i have a lithium battery with 3.7 volts and 9900mAh
i want to know if i am to use two of it and power the arduino using it, wont it destroy y arduino?
im planning to use it directly no, like ill put in in battery holder and connect directly to arduino
please help
No, it probably won't destroy the Arduino, if you connect them in series to make 7.4V and connect to Vin pin it should be ok.
But I have important questions.
What model of Arduino?
What size/shape of battery?
Do the batteries have built-in over-discharge protection? If not, they can be damaged by over-discharging and can then catch fire or explode when you re-charge them.
I suspect you have been cheated. These batteries will not be 9900mAh. A long time ago when I knew little about lithium batteries, I also purchased some which claimed to be 9900mAh. I grew suspicious and measured their actual capacity with a smart charger. The true capacity was around 500mAh, 20x less than they pretended to be. They were 18650 size and the brand was something like "trust" or "fire" or something.
Later, I purchased some major-brand 18650 lithium batteries, I think they were 2400mAh and the brand was LG.
I weighed the LG batteries and the "9900mAh" and the LG weighed about 2x heavier.
I am assuming they are protected batteries. I would recommend connecting them in series and using a SEPIC (Buck/Boost) converter, set the output to about 7.5V and connect to the Arduino's Vin terminal. This will keep the voltage constant through the full battery charge and discharge cycle. I use Vin because of the additional filtering and it current limits the system and may save you from frying an Arduino.
Wire them in series and connect to the barrel connector/Vin pin. I see PaulRB comment about over-discharge, which I don't think is an issue: the 5V regulator will stop after the battery voltage drops below ~7V, which is still well within safe zone of your batteries.
Charging those batteries in series will be an issue, unless you remove them and recharge in an external charger.
That is correct, they will stop regulating, they will just start putting out a lower voltage. They require a minimum overhead voltage to regulate. Once the input drops below that the output voltage starts to drop as well it does not shut off. This can range from 2V to maybe 100mV for a LDO (Low Drop Regulator). Check the curves on the data sheet for a particular regulator.