I've connected the Raspberry Pi's 5V and GND to the Outputs of a buck converter(https://www.droking.com/30W-Power-Supply-Module-DC-5-32.0V-to-0-30.0V-Buck-Converter-Adjustable-Voltage-Regulator-DC-12V-24V-Adapter-Driver-Module-With-Voltmeter) to which I've connected a 7.2 V NiMH battery. I've set the buck converter to output a Voltage of 5.1V. According to the Documentation, the Raspibeery Pi model that I'm using(Raspberry PI 2 model B ) needs 5.1 V and should be supplied with 1.8 A - 3.5 mA of current according to (https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md)my buckconverter can output 1.5A. I've been able to succsessfully power the Raspberry PI with a 9V battery in this configuration, however when I try to power the Raspberry Pi with a 7.2V 2000mAh Rechargeable Battery(https://www.lprcs.com/duratrax-nimh-stick-battery-2000mah-7-2v-6s-dtxc2035) the PI powers up shortly and then turns off again and the Voltage drops to around 3V. Is it because the buck converter cannot supply enought current? If so, why does it work with a 9V Battery? I've even tried powering it with two 7.2 V batteries in series whith out any better results.