Timing battery drain

[drax]

Does anyone know how I can use a digital timer to stop when a battery source is depleted in a simple electrical circuit. I would like to add a certain load and use a timer to determine when the battery dies. Being a mech. engineer, I would greatly appreciate any help and recommendation on a timer.

 

[OperaHouse]

Simple and understandable are two different things. The easiest circuit I know would be to use a LM431 precision zener. This is a three lead device that is adjustable from 2.5V up to about 36V, but this is a little bit of an oddball use. The reference pin is connected to a voltage divider calculated to give 2.5V at the voltage you want the timer to turn off at. The amplified zener conduction will never drop much below 2.5V but this wouldn't be a problem 1f you were monitoring a 12V battery and the relay only had 10V on it. So simple is two resistors, a relay, and a LM431. The relay can turn on something as a simple clock (as long as you look at it at least twice a day). If your voltage is very low, you will need to use an op amp. There are some voltage monitor relays.

 

[OperaHouse]

I should have mentioned that battery voltage will climb again after a load is removed. If your device has a load that varies, you will need to wire an extra set of relay contacts so once the power is removed that the relay can not re-engage.

 

[felixc]

What is the battery that you are using? Will the current drained by the circuit be fairly constant?

 

[drax]

I'm going to use (2) 1.5 VAC batteries, the source would be constant. I was thinking using a small relay with a NC contact while the coil is energized to run a timer. When the battery dies the contact open and stops the timer. This is probably too simple and I'm missing several key points here. If it is possible, how do I set this circuit up if I know the load of the device?

 

[OperaHouse]

A 5V relay will probably just close when the batteries are new and will stay closed until the voltage drops to about 1V. That doesn't really reflect how many battery devices operate. Are you trying to compare brands of batteries or how long a device will operate. Watching a digital voltmeter and making periodic readings will probably give you more usefull information.

 

[itsmoked]

You cannot run any relay with a battery... It will go dead in a very short time while using more energy than whatever else you are running.

 

[OperaHouse]

I was brought up not to waste a battery on foolishness. I still consider them a luxury item. The life of standard batteies at various loads is well documented. Since you are asking for answers without knowing the questions, I can only assume this is a SCIENCE FAIR PROJECT.