I am working on something that I'd like to have a backup battery source for some of the components, it's going to be on a 5V circuit, but everything that needs to be backed up should tolerate anything down to 3V fairly well.
It is all fairly low current, probably under 100ma and wont need to last all that long, so I was thinking about using a single 18650, and when I was looking into doing float charging, I saw that you could do it, if you kept the voltage on the cell low.
I went with this chip for my voltage regulator Digikey Datasheet
By using a 42 ohm 1w or 1/2w resistor, I should limit the current going into the battery to 100ma, and by putting a diode coming out of the Battery to the rest of the circuit I will prevent any accidental charging of the battery from elsewhere, is there anything I'm missing?
Best Answer
I see a spec of 50mA not 100mA
You may consider an adjustable regulator suitable @2A for around same price. ($0.25/1k) and limit current with a 150mA PTC.
http://www.digikey.com/product-detail/en/richtek-usa-inc/RT9025-18GSP/1028-1008-2-ND/2470042
Then design for 3.7 to Vbat and use a 0.2V Schottky diode@100mA for 3.5 Vout to give more headroom for LiPo drop to 3.2V and 3.0 V min standby
When searching for a better diode, you may have to increase Vf a bit and If x10 then come up with a solution like BAT60A 0.2V@120mA@25'C rated for 3A max ($0.10/1k)
High current rectifier Schottky diode with extreme low VF drop (typ. 0.12V at IF = 10mA. http://www.digikey.com/product-detail/en/infineon-technologies/BAT60AE6327HTSA1/BAT60AE6327HTSA1CT-ND/1557845