Capacitor for LIPO 3.7v

I have PRO MINI 3.3v and SIM800L powered by a LIPO 3.7v (max 4.2v).
It's battery based project and I want to last many months. (3-4)

SIM800L is powered directly from battery.

I would like to know if I need capacitor to able to have stable power? Because SIM800L says that can draw 2A. In my case I don't use calls or data, only the SMS but it's possible to happen.
Which capacitor it's suitable (farad)?

An other thought has to use auto adjustment step down/up to able to get more juice from battery.
What you recommend?

Something like Electrolytic Capacitor 25V 1800uF Low Imp

You cannot get more "juice" out of a battery than its rated Ah capacity. (Having made allowances for the variation between specified Ah and actual current drawn due to the Peukert effect)

Cap may help, but only if the pulse of high current is very brief. Good power management (sleeping the cell modem most of the time) will be key here. 3-4 months is a lot to ask of batteries when you have a power hog like a cell-modem in play. How many cell phones do you know with a 3 month battery life?

Something like Electrolytic Capacitor 25V 1800uF Low Imp

Rather small. GSM power spikes are 2A or more for 0.58ms, which requires capacitance of more
like 6,800uF+ to keep supply voltage droop within 0.2V. 1800uF would result in > 0.7V of
droop for full power transmit bursts, which is unacceptable.

LiPo batteries with decent current handling will be perfectly fine with 2A or more. Remember this
is exactly how mobile phones work. There isn't room inside a modern mobile for a large capacitor!

LiPo battery self-discharge rate should be low enough for several months operation, so long as the
average discharge you subject it to is small enough.

It is very easy to add more Ah by getting larger batteries or multiples in parallel.

But we do not know what your average current draw will be.
What is your application? How often will you be transmitting?
With this information, someone here might be able to help you make an educated guess on battery capacity requirements.

If it were me, I would get the project working and attach a battery large enough to make it work. Then let it run for some known about of time, say a day.

Then I would charge the battery with a charger that tracks the charge to see how many Ah were put back in.
Then you know Ah per day used. Multiply by the number of days you want and then get a battery larger than that number.

Hi thanks for the reply.

SIM module is completely off with mosfet when arduino is sleeping.

It transmits 2 times a day. It's active less than 2-3min per day.

I don't remember the consumption. I have to re measure.

So it's pointless to add capacitor?

SIM module works from 3.4 to 4.2v.

What's the C rating for the LiPo. If its more than 1C its completely fine with the ~2A max transmit level.
The power wiring must be sound of course. The SIM module works in mobile phones, phones run off a
single LiPo.

You can add a capacitor if you like but only a large one will do anything much. It might extend
the life of the battery a bit when its getting low of charge, but it won't make huge difference.