# Powering a Raspberry Pi + a display using phone battery

This may be a rookie question and if it is, I apologize.

I want to power a circuit with Raspberry Pi Zero + ILI9341 2.4inch Display with backlight using a phone battery. Intention is to boost phone battery voltage (3.8V 5350mAh with Charging Limit 4.35V) to 5V using WeMoS Battery Shield (https://wiki.wemos.cc/products:d1_mini_shields:battery_shield) , then power the circuit. However, the circuit doesnt work / rpi doesn't boot up correctly. If I simply plug USB port, the circuit works well.

Any pointers as to how to solve this problem is greately appreciated.

• The shield says "Boost Power Supply: 5V(max: 1A)" so a likely reason why your RPi doesn't boot is that it requires more than 1A. Feb 14, 2018 at 16:50
• How much current does the Pi Zero + display draw? Feb 14, 2018 at 16:50
• You can also try adding a hefty electrolytic cap (e.g. 1000µF) at the shield's 5V output, as it may be a transient current issue. Feb 14, 2018 at 17:05
• "if the current draw indeed around 0.3A, it's a mystery why it doesn't work." - youtube.com/watch?v=aND0j2Y2IkM 4:20 Feb 14, 2018 at 17:24
• @BruceAbbott, I can only wish that all chinese products out there had such a well researched and nicely presented review :) Kudos to the youtube guy! Feb 14, 2018 at 18:11

When I connect the circuit, output of 5.020V drops to 4.8V

The lowest value reached might be even lower than 4.8V. If that value was measured with a multimeter (DVM) then it's unlikely the 4.8V reading was the lowest voltage reached, due to the low sample rate of DVM. [See later update below - an even lower voltage of 4V was subsequently reported, and the measurements were done using a UNI-T UT61E multimeter.]

You would need to use an oscilloscope (or similar high sample-rate device) to record the voltage at the RPi itself, looking for short dips in voltage, to understand more about how the power output from that "battery shield" behaves during the RPi boot.

The problem might be mitigated by adding additional bulk capacitance at the output of the "battery shield" (or on the power pins of the RPi). Also minimising the length of any wiring (inductance) between the "battery shield" and RPi can only help.

Update:

As mentioned in comments below, the "battery shield" output voltage was seen to "even drop to 4V once". So, as I said in a reply to that, there is a fundamental incompatibility between the power output from that "battery shield" and the power requirements of the connected devices (RPi Zero and display).

In comments, Bruce Abbott has kindly found a review of that "battery shield" on YouTube by user "Great Scott!" here:

Thanks to Bruce for finding that. The video is worth watching and it shows the output voltage collapses at an output current of only around 300mA (which is the "guess-timated" current drain of the attached devices in this case). Also there is no over-discharge protection and no output short-circuit protection.

Summary:

Therefore the actual output current capability of 5V at 300mA from these "battery shield" boards as shown in that video, does not match their claimed specification of providing 5V at 1A. If the current required for the display and RPi during boot was enough (even momentarily) to cause the "battery shield" output voltage to drop significantly, as shown in that video, then that would explain why the RPi is unable to boot successfully.

If I simply plug USB port, the circuit works well.

A typical PC USB 2.x host port will supply at least 500mA, even without enumeration of the connected device (although host ports are not required to do that). The video gives evidence that the "battery shield" supplies an even lower current of only 300mA, before the voltage drops below that usable for the RPi. This would explain why connected devices which require between 300mA to 500mA could behave correctly when powered from a PC USB port, but not when powered from that "battery shield".

• I also find it fishy that the shield's output drops 0.2V at just 30% of the rated output current. To me this is either 1) very poor regulation, or 2) defective shield, or 3) the current is much higher than 0.3A. Feb 14, 2018 at 17:12
• I saw it even drop to 4V once. So I am definitely considering your suggestion right now. Feb 14, 2018 at 17:17
• @anrieff - Completely agree (and I now see from your comment that we both suggest adding some bulk capacitance, at least as a test). Feb 14, 2018 at 17:18
• Measured with a UNI-T UT61E Feb 14, 2018 at 17:21
• @mozcelikors - "I saw it even drop to 4V once" This confirms that fundamentally, your "battery shield" is incapable of supplying the current required by your hardware. As anrieff said in the comment above, there are a number of possible reasons why it could be incapable of supplying the required current. Feb 14, 2018 at 17:23