How to use Maximum capacity of LISOCL2 Battery ?!

I'm designing a device which works with 1 D size LISOCL2 saft battery, in the pic below, it shows voltage discharge of the battery i.e in 3.5mA current:


you see after about 3000 hours of discharge (point 1), voltage level starts to decrease and almost the rest of time it will go down to achieve 2V (point 2).

the second part ( between point 1 and 2 ) is about 1500-1800 hours.

My whole circuit consumes 2mA current (1mA analog sensor + 1mA digital parts).
if we calculate 17000mAh capacity of the battery (saft LS33600) without self discharge ... device can work up to 8500 hours ideally.
in which almost the last 30% capacity of battery is decreasing voltage so my analog part of device wouldn't work fine.

Now my solutions:

Solution 1:
i have one digital part which can work down to 2V so no problem for this part.
but i have an analog sensor which should consume 1mA current so i adjust the resistor according to VCC 3.3V to achieve 1mA current in sensor.
when voltage drops, i have to change resistor and also sensor output thresholds reading (for counting). that would be little hard to do so, i think i can use 3 or 4 different resistors for 3-4 voltage levels so i switch resistors according to battery voltage ...

Solution 2:
another way is to use step up booster ! for example this circuit with Holtek HT7733A:


first question, i couldn't find in datasheet what happens if input voltage be more than 3.3V (in 3.3Vout model HT7733A or HT7733SA) ?

second question is about efficiency, for about 2mA current, the booster efficiency is about 80%, it means if device consumes 3.3V 2mA at Vout, and Vin be 3.3V, it will consume 2.5mA (Iin) from battery ? (same V, 2.5mA to 2mA = 80% efficiency)

in fact for using the last 30% of the battery capacity, i am adding + 25% current to whole time of device working period (because of about 80% efficiency) ...
means not really good solution !

i've thought of some other changes too, for example using higher efficient step up booster chip ...

or step up boosters with bypass feature like Texas instruments TPS61291 ? ( bypassing down to 3.3V and if lower, booster starts, so almost booster will work on last 30% of battery capacity). Quiescent current in bypass mode is less than 1uA in TPS61291.

Do you have any other idea how to handle this problem ? or any comment about my solutions ?!

Thanks.

- - - Updated - - -

hmmm, i think i was wrong, maybe i use Step-down buck ?!!

i just found it also decreases the input current too ! i.e i can use 2.1 Vout step down buck ( which my digital part works and i adjust resistor for sensor to pass 1mA).

for TPS62740, when Vbat is 3.6V and Vout 2.1V, efficiency is about 85%. for 2mA at Iout, it will be about 1.4mA at Iin.
:grin:

I'm still interested in any comment or suggestion ...

You will probably find the power supply takes more current than it saves. The other option, which may be applicable to your design is to turn off the sensor until it is needed.
In one particularly power sensitive design I power the sensor from an IO pin on the MCU and only wake it periodically to take a measurement. When running the current consumption is ~4mA but during the 'sleep' period it drops to < 1uA giving an overall saving.

Brian.

betwixt said:

You will probably find the power supply takes more current than it saves. The other option, which may be applicable to your design is to turn off the sensor until it is needed.
In one particularly power sensitive design I power the sensor from an IO pin on the MCU and only wake it periodically to take a measurement. When running the current consumption is ~4mA but during the 'sleep' period it drops to < 1uA giving an overall saving.

Brian.

Click to expand...

unfortunately the sensor can not be turned off because of high counting frequency (up to 2000 Counts/s). and it's output isn't digital, it's a sinus wave which i have to monitor all the time to find true count ... one period of sinus wave can be 500 u seconds. and the sensor needed time to be ready after turn off/on is more than that.
anyway, i am focusing on power supply part. i think the step down buck is the best solution to both stabilize Voltage and decrease current consumption from battery.