# Backup LiPo Battery charging circuit

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?

• How are you going to facilitate constant current charging for the battery? That's a 3.9V regulator for those who don't want to look it up – Voltage Spike Sep 16 '16 at 22:39
• This circuit is not that great to be honest about it. First off, lets assume your battery is charged and you have a 32ohm resistor for your load. The voltage at the 3.2 Battery backup point will be ~1.7V from the drop of the 42ohm resistor and the 0.7V drop of the diode. – Voltage Spike Sep 16 '16 at 22:54
• EasyEDA took of the ~ for the 3.2V It's all 5v tolerant, and should be able to operate down to 2.2V – HilarieAK Sep 16 '16 at 23:26
• You say " ... when I was looking into doing float charging, I saw that you could do it, if you kept the voltage on the cell low." -> You MAY be correct BUT where did you read that. There are extremely few references (AFAIK) to float charging LiIon/LiPo at reduced voltage and I'd be very interested to see any. || FYI the various Mars rovers use LiIon at about 3.6V max and get 8000 cycles from them! (at much reduced per cycle capacity). – Russell McMahon Sep 17 '16 at 11:54
• @ Russell McMahon electronics.stackexchange.com/questions/78507/… – HilarieAK Sep 20 '16 at 6:30

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)