# Calculating Battery Usage

Discussion in 'Electronics Homework Help' started by jakekusters, Feb 18, 2017.

1. ### jakekusters

11
0
Dec 2, 2013
Hi,

I have this assignment for a Diploma I am studying and I'm a little stumped. I feel like the method to working it out is right in front of my face but for some reason I can't see it. Here is what it says in the assignment:

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The Radio system will be powered by a D.C. Power Supply, with built-in Back-Up Battery facility. The Power supply is rated at 13.8 V, 25A 100% Duty Cycle.

The Radio System consumes 650 mA when quiescent; 1.75 A when receiving, and 7.75 A when Transmitting. The Duty cycle of the Radio is 40% quiescent, 30% Receive, and 30% Transmit.

There is a 12 V, 50 AH Battery connected to the Power Supply. The client needs 30 Hours of operating time in the event of a mains fail.
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The assignment lacks a lot of direction however I assume that it wants me to work out if a 50AH battery will provide 30 hours of backup and then provide a solution.

I've done a heap of things to try and work it out but I'm not confident in any of it. If someone could point me in the right direction that would be awesome.

2. ### Audioguru

2,906
649
Sep 24, 2016
You might assume that the battery is lead-acid and its 50Ah is 5A for 10 hours.
It is easy to calculate the total average current since you were given the duty cycles.
Fudge the discharge time a little lower since the transmit is higher than 5A.
30 hours?" No Way! Two batteries in parallel will just do it until they age.

3. ### Harald KappModeratorModerator

10,218
2,201
Nov 17, 2011
Start by calculating the average current consumption from the current requirements of the 3 different operating modes of the radio.
From the average current you can
- either calculate the uptime under battery power assuming the full 50 Ah are available (usually you can use only ~80% of the rated capacity of a battery, but lacking any more precise information 50 Ah is what I'd use in my calculation).
- or calculate the required battery capacity from the required uptime (again assuming 100% useable capacity).
The first calculation (above) will give you a yes/no the answer to the first part of this assumed question.
The second calculation will give you an answer to the second part of the question.

What is completely unclear to me is this part of the assignment's description:
This is in my view irrelevant with respect to battery operation. Unless the assignment asks for the 12 V from the backup-battery to be stepped up to 13.8 V which in turn would increase the average current and thus reduce the operating time from backup power.

It is always a good idea to post your attempts and your possible solution. Especially in the homework forum. This will show us that you have done your share of the work, allow us to identify where and why you got stuck and help us guide you in the (hopefully ) right direction.