While loading all the packages and software I need for my battery capacity tester / analyzer on my new pi 3 B+, I would get the undervoltage lightning bolt, and the pi would reboot. Half the time, it would crash again before getting to the desktop. If I was lucky enough to get that far, it would crash again while trying to install packages like apache2 and php7.
I have been using a simple power supply circuit for a long time based on an LM7805 voltage regulator chip in a TO-220 package plus a good heat sink, and with a couple of reservoir capacitors to power my pi projects.
It has worked great for me with no issues using a pi 2B and the pi 3B. On the new pi 3B+, however it simply is not enough. The new pi 3B+ has an aluminum heat sink to improve thermal management issues due to the 1400MHz clock rate of the CPU and requires a 2.5 amp power supply. The particular LM7805 chip I use can supply 2 amps; enough to power on, but cannot supply power under a significant load.
I am looking into using an LM2576, however it requires several external components that I did not have on hand like a 100uH inductor, and I wanted to get this dude up and running right now.
I decided to try and see if underclocking the CPU would get me by. I added the following to /boot/config.txt to throttle the clock speed to 900MHz:
After rebooting, I still got the occasional lightning bolt, but it never shutdown. I then put the pi 3 B+ under a prolonged stress test that maxed out one core of the CPU for over 10 minutes and browsed heavily on chromium during the test. I would reach 100% capacity on the CPU, but it never shut down. I will pursue a better LM2576-based power supply circuit, but this is a good temporary fix.