A few questions about batteries.

Hello,

I am currently working on an Arduino lighting project. Everything is working correctly on it so far. Currently, The Arduino uses an AC adapter to power it. I would like to be able to power it via rechargeable batteries. The problem is that I don't completely understand how to calculate the lifespan of a battery. The batteries I am looking at are 3.7V 5000mah rechargeable lithium. In total it would be 7.4V because I want to use two of these batteries. If I understand correctly, I can calculate the battery life by using this formula?

Battery Life = 5000mah / used mah

Is this correct or would I combine the 5000mah from each battery?

Also, if this is correct, it would appear that I would only get about 25 Hours per charge on these batteries. Would adding more batteries and then decreasing the voltage allow for a longer use?

Thanks in advance,

If I'm reading what I found correctly, I thik I need 4 batteries instead of two.

http://www.batterystuff.com/kb/articles/battery-articles/battery-bank-tutorial.html

It appears that if I do a series and parallel combination, that will increase my voltage and mah. I'm still not completely sure about the formula though.

Is this correct or would I combine the 5000mah from each battery?

It appears you've figured it out, but I'll go ahead and confirm.

Cells in series, voltage adds, current stays the same.

Cells in parallel, voltage stays the same, current adds.

As for the life, remember the voltage is dropping off as the cells discharge. So if the LEDs are driven through a constant current circuit, it depends on if it is a linear or switching regulator. If it is linear, the same current will be drawn. If it is a switching mode constant current drive, then the current drawn from the batteries will go up as they run down in voltage.

Also, that 5000mAhr is really, really generous. The marketing department picks the absolute best rating. That number goes down as you draw power out faster. The good news is that it is often calculated by drawing out 1/10th of that for 10 hours, so what you get may be very close to that.

However, rechargeable cells lose life by not storing as much charge. It is also not really a good idea to discharge Lithium Ion batteries fully.

Thanks for the information!

What would you recommend as the minimum charge that a lithium battery should have?

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

Doh! LarryD beat me to it. I have found Battery University to be a great site for information on lots of different kinds of cells/batteries.

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

Also, how to prolong Lithium Ion cell life:

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

More general information:

http://arstechnica.com/gadgets/2011/02/ask-ars-what-is-the-best-way-to-use-an-li-ion-battery/