# Calculation of battery life for my circuit

I've got a circuit composed of an Arduino Mega that is interfaced with an Xbee Pro Series 1 receiver, Parallax GPS, and a uOLED display screen. I'm trying to figure out an external battery combination that would power this circuit for up to 80 active hours. I've got this theory that I need someone to double-check for me.

The uOLED display screen operates at 5V with 80mA typical current. The parallax GPS operates at 5V with 115mA typical current. The Xbee receiver operates at 3.3 V with 50mA typical current. The Arduino Mega will operate at 5V with about 100 mA typical current. So all our devices except for the Xbee receiver will operate at 5V, and the full circuit will draw a total typical current of about of 345mA. Supposing that the Xbee receiver has a very low power voltage regulator that consumes negligible current, I was thinking about powering this circuit at 5Vdc supply using the following combination:

I plan on using exactly twelve 1.25V, 9000mAH batteries. I will have 3 sets of 4 of these batteries in series. These 3 sets will then be put in parallel combination. Therefore, I will end up having a battery of 5V (1.25 x 4) at 27000mAH (9000 x 3).

Since the total typical current of my circuit is about 345 mA, can I assume that this battery will power my circuit for up to 78 hours (27000 mAH / 345 mA) ??? If you have a battery alternative that could help power this circuit for about 80 hours, please let me know.

Your input on this is greatly appreciated. Thanks in advance...

Your solution and math appear correct to me. What you didn't mention is if you have an physical space requirement, max weight requirements. Is the system fixed or is it designed to be moved around. Will it exposed to any environmental temperature extremes? Do you have an special recharger requirements (AC only, 12vdc vehicle, etc)

Don't mean to hit you with a million questions, they aren't really needed to be answered, but rather to get you to think about all the possible options/requirements one might consider. There is a tremendous number of battery choices these days, all with trade-offs in costs/power density/etc and only you can determine the best trade-offs.

For your amp hours needs, I would recommend looking at used or surplus vendors, possibly E-bay sellers, as 27AH batteries tend to be pricey when purchased new. From a cost point, It's hard to beat a 30AH or higher lead acid car battery for price, followed with a efficient +5vdc switcher regulator rated at say 1 amp. Lots of inexpensive chargers available for car batteries also.

Lefty

Thanks. This system will indeed be constantly moving around from region to region. That's why it has to be powered through an external portable battery. So my circuit could also be exposed to extreme temperatures. Also, I don't have or plan on having any recharger requirement. So once these batteries get consumed, hopefully after 80 active hours, they'll be gone for good. These non-idealities (extreme temp, etc..) will obviously have a negative effect on my circuit. I will research those batteries with high energy capacity, and see what I can find at affordable price. Thanks again

Another important consideretion is that most batteries start to put out a lower voltage when being more than 50% discharged (or even earlier).

If you go for exactly 5V and NEED at least 5V this could be a major problem.