exactly this matter in the context of using either the Ah or Wh displays on the Cycle Analyst for this purpose.

Regarding total battery capacity: Since you are not trying to do precise calculations here but presumably just want run your battery down to 80-90% capacity for reasons of battery health, it's not strictly necessary to calibrate your battery at the outset. If your battery is new and made with A-grade cells, you can get by with the stated capacity as a metric of full-charge capacity and do exactly as you have proposed. On the other hand, if it's from a no-name eBay source instead of a reputable vendor then you may wish to run it down to baseline it.

• FWIW - Wh are a handy means to approximate capacity and range when selecting a battery because the 'Wh capacities' are voltage-agnostic allowing you to easily compare capacity/range of different voltage batteries given an approximate Wh/mi consumption rate. However, this use is quite different than identifying State of Charge.

Excellent post! I will be figuring Ah/mile from now on.

On a slight side note: Should my blue watt meter be fading and lose display around 46 volts?

A watt hour is a watt hour, at ANY voltage. That's why wh/mi is the ultimate way to compare bike A with bike B.
And watts is watts, at any voltage too.

But Ah is fine, if you stipulate. "36v, top of charge 42v, yadda yadda. "

ONLY watts and watt hours eliminates the uncertainty of operating voltage differences. So if your meter measures watt hours, use that for sure.

If it only measures amps, you can still get a very close estimate though, in amp hours. Ride at 10 amps continuous for 30 min, it's roughly 5 ah. Good enough to estimate a half way point reasonably close.

I'm confused by Justin's post. My experience is that when I punch my battery and have big voltage sag, I get less watt hours, and less amp hours. But you have to learn by experience, what your capacity is at 15 mph, vs your capacity at 35 mph. My older batteries are close to original capacity if I discharge them at .5c rate. But if I discharge at 2c, they have half the rated capacity. That battery has a very high internal resistance now, almost 5 years old. A newer battery, if adequate size, should do nearly the same fast or slow, at least in the beginning.

If your battery falls short when you ride full speed, do another full discharge test much slower. Good to know how much stretch you have, when you need to stretch that range. And good to know, your battery is not really up to full speed continuous, if that turns out to be the case.

striider said:

Excellent post! I will be figuring Ah/mile from now on.

On a slight side note: Should my blue watt meter be fading and lose display around 46 volts?

Click to expand...

which watt meter are you using? the GT power RC wattmeter does show loss of contrast as the voltage increases close to 60V but since we don't know anything about your meter it is hard to answer directly.

So please clarify something for me.

I went out for a ride through the woods. 48V9.8Ah battery.

I used up total of 4.92 Ah and 95Wh. The numbers totally do not match at all. What's going on? Is there something wrong with my meter?

Unfortunately, meters (especially cheap ones) can exhibit problems. Maybe that's the case here? Shutting off over 48V isn't good. How are your connectors setup? Good crimps? Solder?

I go with the cheap too but I always have more than one so if something doesn't add up or seem right, at least compare it to another one.

Wh should calculate close to Ah using the average voltage throughout your discharge curve. As you noticed, 4.92Ah running 48V doesn't add up to 95Wh. Something's weird...

the contrast should be able to be hacked. it just requires changing the value of the resistor divider bridge on the pcb. the first three pins are 1, low with a surface mount resistor to pin 2, voltage signal for contrast, and connected through another surface mount resistor to pin 3, the high voltage, i think 3.3V for the top of the resistor divider.

so change the value of the resistor by soldering another surface mount resistor on top or use a trimpot soldered across the resistor in the divider. i have one somewhere that has the pins exposed so i will find it and measure the resistor values.

Sounds like your wattmeter had its brain drained. This is one reason I bought one cheap ebay wattmeter, and when I fried it by mistake, replaced it with the second Cyclanalyst, and today I own four of them.

Not at all weird for even a CA to need a recalibration, but this is the first time I ever heard of a mismatch between ah and wh used this bad.

Chuck it, get something better. Or at least, a new cheap one.

I thought this was the same user but it’s just another unlucky GT Power meter eBay customer -

dakh said:

Ok I'll re-use this thread for trying to resolve various issues.. Until I have time to create an actual bike build thread with pictures and all that.

GT Power Meter. Says can handle up to 60V. So I made this nifty thing that goes between the battery and the controller. With a fuse on one wire and a shunt on the other. The bugger doesn't work at 56V (14s battery). Screen lights up but nothing is displayed. Works just fine off a 6s brick. Actually works for a second off the 14s battery just after I disconnect it

Did I get a dud or they're all like that? I'm looking for a simple way to monitor whole pack voltage and a current meter.

Click to expand...

Gotta quit suggesting those meters from now on, I guess? They used to be pretty decent for the money but they got cheaper and cheaper and now they're unable to work even over labelled ranges. BS…

My experiences with them have been crappy so far.

This one loses display below 46 volts but works well otherwise:

This one totally resets itself and blinks and freaks out over 48 volts:

puregsr said:

So please clarify something for me.

I went out for a ride through the woods. 48V9.8Ah battery.

I used up total of 4.92 Ah and 95Wh. The numbers totally do not match at all. What's going on? Is there something wrong with my meter?

Click to expand...

Wh's can be very misleading and are not a true indication of range so always use ah's this will be a much better indicator to how much battery capacity have been uses..

teklektik said:

puregsr said:

What do you do with Ah? Is it a better gauge for range than Wh?

Click to expand...

puregsr said:

What about Ah? Can I ride until 8-9Ah?

Click to expand...

Absolutely. That technique will give you the best results across varying riding styles (discharge rates).
Do not use Whs if Ahs are also available.

• By definition, Amps are an exact measurement of the number of electrons (energy) that pass in or out of your battery - this is the measurement of charge that you seek.
• Watts (power) are a derived value - and again by definition - are altered by the voltage - which due to resistance will vary according to how rapidly the amperes are withdrawn. Because of this, you can get different Wh readings for the same Ah expenditure.

• Here's a post by Justin on exactly this matter in the context of using either the Ah or Wh displays on the Cycle Analyst for this purpose.

Regarding total battery capacity: Since you are not trying to do precise calculations here but presumably just want run your battery down to 80-90% capacity for reasons of battery health, it's not strictly necessary to calibrate your battery at the outset. If your battery is new and made with A-grade cells, you can get by with the stated capacity as a metric of full-charge capacity and do exactly as you have proposed. On the other hand, if it's from a no-name eBay source instead of a reputable vendor then you may wish to run it down to baseline it.

• FWIW - Wh are a handy means to approximate capacity and range when selecting a battery because the 'Wh capacities' are voltage-agnostic allowing you to easily compare capacity/range of different voltage batteries given an approximate Wh/mi consumption rate. However, this use is quite different than identifying State of Charge.

Click to expand...

That particular watt meter is only indicating that it's broken.

There should be a pretty close match between capacity in AH and capacity in WH. That one is off by a huge margin. No telling which measure is correct, and my guess is both are way off.

These meters are cheap, mine died pretty quick and I now have 4 Cycleanalysts.

dogman dan said:

That particular watt meter is only indicating that it's broken.

There should be a pretty close match between capacity in AH and capacity in WH. That one is off by a huge margin. No telling which measure is correct, and my guess is both are way off.

These meters are cheap, mine died pretty quick and I now have 4 Cycleanalysts.

Click to expand...

Wh's used on a watt meter means nothing for distance covered. Ah's is a much more meaningfull measure ( easier to get your head around ), but still has its flaws as do Wh's

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

puregsr said:

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

Click to expand...

I am sending the second one back (GT power meter) - bought through Amazon and they even give you a label to ship back with. I gotta figure out how to rescue at least the plastic parts of my Anderson power pole connectors off of there - they aren't getting those for free damnit! I need to figure out where to buy those things for less than a buck each, I am going through them like butter.

Thanks for reminding me that I still got time to file a case and return it. Today it gave me 2Ah and 15Wh.

I wonder how inaccurate the LCD battery indicator is. I guess I'll just have to use my trip meter to calm my range anxiety.

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.

And with one flaky watt meter, how does he know the Ah is correct?

striider said:

puregsr said:

I need to figure out where to buy those things for less than a buck each, I am going through them like butter.

Click to expand...

Click to expand...

I use this eBay seller for APP's. http://stores.ebay.com/Anderson-Power-Pole-Store?_trksid=p2047675.l2563

Once you have a good number of housings, then start buying large amounts of loose contacts. I also highly recommend the crimp tool. Shouldn't be $40 but I'm always glad I bit the bullet....