Abeer_engjbr

Hi everyone,

I need to build a circuit that could measure a buttery charge, and I want to use three Leds that would be on (as the princible of mobile buttery charge), if the buttery is weak one led will flash ,if it's little bit stronger two will flash and if it's stronger(as full) three leds will be on .

I hope you got the idea , the three leds would be in series.

Would this circuit be usfull to measure a car buttery if impelmented?

Thanks

crutschow

Yes, but it would only properly montor the battery charge after setting for awhile with the engine off and no battery load. It would not work when the engine is running and charging the battery. Also it takes several minutes after charging stops before the battery stabilizes to its final voltage to determine its state of charge.

Abeer_engjbr

Ok, but first what would the circuit look like? Have you got any information or usfull sites?
Thanks

crutschow

You could use a circuit such as an LM3914 Dot/Bar Display Driver. It has a built-in voltage divider you likely could use to select the voltage levels that the lights turn on. It can drive up to 10 LEDs but you can use as few as you want.

audioguru

Nobody said if the battery is lead-acid, Ni-Cad, Ni-MH or lithium-poly.
Their voltage vs amount of charge are all different.

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Uncle $crooge

Leftyretro

Most correct. Ni-cad and Ni-MH have pretty flat charge/discharge curves, where Li-poly and lead acid are pretty linear in charge/discharge characteristics.

Lefty

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Measurement changes behavior

Ubergeek63

Lithium Iron Phosphates are pretty flat as well... And stable to boot!

crutschow

Abeer_engjbr's initial statement was a general question about batteries. But his last sentence mentioned using it for monitoring a car battery so I assumed that was what he was concerned about and the battery was lead-acid.

But if his inquiry was for batteries in general then, as you noted, a voltage measurement by itself will not necessarily work.

Boncuk

Hi audioguru,

seems you need corrective lenses when reading. I never heard that cars are equipped with Ni-Cads, Ni-MH or lithium-poly batteries.

Hans

audioguru

Ni-Cads are old and toxic. Many battery manufacturers don't make them anymore.

Most car manufacturers use Ni-MH cells in their hybrid cars and the car's computer knows the exact amount of charge. The driver does not need to know.

I have seen tiny cars called SMART. Maybe they use rubber bands or a Li-Po battery for power.

Lead-acid batteries in cars are charged by the alternator so there is no reason to know the state of charge.

__________________
Uncle $crooge

Papabravo

Ahhhh...could I get some of that buttery charge for my popcorn?

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We never have time to do it right; but we always have time to do it over.

Boncuk

I can assure you they are using combustion engines.

They are the non-plus-ultra cars for shopping. Easy to park (you can park three SMARTs on the same parking lot you are parking your car.)

Gayan Soyza

Try this.

In the circuit below, a quad voltage comparator (LM339) is used as a simple bar graph meter to indicate the charge condition of a 12 volt, lead acid battery. A 5 volt reference voltage is connected to each of the (+) inputs of the four comparators and the (-) inputs are connected to successive points along a voltage divider. The LEDs will illuminate when the voltage at the negative (-) input exceeds the reference voltage. Calibration can be done by adjusting the 2K potentiometer so that all four LEDs illuminate when the battery voltage is 12.7 volts, indicating full charge with no load on the battery. At 11.7 volts, the LEDs should be off indicating a dead battery. Each LED represents an approximate 25% change in charge condition or 300 millivolts, so that 3 LEDs indicate 75%, 2 LEDs indicate 50%, etc. The actual voltages will depend on temperature conditions and battery type, wet cell, gel cell etc.

Attached Images

Lead Acid Battery Monitor.gif (3.4 KB, 14 views)

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Gayan

My Website
http://gsmicro.blogspot.com/

Boncuk

SMART:

SwatchMercedesart

3 cyl. 599ccm, 61DIN-HP, max. speed 135km/h (85mph)