# Battery Run Time Calculator



## .40AET (Jun 7, 2007)

A couple of guys have asked about a run time calculator. Rockymtnway sent me the math to figure out the run times of different batteries. I put it together in a spreadsheet to make it easy to test different voltages and mAh ratings. Could one of the seasoned vets please check it for accuracy?

View attachment Battery Run Time Calculator.zip


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## mofoki (Feb 1, 2005)

Wish you could have created it using a program I didn't have to purchase.


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## .40AET (Jun 7, 2007)

mofoki said:


> Wish you could have created it using a program I didn't have to purchase.


Is there a piece of freeware, similar to excel, that you recommend? I'd be happy to do it again.


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## Tom Brown (Jan 18, 2008)

mofoki said:


> Wish you could have created it using a program I didn't have to purchase.


I just opened it with OpenOffice v2.3 on Ubuntu Linux. It doesn't get more free and open than that. :thumbsup:

Thanks, .40AET. 

BTW, seems right.


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## scar (Jun 2, 2005)

Here is a link to a free runtime calculator.

LED Pro
http://www.jtice.com/led_pro/led_pro.htm


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## Hardtails Are Better (May 4, 2005)

mofoki said:


> Wish you could have created it using a program I didn't have to purchase.


http://www.openoffice.org/

That'll get 'er done.


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## crisillo (Jul 3, 2004)

you can also put the document in Google Docs

but it gets complicated about sharing and simultaneous editing etc

I've uploaded a copy of the excel file:
http://spreadsheets.google.com/ccc?key=pSYI4OwJNq90sGqr9e7dQKQ&hl=en


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## pimpbot (Dec 31, 2003)

*Yeah....*



.40AET said:


> Is there a piece of freeware, similar to excel, that you recommend? I'd be happy to do it again.


Openoffice rules. I've been running OO at work on my Windows Vista (POS-OS) laptop instead of buying M$ Office. I don't miss Office at all. Well, except for Outlook, so I bought that. Better to buy $100 Outlook than $400 Office when you don't need it.


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## Hardtails Are Better (May 4, 2005)

The one thing about OO that pisses me off is that their spreadsheet program dosn't do trendlines. At least not on my version. Have they updated?


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## Tom Brown (Jan 18, 2008)

Hardtails Are Better said:


> The one thing about OO that pisses me off is that their spreadsheet program dosn't do trendlines.


I've never used them in OO but I understand v2.3 supports them.


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## mofoki (Feb 1, 2005)

OK. I downloaded the free version of Open Office, then went back and opened the spreadsheet files. Everything looked ok but I'm clueless on how the program works. I just don't have any experience using speedsheets. Is this actually a calculator where you type in the variables and it displays an answer? I have to do some reading and I guess there's no better time to learn than now.


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## .40AET (Jun 7, 2007)

mofoki said:


> I just don't have any experience using speedsheets.. I downloaded the free version of Open Office, then went back and opened the spreads Is this actually a calculator where you type in the variables and it displays an answer?


No problem. You enter the information from the battery in the first 2 boxes at the top that are in red. The spreadsheet will fill in the rest of the boxes. I could have done all of the math in a single cell, but for the ease of someone here checking it, I laid it all out so it can be seen. The calculator great for when you want to quickly know the run time difference between 2400mAh batteries and 4400mAh ones. Same with voltage. Play around with the first 2 numbers and you'll see how the run time is affected.

Battery amp hours= (From your battery)
Battery voltage= (From your battery)

Run Time of Lights= (Your answer in hours)


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## mofoki (Feb 1, 2005)

That's pretty cool stuff. Thanks for the help guys. Do you anticipate expanding this to do parallel and multiple driver circuits?


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## .40AET (Jun 7, 2007)

mofoki said:


> That's pretty cool stuff. Thanks for the help guys. Do you anticipate expanding this to do parallel and multiple driver circuits?


If you can send me a PM or post the math, then I can put it in a spreadsheet for you. An example would help so that I can follow along first and then check the answer. Everyone has done a lot for me posting all of the lighting info. This is the least that I can do.

Thanks:thumbsup:


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## Coldass (Nov 23, 2005)

Thanks needed this right now for my project and this was a big help.

Coldie.


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## mofoki (Feb 1, 2005)

Question. I hooked up, but haven't done a complete run time test, two K2's in series just to get the real values of Vf and current in and out of the driver. I plugged that info into the calculator and then I wondered, is this runtime estimate factored for the batteries to run until dead or the safety cut off point of the Li-ion battery's circuit. Because my battery packs shut off at somewhere around 6.8 volts I think. I don't know what's left in them because once shut off you can't get a measurement out of them until you've put them on charge again. Also you have the option of choosing how many leds in the circuit. Since I had the actual value of the voltage drop of the leds could I plug that value in and only chose one led.


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## .40AET (Jun 7, 2007)

Mofoki- I couldn't begin to answer that one. You might want to call the battery manufacturer and see what they say. The run times are estimates and I'm not sure what they are based off of. Sorry


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## super-fast (Sep 28, 2006)

@Mofoki. When the safety kicks in there battery should be almost empty. The voltage is dropping fast and the cells can't deliver enough current anymore to fully power your leds. Without the battery protection your lights will burn a lot longer, but only very dim.


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## mofoki (Feb 1, 2005)

For some reason the pack didn't shut off but I noticed the light was considerably dimmer. This was after 4 hours of running it. That's still less than the 4.72 hours the calculator said it would run. I checked the battery pack and the meter reads 6v. 

The measured values with a DMM at the start were;
7.68v .91amps before the driver
6.77v 1amp at the leds


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## brent878 (Apr 17, 2007)

ya i don't know how accurate the cacluator is becuase i don't get the run times it says also. Says with my setup i should be getting 2.82 hours when mine shuts off before 2 hours. But i think its the pcb board in the battery pack as the led's don't dim at all before shutting off. Is there a way to adjust the calculator to factor in the protection circuit? Because as far as i know if we are using li-ion batteries there is no way around the protection circuit right?


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## super-fast (Sep 28, 2006)

The calculator is correct, the problem is that you don't know the exact efficiency of your driver and the capacity of your battery pack. When you buy for example ultrafire 18650's the capacity rating is far to high.


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## .40AET (Jun 7, 2007)

I did some playing around with the calculator. In the example that is posted, best case data was used. If your LED are running at 3.7v instead of 3.5v, that will lower the run time. If the driver efficiency is 80% then you need to change the 1.1 to 1.2. Both of these values will decrease the final number. I think that I made it sound a little too simple in the intro. I was using it to compare a 11v battery to a 14v and the same with the mAh ratings for all of the different size batteries as well. Thanks for all of the input.


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## mofoki (Feb 1, 2005)

The calculator probably is right. And you are more than likely correct about the capacity of the batteries from Batteryspace. I know everybody hypes up the specs to make their products more attactive. The Vf and other values I posted are measured values. My calculations put the efficiency of the driver at 88%. I would tend to believe the best way to factor run times is use specs and the calculator and and avg of both then multiply by a fudge factor of .9 . That should be about right but you'll know for sure once you do the actual run test.


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## Ultra Magnus (Jan 13, 2004)

There was a link somewhere on mtbr to a thread on candlepower forums about some accurate drain mAh tests of AA NiMH batteries. From what I could tell on the charts, the actual mAh rating changed depending on the drain rate. So, I assume from that information when a battery manufacturer rates a battery, they find whatever drain rate runs the battery at it's peak efficiency, even if it's like .1A discharge rate. So, a 4400mAh battery at a higher drain rate may only achieve 3800 actual mAh of performance. 

Of course, this is my unproven, untested, hypothesis.

BM


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