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# Battery power

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#### jonathand1000

##### New Member
Hi,
Firstly, erm, I've kinda been asking alot of questions recently...I'm sorry if you're getting fed up! Please say if you are! It's just I'm a bit of an enthusiastic newbie...

Anyway, I was wondering if anyone can help me with something. I was trying to figure out with some school friends how to determine when a battery is going to run low in x amount of time or less. For example, you may find this feature on, I dunno, perhaps medical equipment or a smoke alarm or something. I was thinking that perhaps you could have a seperate battery that kicks in once the normal one dies to light an LED or sound a buzzer or somthing...but then what if that battery ran out...

Thanx in advance for any ideas on this,
Jon

jonathand1000 said:
Hi,
Firstly, erm, I've kinda been asking alot of questions recently...I'm sorry if you're getting fed up! Please say if you are! It's just I'm a bit of an enthusiastic newbie...

Anyway, I was wondering if anyone can help me with something. I was trying to figure out with some school friends how to determine when a battery is going to run low in x amount of time or less. For example, you may find this feature on, I dunno, perhaps medical equipment or a smoke alarm or something. I was thinking that perhaps you could have a seperate battery that kicks in once the normal one dies to light an LED or sound a buzzer or somthing...but then what if that battery ran out...

Thanx in advance for any ideas on this,
Jon

Well, if you are to generate a signal of somekind to let someone know that the primary power source in the device is "low" then, you have to have a source of power. If your backup dies too, then you are out of luck if you are monitoring by electronic means with another battery.

Some smoke alarms just have a low voltage detector circuit that when the battery gets low, the circuit "chirps" the buzzer to let you know. This circuit consumes so little power, that chances are very good that you will hear this and replace the battery long before the battery dies due to the "chirping" circuit. and that is typical in devices that detect low battery conditions. They still use the main battery for the monitoring but the detection design is very low power consuming.

A low voltage comparator may be able to do the job if designed properly.
If the instrument you want to control is hi-tech then a microntroller can very easily work as your comparator and at the same time consume very less current (the one's available with a sleep mode)

I looked into this some time ago. One of the first things I did was atttempt to understand how a battery behaved as it was depleted from full charge. What I learned caused me to think that the project was not quite as easy as it first seemed. A few of the more significant learnings -

A. For the batteries of interest, the full charge voltage, from battery to battery (or cell to cell) was quite variable - not of much consequence normally but if you are trying to measure how "full" it might be the variability was significant.

B. The voltage would drop ever so slowly as the battery discharged - until it starts to drop off rapidly - that "knee" in the curve was quite profound. The difficulty was in seeing the approach of the knee and estimating how far away you might be from it. As I recall the knee was a good indicator that the battery was empty.

C. Cell or battery temperature made quite a difference - so much so that I felt that you'd have to keep the battery at a stable temperature for quite some time to make even moderately precise measurements.

I put the project down. If I ever picked it up again I might consider using some form of power consumption logging ( a PIC maybe) as a way of indicating battery status.

Re:

Hi,
Yeah, that late 'knee' was the problem. I reckoned that there would not be enough of a voltage drop to really detect when it was going to happen. The comparator was my initial reaction, but then I thought that it might not be accurate enough if their was only a small voltage drop for most of the battery's life. Anyway, thanks for the suggestions, this was originally only a discussion, but I think I might just try it out for real...

Interesting one.

Cheers,
Jon

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