# determine battery level

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#### electricano

##### New Member
how to determine the battery level when it is low?can we measure the capacity of the battery or we determine from its voltage level?i need it for my robot coz when the power is low, it will alert me.
thank u

#### Nigel Goodwin

##### Super Moderator
electricano said:
how to determine the battery level when it is low?can we measure the capacity of the battery or we determine from its voltage level?i need it for my robot coz when the power is low, it will alert me.
thank u

You need to know exactly what type of battery you have, and it's discharge curve, there are some discharge curves here which I plotted years ago. It's up to you to decide at what point you need to initiate a low battery warning - as you can see from the Alkaline and NICAD (particularly the NICAD) they maintain a reasonably constant voltage and then suddenly drop. I haven't tested lead acid batteries (they don't make AA ones!), but I suspect they may well behave in a similar way to NICAD's.

#### eemage21

##### New Member
I'd just go with a voltage measurement.

#### electricano

##### New Member
how to implement voltage comparison to determine the battery level.if can, plz provide me links or circuit diagram.thank you

#### electricano

##### New Member
can anybody helps me with this one?????

#### dingo

##### New Member
Adjust R2 for whatever voltage you want to LED to light up at when it gets below that set point.

#### dingo

##### New Member
Here is another circuit that is very simple.

You get about .6 of a volt drop over a doide, a transistor needs at least .6 of a volt to switch. The buzzer in the diagram below will sound if the voltage gets below 2.4 volts (that is if you can find a buzzer that works off less than 2.4 volts).

To make this work at 12 volts you will need around 19 doides and to adjust you just add or remove doides to give you .6 volt steps.

all resistors are 10k, transistor BC547 or any general NPN one, doides you can use any, a signal doide is ok.

#### Noggin

##### Member
hahah funny 'high power' batteries. Thanks for the link, I considered something like that for a high school science project but decided to give beer to newborn mice instead (no kiddin)

##### New Member
electricano said:
how to determine the battery level when it is low?can we measure the capacity of the battery or we determine from its voltage level?i need it for my robot coz when the power is low, it will alert me.
thank u

Hi

why don't you use a comparator. You can easily build it with simple opamp circuit. Please see LM301 data sheet. (National Linear series). You may find lots of application in the data sheet.
Regards,

#### jem

##### Member
Not saying that these comparator circuit will not work for voltage comparisons, I think that Nigel is absolutely right. A battery voltage-capacity curve depends on the type of battery, the ambient temperature, the discharge type (heavy or light), and the age of the battery. Since you did not mention the type of battery you are using, you could be up to 50% off on some types of battery chemistries.

Jem

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