[PLUG-TALK] RPI4B and power bank requirements

[Keith Lofstrom]

 On Fri, 7 Aug 2020, Tomas Kuchta wrote:
> I would make sure that your power supply (battery bank) is capable of
> supplying the recommended current 3A or close to it.

On Sat, Aug 08, 2020 at 05:52:56AM -0700, Rich Shepard wrote:
> That's the RPI recommendation (5V and 3A) and why I want to learn how to
> translate a power bank rated in 5.5V and mAh to the amperage it provides.

Tomas is correct.  There is not enough information to compute
amperage, which is determined by the power ratings of the
components.  You MUST consider the maximum designed POWER
rating for the device (WATTS, or AMPS at given voltage),
and make sure there is AMPLE margin.

If there is no specified maximum power/current rating for
a power supply or battery, don't use it for anything (such
as a Pi4) that draws more than a trickle of current.

You can't compute a car's top speed from its maximum miles-
per-gallon numbers --- though you CAN be sure that at top
speed the car won't achieve anywhere near maximum mileage.

For example, an inadequate 1 amp power supply might briefly
produce 3 amps.  However, power losses and internal heating
tend to be proportional to AT LEAST current squared, often
higher, so the power supply will heat up AT LEAST 9 times
as fast as it was designed for.  Heat increases resistance
and loss, so "often higher" usually applies.

Rule of thumb: Electronics fails twice as fast for every 10
centigrade increase in temperature (that includes your CPU;
overclock means faster failure).  For way-overloaded power
supplies, the heat can increase exponentially over a very
brief time (40 factors of ten is a factor of a trillion),
until you blow a fuse (lucky you) or something explodes or
catches fire.

As a teenager, I destroyed electronic components learning
how to design (well, sorta) power electronics.  I still
have a few scars from the shrapnel of those explosions.

Off-the-shelf power supplies are probably safer than my
juvenile designs; when overloaded they will merely emit
hazardous fumes or catch fire.  Since that leads to
lawsuits and liability insurance, the price of a power
supply reflects that additional cost.  Unless it is a
Chinese eBay special, practically immune to US tort law. 

Don't skimp on power supplies.  

Keith

P.S. the 10 centigrade "rule" is an approximation; the
actual rule is a sum of Arrhenius chemical reaction rates.
Some older electronics (germanium!) had faster rates.

-- 
Keith Lofstrom          keithl at keithl.com