# How can I find out the "Estimated Time to Empty"of a battery?



## skooler (Mar 26, 2011)

You need to now how much power you are drawing, in watts and how much usable energy the battery can store, in watt hours. 

Divide energy by power for the time in hours.

As an example. a 2KWH battery (Usable energy not total) with a 200w power draw will take 10 hours to discharge.

theres a bit more to it like peurkert and %DoD but I'll let you work thse out 

Cheers,

Mike


----------



## electronics_guy (Aug 6, 2012)

skooler said:


> You need to now how much power you are drawing, in watts and how much usable energy the battery can store, in watt hours.
> 
> Divide energy by power for the time in hours.
> 
> ...




Thanks Mike.

Any more suggestions from others???


----------



## major (Apr 4, 2008)

electronics_guy said:


> Any more suggestions from others???


Time remaining = (Capacity * SOC) / current


----------



## Ziggythewiz (May 16, 2010)

Time to empty is a common metric for showing golf cart battery capacity. It factors in how much power you lose by drawing over the current rating (the 20 hr rate) from the battey. These numbers are published by the manufacturer. One way to estimate it is using the sticker capacity * .6, divided by the current you want out.

For lithium, the sticker rating is much closer to the actual usage rate so just use the formulas skooler and major posted.


----------



## batterytang (Nov 17, 2011)

For Lead Acid batteries, cell voltage has a very close relation with reserved capacity (reserved energy), thus many BMS can monitor volt for SOC calculate.

For Lithium based batteries, you will see that voltage curve is very flat during the whole discharging platform, so if take volt for reference, you will meet a big mess.


----------



## electronics_guy (Aug 6, 2012)

Thank you everyone for your valuable replies. Where do you think I can get to know more about this concept? Plz suggest some white papers or may be links where I can find in depth information about this.

Thanks


----------



## green caveman (Oct 2, 2009)

major said:


> Time remaining = (Capacity * SOC) / current


I think that the problem is that "Capacity" is a variable. It will drop over time. Is there any way to estimate the Capacity at any point in the cell lifecycle?


----------



## major (Apr 4, 2008)

green caveman said:


> I think that the problem is that "Capacity" is a variable. It will drop over time. Is there any way to estimate the Capacity at any point in the cell lifecycle?


Hi green,

Put a meter in the EV and keep a log 

I have seen and you can find Lithium and other type of cell manufacturers that list capacity vs cycle number charts on their websites. These would be under controlled conditions and likely to a standard 80% DoD. It would be unclear as to how to relate this to your actual dive cycle use of your EV.

The best you can do is to instrument your EV and keep a log showing the usage, miles, charge, etc. After a short time you will get to know how she behaves and what to expect. If you keep on doing this, you may start to see trends as your battery ages and from that make some reasonable predictions for your particular case. But I don't think anyone can accurately tell you how your battery capacity will change 2 or 3 or 4 years from now.

If you figure it out, let us know 

major


----------



## dtbaker (Jan 5, 2008)

the 'gentle' indicator would be to drive around till the pack is sluggish to 'recover' to nominal voltage at a stoplight. You can then conclude that you are in the knee....and probably deep into 85%-90% DOD, thus you can 'adjust' actual capacity if you have a decent AH, or just use the trip odometer, and back off 10% for safe max range.


----------



## green caveman (Oct 2, 2009)

major said:


> If you figure it out, let us know


Actually, the discharge curves for Lithium are not totally flat. Given current usage in C and voltage of each cell you could map them to the curve. 

So, if you're pulling 1C and have a voltage of something like 3.21V you're at 60%. Better curves would be helpful because the difference between 60 and 40 is some 10s of mV. (It's easier if you routinely pull 3C+).

That means it's a data mining operation - you're not going to get the number instantaneously. Over a number of trips, and some thousands of data points,you should be able to collect enough data to analyze and find, say the 60% point and so the current capacity of the batteries. (assuming you can get decent curves from the manufacturer to know what you're looking for). And don't forget the temperature dependencies which have at least as much impact as the discharge rate.

Without the curves, over many trips you should see the degradation, but would have no idea how much the cells have degraded.

This is all speculation, but people keep telling me that large amounts of long-term data has no value. I have to remind them that if that were the case Facebook would not have data that amounts to 1000+ pages of information on their users.


----------



## onegreenev (May 18, 2012)

> This is all speculation, but people keep telling me that large amounts of long-term data has no value. I have to remind them that if that were the case Facebook would not have data that amounts to 1000+ pages of information on their users.


It's not speculation but its not totally false that long term has not value but for the most part it does have no value. If your building a HUGE DATA BASE for test purposes then by all means go for it. For the average Joe it means squat and if you did have it what would you do with it? Do you really care about all the DATA that was collected over the last 100 years on how engines run? When you went to buy your car did you inquire about all the DATA and insist that you be able to collect data because you don't believe what you read? Doubtful. Be realistic. Data mining is fine for those WHO CAN AND WILL do something with the information. Beyond that it is useless fluff. 

Bottom balance and use a good AH counter and volt gauge and don't over charge or over discharge and you will be happy to know that you can safely go to and fro with a happy grin on your face. When is enough Good enough? The information that is in is good enough to get started safely then slowly improve it a bit at at time.


----------



## green caveman (Oct 2, 2009)

onegreenev said:


> It's not speculation but its not totally false that long term has not value but for the most part it does have no value. If your building a HUGE DATA BASE for test purposes then by all means go for it. For the average Joe it means squat and if you did have it what would you do with it? .


We're getting kinda off topic, but I think you're looking at it wrong.

In and of itself any large dataset is useless, uninteresting.

No dataset really has any value unless you can analyze it and pull meaningful information out of it.

So what if there are multi-GBs of raw data stored on a flash drive, the user doesn't care and doesn't need to see it.

BUT if those GBs of data result in one number on a display that is the current pack capacity (or anything else useful) then that dataset suddenly has more value than the $2 worth of flash space that it's using.

Whether I end up with GBs of useless data, or something that has some value is REALLY hard to determine unless you have the data in the first place.

And, you're right, I'm not asking you to collect it or figure it out, but I'm willing to, and if I can figure out any value I'll let you know.


----------



## onegreenev (May 18, 2012)

The data is already there. Why do it again. So what ARE you going to do with the information? 



> Data mining is fine for those WHO CAN AND WILL do something with the information.


Can you? Will you? Like more than one person has said. What are YOU going to DO with the information? There is more than enough already gathered by users.


----------



## green caveman (Oct 2, 2009)

onegreenev said:


> The data is already there. Why do it again. So what ARE you going to do with the information?
> 
> Can you? Will you? Like more than one person has said. What are YOU going to DO with the information? There is more than enough already gathered by users.


We really should start a thread for this. Once again you CANNOT KNOW what you can do with a dataset until you have it.

But since you asked here are the obvious things:

So, the data set I'm aiming for. pack voltage, current, vehicle angle (accelerometer), acceleration (accelerometer/gyro), speed (GPS), direction(magnetometer), temperature, barometric pressure. The most tedious - the voltage of each cell. It should be clear that if you're going to collect, say, voltage and current, adding a board with accelerometer, gyro, magnetometer and barometer is $25.

Now, remember this is the DIY EV group - it's a hobby, not something where I expect a financial return on investment. Also my goal is to return something to the community, I'll let you know what I get out of this data collection effort, even if it's nothing - a negative result also has value (at the very least it means you shouldn't bother collecting the data but it could mean that not enough, or not the correct data was collected).

So, the first thing you can get from this is cool display of interesting numbers. In my case an Android Tablet. Does the Tesla really need a 17-inch flat-panel screen? No, it's just cool and I want one (well probably a Nexus 7 but you get the idea). You could do OBDII error codes to, but I don't have ODBII in the EVs.

So you can display all these things plus some easily derived values such as power usage - instantaneous and total pulled from pack since charging (or ever).

There are some values that can be derived fairly quickly from the historical data - power usage vs. incline, how much power do you use going up a hill. Power usage vs. speed. Power usage vs. Temperature.

If you know the pack capacity these all useful in predicting remaining capacity accurate. But, frankly, who cares to predict with that degree of accuracy.

You can quantify lead-foot vs smooth start, when to coast and you could optimize the amount of regen.

All fun stuff but arguably nothing but fun. 

One area I'm interested in, if you know these number, then you can start trying to increase your range and actually know whether it makes a difference. Replace the side-mirrors with cameras? You're probably not going to see that change in instantaneous power, there's too much fluctuation. Even long term you probably won't see it from eyballing since any of these will make probably 1-2% difference. Belly pan, front skirt, wheel covers? Each one should make a quantifiable and small difference. If I can figure out the values for each, you can decide what's worth your effort (you can thank me later!).

Preventive maintenance is almost always the goal of data collection, and almost never achieved.

If you can model these numbers, so you can predict, given the temperature, incline, speed, (air pressure?) etc. how much power you should be using and you are using a significantly different amount, then you have a maintenance problem. You actually don't have to be quite this clever. If you always drive the same route everyday, then you could get accurate numbers just from that. Average power usage, like MPG in a gas car, gives some indication, but is not as good.

So, those you can see deriving from the dataset. But, until you have the dataset you still don't know what else you can get from it. I've presented a theory that Capacity might be hidden in there. Probably is, but can it be extracted? What else?


----------

