# Low Battery Indicator

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#### Mohamed Najiullah

##### New Member
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?

#### ericgibbs

##### Well-Known Member
hi,
Look at the MAX8211 and MAX8212 IC's

#### Mohamed Najiullah

##### New Member
hi,
Look at the MAX8211 and MAX8212 IC's
I can certainly use MAX8211 for either undervoltage or over voltage indicators but i was kinda hoping for a solution for both under and over voltage indicators separately in a single IC or circuit

#### crutschow

##### Well-Known Member
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.

#### Mohamed Najiullah

##### New Member
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.

Because NiMH batteries maintain their nominal voltage of 1.2V until they are completely discharged?

#### crutschow

##### Well-Known Member
Because NiMH batteries maintain their nominal voltage of 1.2V until they are completely discharged?
That is correct.

#### Mohamed Najiullah

##### New Member
That is correct.

Ok. But still i suppose i can use the MAX8211 to indicate the state of charge.. That is whether the battery can still be used or not.

#### MikeMl

##### Well-Known Member
TL431/LM431 will make a simple low-voltage detector from any voltage ≥ 2.495V. Look up the data sheet.

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#### crutschow

##### Well-Known Member
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.

#### Mohamed Najiullah

##### New Member
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?

#### Mohamed Najiullah

##### New Member
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?

#### Mohamed Najiullah

##### New Member
Any ideas for charge status indicator? Like green for fully charged, orange for partially charged and red for amost discharged..

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