Calculating Battery Life based on voltage

kcuf said:

many variables estimate voltage vs % remain

differences between loaded and resting voltage measurements

cell chemistry bms oddv

numbers from my ass

16s pack around 46 48v loaded

minimum it should probably go

soc voltage roughly between 67 and 50 52v resting

if linear usually not

careful discharge measurement and noting

between hi lo points

can reveal your normal system % vs voltage

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speedyebikenoob said:

I'm running a 60 volt battery on my e bike, and fully charged it's about 67 volts. My battery meter does read in percentage but it also reads out the battery voltage, and I think I would prefer to calculate my range based on that. Does anyone know how I could calculate the amount of watt hours left for a given voltage, for example 62 volts?

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Also, my battery has 24 amp hours in it, so 60 x 24 should give me 1440 wh right? But what about if it's completely charged? Shouldn't it be 67 x 24 which is 1608 wh?

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kcuf said:

some folks ride 20wh/m

most dont or won’t

cell age ambient temps

terrain wind soc

use of legs desired speed

all contribute

good luck

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billvon said:

speedyebikenoob said:

I'm running a 60 volt battery on my e bike, and fully charged it's about 67 volts. My battery meter does read in percentage but it also reads out the battery voltage, and I think I would prefer to calculate my range based on that. Does anyone know how I could calculate the amount of watt hours left for a given voltage, for example 62 volts?

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You can do it but it's not easy.

First get the discharge curve for your battery at a few standard loads (say, .25C, .5C, 1C.) Then do the same at a low rate but at different temperatures (three temperatures that represent lowest, highest and average temperatures.) With that data you should be able to do a linear interpolation at each point to synthesize a discharge curve for your current temperature and discharge rate.

Then you measure temperature, current and voltage, plug them into your curve, and look up remaining energy from the curve.

You may also want to add an ESR and a total-capacity variable, since ESR increases and capacity declines with time. Then you tweak them as the battery ages. Alternatively repeat the tests again.

I've gotten within about 5% of coulomb counting using the above method.

Also, my battery has 24 amp hours in it, so 60 x 24 should give me 1440 wh right? But what about if it's completely charged? Shouldn't it be 67 x 24 which is 1608 wh?

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To get an accurate number, integrate the area under the discharge curve. A simpler way is to just use the average voltage and you'll be close.

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d8veh said:

It's best to get a wattmeter, which will give you a fairly accurate and repeatable indication of your battery's capacity and how many watt-hours you used. Voltage isn't consistent enough.

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dogman dan said:

The more affordable DC watt meters on E bay are well under 20 bucks, worth every penny. The cycle analyst is pricy, but a good wattmeter is so great to have, I had 4 of them at one point.


If you do a fairly consistent ride, then you can actually get by pretty good with just an odometer. If your pack consistently delivers 40 miles, no brainer that at 20 miles turn back for home. Works fine on a commute you repeat a lot, or a regular route you like to ride.


But in new territory, with different hills or weather, you need that watt meter. Half voltage is kind of a wide band. With the lipo cells I use, 3.8v per cell is the middle. But the voltmeter reads 3.8v for about a full third of the ride. So I never know the middle by volts, I know the first third and last third marks, but no idea of the actual middle.

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