battery power indicator circuit 1

[E2005]

I am trying to find most efficient way to detect the voltage of a battery without drawing too much power from it(as that would defeat the purpose).I have seen some comparator circuits which do that but they all used zener(power wastage). Is there another way or better way to do it? Thanks

 

[VisiGoth]

More informatino on your requirements would help in getting better suggestions.
Do you need continuous monitoring, or is once a month (day year, millisec?) good enough.
How accurate?

A switched circuit using relays would eliminate drain when you are not monitoring.
Relays may be low reliability, FET's may do.
A high Z bridge set to near whee you expect the voltage to be could be a very low drain (and possibly a charge instead of a drain) possibility.

Do you need to know the voltage, or do you need a trip point indicator? If all you need is a trip point, what latency can you afford? Within an hour or a few seconds?

Without further information a guess would be a timed relay to a beridge with an accurate monitor. This solution could be much more expensive than you need, so better requirements can produce more cost effective solutions.

 

[IRstuff]

Then, YOU need to determine how much power you're willing to lose to achieve your desire.

TTFN

 

[E2005]

Thanks VisiGoth. Sorry for not providing more information. Its a microprocessor based sensor system, run by a 12V DC battery. I want to measure the battery power every 30 seconds and feed it into the ADC to keep a check on it. As the system will be operating 20 hours per day. It should draw very small current for measurement purpose.

 

[E2005]

I need to do it with as less wastage of power as possible. I am shooting for micro amp hours(But if not then will go for whatever is possible). I am not sure about my calculations but even a 10uAH current will only consume 0.00001X24X365=0.0876A (or 87mA) in one year if run continuously. That would be less than 10% of the 1A battery capacity. But do let me know if my calculation is wrong or if there are any other options

 

[itsmoked]

E2005; measuring the remaining battery life is f a r from a trivial "measure the voltage". If you are using lead acid batteries then there is a current knee that you must exceed before you get any meaningful info. Because of this you usually want a load on your battery system when measuring the "remaining" capacity. Because of this you may be able to hook in a run-of-the-mill voltage divider to scale the battery voltage to your micro's A/D only when your micro is actually doing something. Does it "do" something on a schedule? Perhaps if you can only make reliable readings during higher current usage times a divider will work.

 

[Warpspeed]

There are many very low power CMOS chips that already have as an inbuilt feature some sort of low battery warning indication.

To build something similar, all you need is a very low power op amp or comparator and a voltage reference from somewhere. A voltage divider of high value resistors, and some hysteresis, and bingo !

 

[fsmyth]

Use a TL431 or similar instead of a zener as reference.
Or check Maxim or TI - both have battery management
IC's which might be used for the purpose. Some have
"sleep" modes (i.e., very low current drain) if you
have a way to control it from your embedded MPU.
<als>

 

[zappedagain]

Here is a cheap and dirty technique that's low current:

If your micro is running directly off battery (no regulator, we are talking cheap here) and you have an LED indicator, enable the LED for a very short time and read the LED voltage into an A/D input that references the micro's power supply. The LED will maintain a fairly constant voltage at only a few milliamps (much less than a zener) so as your battery voltage drops the constant voltage off the LED will give a higher reading. Pass this through a look up table and you have a fairly accurate (I vaguely remember about 5%) battery monitor that consumes much less power than a zener circuit.

 

[itsmoked]

zappedagain that's pretty sweet. And Cheap!

 

[E2005]

zappedagain my microcontroller does use a regulator. I was thinking how would a simple voltage divider consisting of two (0.25W ,30K resistances). I can continuously measure the voltage drop across any one of the resistors(and feed it to the ADC).The current drawn is very less too 12/60,000=0.2mA. This seems too easy a solution for it, is there a catch to it.

 

[itsmoked]

Yes the 0.2mA will actually drag a battery down rather quickly. It's the little drains that run constantly that really hurt the o'l battery budget. If you just run that string with an NPN at the bottom so you can turn it on, and off, it would probably work pretty well.

 

[zappedagain]

I agree with it, the load would be excessive without a switch. With a switch it should work fine.

One caveat on my non-regulator technique - watch out for the micro's operating voltage. This initial design had a PIC18 and with fresh batteries we ran it above the specified voltage (5.5V) but well below the absolute maximum voltage (7.5V). Unfortunately the EEPROM had intermittent failures in this range so we ended up discharging the battery packs slightly before shipping until we could get a design tweak in place :-(.