Is this circuit good for watching a 5V battery?

Goodmorning everybody,

Recently I've purchased a powerbank for my Arduino and I need to check if the powerbank is running low on power (at runtime).

I've found a example of a 12V voltage circuit: http://www.janspace.com/b2evolution/arduino.php/2010/06/26/scooterputer

I suppose when the lights go red, the battery is dying, and need to be replaced/charged. I want to make such a thing for my 5V powerbank, but can I use the same resistors?

If not, can you show my how to calculate the resistance values for the resistors, so that I can do it right in the near future?

Thank you in advance,

Tim

0 peterjcs23 over 9 years ago

Tim, if your Suuply voltage is VSup, RA is the 10k reistor, RB is the 4.7k resistor then VAn is the voltage applied to the analog input in the link you gave us, then the equation is:

VAn = VSup (RB/(RA+RB))

with the original numbers VAn is 12 (4.7/(10+4.7)) = 3.84V, which is the voltage the software will detect to put the light on

with VSup=5V, VAn will be 1.60V

Peter
Cancel
Vote Up 0 Vote Down

Sign in to reply

Verify Answer

Cancel
0 tim687 over 9 years ago in reply to peterjcs23

Thanks!!

I'll write down the formula and use it next time!

Do you think I've got plenty of range? So if the VSup is low because it's empty, do you think VAn won't be 0 V?
Cancel
Vote Up 0 Vote Down

Sign in to reply

Verify Answer

Cancel
0 peterjcs23 over 9 years ago in reply to tim687

You need to decide what is the minimum voltage at which your circuit works correctly but you need to warn about the battery. If the battery drops too low then the circuit stops working and you can't run the monitor anyway. For example, If say 3V is the minimum working VSup then your monitor software will be looking for VAn input of 0.96V.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Verify Answer

Cancel
0 artful_bodger over 9 years ago

The 2 resistor circuit is called a potential divider and is used in lots of places in electronics. This link http://www.electronics2000.co.uk/calc/potential-divider-calculator.php is to a calculator, so you can play with the resistor values to get the output voltage you want.
I'd check with the manufacturer of the powerbank to see if they specify a minimum voltage, as most types of battery are permanently damaged (aka life shortened) by continual discharge to a too low a voltage. You might even find your powerbank shuts itself down, to protect the battery, once it can no longer keep the USB 5V line within specification, which IIRC is 4.75V. This would mean you need to shut down at say 4.9V to give yourself a bit of margin on ADC and reference errors.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Verify Answer

Cancel
0 tim687 over 9 years ago in reply to artful_bodger

Thanks!
I'll lookup the voltage for various battery charges.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Verify Answer

Cancel