Charging coin cell battery and feeding the board circuit at the same time

[Tizana]

Hi, i'm designing my bluetooth low energy board and i need your help regarding the power management.
i use a coin cell lithuim battery LIR2450 to supply my board. this battery is a rechargeable one. here's the datasheet of the battery : http://www.farnell.com/datasheets/1475807.pdf

I'm using a switch to select between charging the battery and letting the battery supply the circuit board.
Here's a simple schema of the system:



switch positon (1) : the battey is connected to the charger circuit. and the charger circuit is supplied by a micro USB port.
switch positon (2) : the battery is disconnected from the charger circuit and conneted to the circuit board.

the charger circuit is the MCP73831T and it deliver 4.2V to charge the lithuim battery. here's the datasheet : http://ww1.microchip.com/downloads/en/DeviceDoc/20001984g.pdf

When the battery is fully charger the voltage is equal to 4.2V and when is completly discharged the battery voltage drop to 2.75V. this is the reason why i use a Ultra-Low Power Buck Converter, the TPS62737 , to drop the voltage to 2.5V here's the datasheet : http://www.ti.com/lit/ds/symlink/tps62736.pdf
this buck conveter is good for my board because it has a good efficiency in low current and my board consume a low current.

You can notice that when the battery is charging the circuit is turned off using the switch. so i want to modify the circuit to let the board charging and supplying the circuit at the same time. below the new schema :



so lets suppose that the USB is not plugged in, meaning the battery is not charging and it's only feeding the board circuit through the diode D1 .


when we plug in the USB the diode cathode will have a higher voltage then the anode (5V vs 4.2V). this will cause the diode blocking. in this case the battery is charging and the circuit board is feeding directly by the USB through the TPS62737 buck conveter.



I want to know if this schema is OK and how the TPS62737 buck conveter will react with this brutal variation of the input voltage. moving from 2.9V when the battery nearly discharged to 5v when we plug in the USB?


and i want to know if the voltage drop of the diode won't deteriorate the power efficiency?? because i want that the battery will hold energy for a long time. if there is another solution for my problem i'll appreciate that.


note that the circuit consume in average when all the circuit are idle mode 20uA.

regards,

 

[Bluejets]

Only issue I can see is that the Lithium Iron cell has no protection and given they have a history of bursting into flames for no apparent reason, I'd be thinking about that.

 

[Tizana]

Only issue I can see is that the Lithium Iron cell has no protection and given they have a history of bursting into flames for no apparent reason, I'd be thinking about that.

there is no way to protect it ? like add another diode between the charger circuit and the battery ?

 

[AnalogKid]

Your overall plan looks good. The TPS datasheet should have some transient response information or plots. Another approach is to eliminate both diodes and connect the battery charger directly to the battery, which is connected directly to the TPS. This only works is the battery charger has nothing after its output rectifier other than filter capacitors. anything else will drain the battery when the USB is unplugged, requiring a diode between the charger and the battery.

ak

 

[Bluejets]

I was thinking along the lines that other manufactured packs have i.e. encased monitoring with feedback to the charger or cut-out in event of over-temp or over-charge.