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 : https://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 : https://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 : https://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,

 

[FvM]

Looks basically feasible.

I believe, you'll need another diode to prevent current flow back to the USB connector.

 

[D.A.(Tony)Stewart]

You could put a Silicon power diode across the #1 switch to supply current if the coin cell drops below 3.6 and charger happens to be on.

 

[Tizana]

Looks basically feasible.

I believe, you'll need another diode to prevent current flow back to the USB connector.

Thx for the answer.


Do you think the diode D1 is a good choice here's the datasheet : **broken link removed**
i think it has the lowest forward voltage drop i've seen till now.

the second diode that you suggest me to prevent the current flow back to the USB it can be the same reference as the diode 1 ?? and should i put it in which position ?



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You could put a Silicon power diode across the #1 switch to supply current if the coin cell drops below 3.6 and charger happens to be on.

Thx for your answer.

can you explain me why should i add a silicon power diode if the battery drop below 3.6 V ?

this is the modification you mean :



by the way the lithuim battery whan fylly charged is equal to 4.2v and when its fully discharged the voltage drop to 2.75v. according to the battery Typical Discharge Characteristics curves :

 

[FvM]

A disadvantage of the shown diode is the large reverse current which will considerably reduce battery lifetime. You should definitely choose a diode with guranteed reverse current in the low µA range.

You should use the diode configuration that isolates the battery charger inout from the battery.

 

[D.A.(Tony)Stewart]

I am not sure why you show a switch for the coin cell to be (1) charged with no load or (2) drive Buck regulator with no charger.

Your Title suggests you want to do both.

I only suggested a silicon diode so that battery won't drop below 20% SOC by haveing 4.2 charger-0.6V diode= 3.6V to prevent excessive underchargeof coin cell if charger is connected.

In reality, LiPo cells charger cycles need to be ended at 100% SOC so that 4.2V drops to 3.75. I suggest if charger must be designed to shutoff, why not design it to reduce to voltage of 3.7 to 3.75 to drive load with LiPo connected drawing very little current except during initial stage of discharge.

Just a suggestion.