Best way to implement battery backup and charger circuit?

Thread Starter

summersab

Joined Apr 8, 2010
152
I have a number of project that I would like to make available in the event of a power outage. None of them are especially power-hungry, but some are sensitive to voltage ranges and require a regulator (others do not). Let's take a RPi and a ESP8266 as examples. The Pi will work just fine from a Li-ion battery while the ESP needs to have a regulator to get it down to >3.6V. What is the proper way to hook up either board, a Li-ion charger, a Li-ion battery, and a regulator (for the ESP) so that the boards run from the battery when a charger is not present but run from the wall and allow the battery to properly charge? Do I simply hook up the board (plus regulator) and the battery in parallel to the charger, or is that unsafe? I always get a little nervous when messing with lithium batteries (or any rechargeable battery, for that matter - they're all a tad volatile).

Thanks!
 

Papabravo

Joined Feb 24, 2006
19,023
I'm a bit confused. What supply voltage are you trying to get down to 3.6V? A typical LI-Ion Battery works between about 3.0V and 2.7 Volts. Maybe we should be a bit more clear on exactly what your power supply arrangement looks like. Then you should spend some time at Battery University to educate your self on battery chemistry and charging methods. Having multiple supply sources is usually done with diodes.

http://batteryuniversity.com/
 

Thread Starter

summersab

Joined Apr 8, 2010
152
I had a feeling that diodes were going to be involved. Headed to Battery University, but I'm kinda confused about your comment that a Li-ion works from 3.0-2.7V. I'm definitely a novice with electronics, but I'm pretty sure that lithium cells are 3.7V nominal, 4.2V max, and 3.2/3.3V min (depending on the protection circuit). I know that much, at least.

UPDATE: Looked at Battery University. It's definitely good material, but it's also pretty introductory and doesn't cover much circuitry (if any). I don't see any discussion of how to properly implement dual power sources with a battery as a secondary source.

UPDATE TO THE UPDATE: Okay, all I had to do was add the word "diode" to my Googling. Problem solved:
https://www.allaboutcircuits.com/projects/battery-backup-power-supplies/

D'oh!
 
Last edited:

Papabravo

Joined Feb 24, 2006
19,023
I had a feeling that diodes were going to be involved. Headed to Battery University, but I'm kinda confused about your comment that a Li-ion works from 3.0-2.7V. I'm definitely a novice with electronics, but I'm pretty sure that lithium cells are 3.7V nominal, 4.2V max, and 3.2/3.3V min (depending on the protection circuit). I know that much, at least.

UPDATE: Looked at Battery University. It's definitely good material, but it's also pretty introductory and doesn't cover much circuitry (if any). I don't see any discussion of how to properly implement dual power sources with a battery as a secondary source.

UPDATE TO THE UPDATE: Okay, all I had to do was add the word "diode" to my Googling. Problem solved:
https://www.allaboutcircuits.com/projects/battery-backup-power-supplies/

D'oh!
You're right, I was remembering the conditions for the end of the discharge cycle. A fully charged LI-Ion cell is at 3.7V-4.2 but drops rather quickly at the beginning of the discharge cycle. The greater the demand the faster it drops.
 
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