Arduino mega+sim808 power consumption

I'm trying to feed arduno mega and sim808 module (for tracking and texting purpose) with 2s 18650 battery pack, i'm using two Sanyo NCR18650GA which should have a capacity of 3200-3400 mAh.
The system is about half the time in normal mode (fetching gps data and, eventually, sending an SMS if coordinates change) and half the time in sleep mode (arduino in pwr_down mode and sim808 in AT+CSCLK=2 mode), I was guessing that the system can stay on for about one day/one day and a half with that power source (assuming an average consumption of 150mA), but about after 12 hours the battery pack reaches a critical state (about 7.2V, that is about 3.6V for each cell), I have no way to measure current consumption directly, but how much could it be? Am I doing wrong anything?

How are you charging the battery , are you sure it’s fully charged ? How many mAh has it taken when charged ?

You really do need to get a meter to measure the current consumption to identify where the issue lies .

The capacity of the battery is probably calculated to a voltage lower than your 3.6 volts ( 3.0? ). 3.0 volts is the voltage used in the likes of RC vehicles and I recall this aligned well with stated capacity when recharged .
3.6 volts seems a bit high , I would take them lower . If you google battery characteristic 3.6 v does align with about 50% capacity

You should be measuring your battery voltage without a load , with an accurate meter ..

Thanks for your advice.
I think the batteries are fully charged because I use a regulated charger and, when I measure voltage after charging it is 4.2 V each, which should be the value for a fully charged cell.
You could be right: from this chart it seems that with a voltage of 3.6-3.7 the cell should have a charge between 33-50% (third column) of the maximum capacity image

But I don't have a bms yet and I don't want to let the batteries go too low, to avoid any possible damage.
When I have a protection circuit I'll test it till the bones!

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