Daily Ramblings

There are hardly any usecases for having such a large power bank. So I did some tests to figure out how much capacity this really is:

I powered a Raspberry Pi 4B 8GB with a USB HDD and prometheus and grafana running on it, and used a USB power meter to measure total energy

The following is the total data:

$time = 28:20:09 (hh:mm:ss)$

$energy = 106.84Wh$

$charge = 21078mAh$

What the above numbers mean: The pi was on for a bit over $28 hours$ It consumed $21078 mAh$ of charge, which amounted to $106.84 Wh$ of energy when the voltage at which the current was consumed is considered.

But that is way off form the $30000 mAh$ mark right?

So, the $30000mAh$ is at the battery not at the output of the powerbank. The typical voltage of the battery is around $3.6 V$ (with $4.2 V$ being the full charge voltage and $2.8V$ being cutoff voltage – these numbers can vary a bit). On the contrary, the power bank output is always $5V$ or higher

So, to get the charge capacity at battery, I would do $$ {106.84Wh \over 3.6V} = 29.6777777778 Ah = 29677.78 mAh $$

That number is almost equal to the advertised capacity.

We can also find the $P_{avg}$ – average power consumed by the raspberry pi over the time it was powered on:

$$P_{avg} = {energy \over time} = {106.84Wh \over 28h 20m} = {106.84Wh \over 28.33h} = 3.77W$$

So, that means our PI used almost $4 W$ of power on average, which means that the instantaneous power can be way higher (As we are not even stressing the CPU a lot). I will be using a $10W$ power adapter to power this pi – just in case.