Battery Source

Hi everyone! My name is Jacky Ang, recently i am going to make a final year project about the security system.
my problem is the backup battery source, i am intend to use this battery as my secondary power source.http://www.amazon.com/Panasonic-7-2Ah-Sealed-Lead-Battery/dp/B000PHD6CC

so how to i calculates the battery life that can lasting for my system? it's current draw is my main concern?

if i want to measure the capacity or battery level by using arduino, voltage or ampere should be measure?or any other suggestion? thanks a lot.

First you should assume that the actual battery capacity is 50% of what the sticker says. However if the battery is always on charge and will only be discharged in a very occasional emergency (like once every 6 months, and then immediately recharged) you could probably assume 75%.

Then you need to calculate how many amps your circuit draws and divide that into the Amp-hrs capacity to find out how many hours usage you will get.

I don't think there is any value trying to meaure the capacity with the Arduino. Even if you go to a lot of trouble you won't get a very accurate indication.

You could measure the voltage and arrange for the system to shut down if the voltage falls below a certain level.

...R

Hi, thanks for your response
About the measurement, it's mean by measure the voltage so I can roughly know the capacity of battery?
Sorry for my poor English

by measure the voltage so I can roughly know the capacity of battery?

The voltage does not tell you anything about the battery capacity. But you can tell if the battery is nearly dead.

The voltage is a reasonable guide (well, probably the only one available) to the state of charge but the capacity depends on the size of the battery and how much use and abuse it has had.

And there is a big difference between the capacity in a battery if you only want to use it once compared with the capacity that is available if there is a substantial discharge and charge every day. Capacity is also affected by the rate of discharge - higher discharge rates reduce the capacity - at least for that discharge period.

The Battery University has lots of information.

...R