Low Battery Indicator

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Mohamed Najiullah

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We are trying to design a low battery indicator and full battery indicator for 1.2V NiMH batteries. Possible types are two batteries(2.4V) in series or three batteries(3.6V) in series. Can someone give me a few pointers or if possible a circuit diagram?
 
The charge state of NiMH batteries can not be reliably determined by measuring the voltage expect that when the voltage drops to about 1V, they are essentially discharged.
 
crutschow said:
The charge state of NiMH batteries can not be reliably determined by measuring the voltage expect that when the voltage drops to about 1V, they are essentially discharged.
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Because NiMH batteries maintain their nominal voltage of 1.2V until they are completely discharged?
 
TL431/LM431 will make a simple low-voltage detector from any voltage ≥ 2.495V. Look up the data sheet.
 
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According to a typical NiMH discharge curve at low current levels, if you detected a low voltage of 1.15V, the battery would have perhaps 5-10% of it's charge remaining. Beyond that it's not possible to reliably determine the state of charge of a NiMH battery from its voltage.
 
We decided on using the Energizer 2450mAh AA batteries. I looked up a few graphs of the voltage Vs Time and saw that the fully charged battery can have upto 1.6V.

Assuming we use 2 such batteries I can make a Low Battery Indicator with a MAX8211 for 2.3V minimum input(1.15x2).

But what about a charge level indicator for the actual charge on the battery like green for fully charged, orange for partially and red for exhausted battery? Can someone help me design a circuit for that?
 
We decided on using 2 Energizer 2450mAh 1.2V batteries.

I checked out its graph for voltage Vs Time for varying discharge currents. It says its voltage can go upto 1.6V when fully charged.

So I can make a Low Battery Indicator for 2.3V(1.15x2) using a MAX8211. But i was kinda hopin for an actual charge indicator like green for full charge, orange for partial charge, and red for complete discharge. Can u help me design something for that?
 
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