Powering circuit directly from USB while charging li-ion battery side by side

Question

What I am thinking of implementing:
1. I have remote monitoring circuit using gsm which senses electrical parameters like phase, power, ac supply etc. Now considering case of no ac supply available then also device should be able to send notifications. to achieve this I am thinking of running module on battery(3.7v Li-Ion).
2. As it is remote monitoring system, so battery will always be connected to supply for charging.

As per my knowledge one should avoide keeping battery connected for charging for all the time, it will reduce life of battery/overheating/overcharging are the parameters comes into the picture.

• If I do like |Wall Adapter((ac-dc smps)|-->|battery charger|-->|circuit| above mentioned parameters will participate.
• If I do like Wall Adapter and battery to use in parallel using OR ing with two diodes or using relay or using IC's like TPS2104/MAX 6326/TPS2110 etc then how can I take care of side by side battery charging?

I googled so many things to implement this but not found useful or may be I am not looking into right direction. I referred similer link 'http://forum.allaboutcircuits.com/threads/how-to-charge-a-battery-while-supplying-power-to-my-device.67530/' but no luck.
Even I looked into the data sheets and videos for li-ion charging circuits using MCP73831 or TP4056.

I am totally confused how to proceed further. Is it a good idea to use |Wall Adapter((ac-dc smps)|-->|battery charger|-->|circuit| or
using Wall Adapter and battery in parallel and side by side charging battery. or to use wall adapter when supply is available and side by side charge battery so that I can use battery when power will be absent.

I know UPS works same way like it powers directly from supply all the time and charging battery side by side. At the time supply is interrupted it switched to the battery.
I am looking for simple solution, and I think this is a right forum I am looking for a solution. Thanks in advance...

Answer 1

I don't know whether this is a 100% perfect solution as per my requirement or not but seems to be working good and very much similar to the solution I was looking for.

Solution is Load sharing: charging the battery and have the main circuit run normally
We can use any charge controller ic I am considering here is MCP73831 because of low cost and good features and 5-Pin SOT-23 package, just simple to use.

• When USB power is applied this circuit will turn off Q1(P channel MOSFET), and as long as (Vusb – D1 VF) > (Vbat – Q1 VSD) then the load will instead use power from USB through D1.
• When USB power is applied Q1 will turn off and stop current flowing from the battery to the load, effectively disconnecting the battery. The load will then use power from USB through D1.
• The time USB power disconnected, Q1 will turn on and battery will get connected to load.

Will have to test this for charging cycles per day because my requirement is to have low number of charging cycles(as charging/discharging continuously is not preferred and my circuit will be at remote position and will be powered ON 24x7).

For time being this solution looks good to me.

References: Thanks Microchip for good explanation on Load sharing concept ww1.microchip.com/downloads/en/AppNotes/01149c.pdf
and a good blog from
blog.zakkemble.co.uk/a-lithium-battery-charger-with-load-sharing/

as I have lower reputation so cannot share more than two links hence links posted as a string(sorry...)

Please correct me if I am wrong.

Answer 2

I would post this as a comment to your own answer, but I can't because I don't have 50 rep...

Anyway, your circuit will technically work for what you're trying to do in that your load will be powered from USB. It seems as though your PFET drain and source terminals should be switched. Right now, your load is being powered by USB only because of the inherent diode from drain to source in your PFET, not because you've gone out of the Vgs threshold as you expect.

Switching your PFET drain and source will indeed cause your circuit to perform as expected.

EDIT: DON'T SWITCH THE SOURCE AND DRAIN. The reason they are oriented the way they are in OP's answer is so that the inherent diode in the FET is used as a blocking diode to protect the battery and avoid shorts. In the case of Lithium batteries, the USB voltage (5V) will always be higher than the voltage of the battery (~4.2V max). If the drain and source are reversed as I have previously suggested, the USB input voltage will be shorted with the battery through the inherent diode. Sorry for any confusion.