Anyone here who has powered raspberry pi with batteries like lipo or alkaline batteries? I need to know how to do it.
You will need a voltage regulator to properly supply your pi (PSU @ elinux.org, there is related questions on RPi SE). Additional protections should be considered if you feed through GPIO.
Basically to answer you question, you can use the Peukert's Law as a first approximation:
C = t*I^k
k is an adimensional constant ranging in [1.1;1.3] depending of the battery,
I is the current drawn (you need to sum all components of your circuit, including your GPS. RPi draws 1A when idle),
C the battery capacity (the fundamental specification of the battery with a dimension in A.s) and
t the discharge time.
t = 1/2h, I = 2000mA and
k=1.2 leads to
C = 0.5*2000^1.2 = 4573 mAh, then you better to select a
Battery voltage must be chosen accordingly with the voltage regulator you selected. Do not forget that you need a good 5V supply in order to service your RPi.
Caution: LiH battery needs to be handle with care, it may explode and cause injuries. You need a dedicated circuitry to charge and discharge it. If you are not used to, select regular NiCd or alkaline Battery.
Something that works well for me is using a portable power pack as opposed to a battery. These are convenient due to the fact that they can plug up via the usb port, and they are easy to charge. I have a solar powered one thats around 2000 mAh that can run my pi for an hour or 2 (havent timed it just ball parking it). Or recently i got a portable charger that has a 6000 mAh battery that can power it for at least 4 hours (once again just ball parking it). I have tried the battery method before but it seems a bit overly complicated and runs a risk of ruining my Pi. So thats why i opted for this. Anyway i hope this helps you out. I highly recommend this route above any other. Cheers!!!
As already pointed, battery voltages will decrease with utilization (consumption), so you will need some sort of circuit to regulate such voltage to 5V for the Pi.
Lipo cells (single ones) for example will have voltages around 3,7V so you will need a boost (step up) circuit; putting them in series (say three of them) will give you around 11,1V, so you will need a buck converter (step down).
There is also the case when you may need a buck-boost circuit, since the voltage of the batteries can go below or above (say) 5V. I recommend a UPS I found on Aliexpress, which will recharge lipo's and provide a clean stable voltage output for the pi: https://pt.aliexpress.com/item/32783727347.html .
Also keep in mind that you will need to calculate the quantity of batteries based on how long you want to power your system for. In my specific case I am supplying a Pi 4 + 7" HDMI display (for a total of 7W) for 7 hours on 6s (cells) Lipo.