[Solved] Dummy Load Confusion (Witty Pi 4 L3V7)
I am still trying to discern why the Rpi Zero 2 will not always start when I apply a new charged battery.
I use a USB Power Bank.
Set low volatge theshold is 4.2v
Auto-On is Yes
Power Cut is 25 seconds.,
All the other settings are left at default.
I am testing 3 units. When a usb power bank is fully discharged I replace it with a fully charged one. I have found, sometimes, that the Rpi does not start up.
When in this state the Witty board flashes blue and the red light pulses.
If I leave it for a few minutes and then reconnects the battery it will start up.
In desperation I have looked at your other settings. I am looking at dummy load.
In your https://www.uugear.com/doc/WittyPi4L3V7_UserManual.pdf it is specified that if using a USB Power Bank it states this:
If you are using a power bank to provide 5V to the USB-C connector on Witty Pi 4 L3V7, in
the majority of cases you don’t need the (pulsing) dummy load: if the battery is being
charged, it will draw enough current from the power bank and keep the power bank awake; if
the battery is fully charged, it will keep Witty Pi 4 L3V7 functional even if the power bank is in
But in one of your replies for
WittyPi and RaspberryPi wake up problem
You have said:
You don't need to turn on the dummy load unless you are using a power bank to power Witty Pi, also the dummy load is just a trick and there is no guarentee that it will work with all power banks.
So I am confused if to use it or not?
My previous answer you referred is for the case that you use power bank as the only power source (it can be the case for Witty Pi 4 and Witty Pi 4 Mini, although not a typical use case for Witty Pi 4 L3V7), that if power bank goes to sleep mode your device will not startup.
For Witty Pi 4 L3V7, we assumpt the 3.7V Li-Po battery is connected. So even if the power bank goes to sleep mode, the device can still startup with battery. If the battery goes low, it will draw enough current from the power bank to keep the power bank awake, as described in the referred paragrah in the user manual.
So whilst I mentioned the Dummy Load the underlying question was that the Rpi did not always startup and if it did ot leaving it a few minutes and then reconnecting the power bank did. What could be causing this affect please?
Well most USB Power Banks fluctuate. But this board of yours uses a step up so not really relevant?
I have read elsewhere that setting the threshold to 4.2V is not a good idea so I have set it to 4.0v.
Also, I read electrical interference can stop this unit starting up? I will replace each unit in my design if that is the case.
I appreciate you reply.
@andrew-simpson17 I mean you can confirm if your power bank was indeed outputing voltage (not in sleep mode) when you plug it in. I was not implying it could be a voltage lower than 4.75V (according to USB standard, it should output 4.75~5.25V).
However if the power bank really outputs a voltage like 3.8V, it will not turn on your Pi. This functionality can be triggered only when the power mode has changed (from battery to external 5V), and it required the external voltage to be higher than 4.35V (see the firmware source code here). In reality it will require a voltage higher than 4.5V or more because there is a diode between.
The voltage input via USB-C will not go through DC/DC converter, instead it go through a charging manage circuit and a MOSFET switch to power the device.
yes, I knew what you meant. I use reliable USB Power Banks (have to for the retail) and I use usb-meters to determine the output voltage.
I have 4 units on all the time and have done for weeks. It is only that one particular unit which implies it is not your HAT. I have changed everything but the Rpi. If is it fails again I will replace that and retest.
This morning I changed the battery. Lets see if reducing the low threshold from 4.2v to 4.0 is the 'helping' factor. Cannot see how it can be but I have no choice at the moment as have no other ideas.
Thanks for your thoughts.