Non rechargable backup battery

[arpa123]

If I'm using an Arduino or an ESP8266 NodeMCU board and have it connected to a power supply, how can I set up a backup battery?

Let's consider a scenario: the board has been running on the power supply for one year, and then a power outage occurs. In this situation, I want the board to switch to a backup battery, not necessarily for an extended period, but just long enough to send a notification indicating a power outage.

Please note that the battery is not rechargeable and is meant to be in place solely as a fail-safe measure in case of power failure.

What components or methods can I use to accomplish this?
the point is while battery not used, not to discharge it

[ShermanP]

What is the voltage of your power supply, and what battery will you be using?

[Paul_KD7HB]

@arpa123

the point is while battery not used, not to discharge it

In the real universe, ALL batteries are chemical devices and will self-discharge. Even atomic backup devices have a limited lifetime.

[LarryD]

Feed the power supply and battery to two common cathode Schottky diodes.

Your power supply needs to be greater than the battery voltage.

Feed the cathode output voltage to a Boost/Buck converter set to the controller supply voltage.

[alto777]

@arpa123

just long enough to send a notification

is quite possible with a largish capacitor and a few other components. The technique is used often to afford an opportunity to write to EEPROM.

Google and or see

Saving to EEPROM on power-out

You need to get the power out event to trigger an interrupt. Detecting it is tricky. One way is to feed the power through a diode with a large capacitor on the arduino end. Then use a voltage comparator on the input end to detect when the power is removed. The size of capacitor used depends on how much current your circuit draws.

a7

[traisjames]

@LarryD

Feed the power supply and battery to two common cathode Schottky diodes.

Why Schottky diodes as opposed to regular? As long as both the external supply is a slightly higher voltage won't the Arduino just run on that, but if it drops low or out the batteries take over?

[jhaine]

They have lower forward voltage drop.

[LarryD]

Not sure what batteries you are using.

Here is one way to connect things.

Schottky diodes have a Vf of about .2 or .3V where Silicon is .7 to .8V

• When the UNO is connected to the PC via USB, disconnect the Boost/Buck converter.