There's a fly in your ointment. The Ampere -hour ratings for powerbanks are at the nominal cell voltage of 3.7v. So a 20Ah unit will supply 20Ah*3.7v= 74Wh. A Pi3B+ at full load will draw 2.5A*5v=12.5W. So rounding up slightly, a 75Wh source will provide 75Wh/12.5W= 6 hours.Z80 Refugee wrote: ↑Fri May 25, 2018 9:09 amThe output of the powerbank will be regulated to exactly 5V. When the charge runs out, it will drop below 5V very rapidly (and cut out).
With a 20Ah capacity, the powerbank should supply current at 5V for however long it takes to use up that capacity. 1 amp for 20 hours = 20Ah (amp-hours). 0.5A for 40 hours = 20Ah. Etc.
No, for several reasons. The first being that the step-up converter built into the powerbank (the actual battery cell only provides 3.7v) is very unlikely to be rated to provide 5A. The highest rating I've seen so far is 2.4A and many powerbanks are only rated to deliver 1A. The other issue here is that no system is perfect. There will be losses. A really good system may convert 90% or more of the battery power to usable 5v current. But it might be as low as 70% efficient. And, as already noted, you can't draw a battery down to zero charge. At least not more than once. Plus, if you try to drain most batteries too fast, they heat up and their internal resistance (and losses) increase. Rather specifically, you don't want a Lithium-based battery to heat up...because it may continue to do so until it spontaneously combusts.
Well... A Lead-Acid or NiCad battery can supply a *lot* of current for a relateively short time. In commerically available cases, over 500A. That's one of the reason why Lead-Acid batteries are used in automobiles to start the engine. Basically, the battery is shorted through the starter motor. NiCad can do that too--and better than Lead-Acid, but they are far more expensive. The problem is that the output voltage of the battery drops. For a 12v Lead-Acid, it drops to about 7v under that kind of load, but starter motors (and ignition systems) are designed knowing that that happens. A 12v NiCad will still deliver about 10v under those conditions. On the other hand, the NiFe Edison cell will only deliver about 1.2v from a 12v battery when given that kind of load, which is a big reason why we *don't* use Edison cells to start cars.
Correct, and unless you can find a specification telling you exactly what current the unit can provide - don't buy. That's the essence of my first reply.
Yes. So what? They are not used separately by the power bank.
What gives you that idea? The two batteries are used inside the power bank to operate it. There are some control electronics which take a 5V input to charge them, and some more electronics which take their output to supply the 5V outputs. If you want to use the batteries separately, you will have to open up the power bank and remove the batteries, and then provide your own control electronics to charge them etc etc. Wouldn't it be easier just to buy some batteries?
Two separate power banks, yes.
That is very specialised. If your "load" communicates with the USB port that it can accept the higher voltage before the 12V is made available. It would be dangerous to do so unless the load was 12V-capable. I don't know what the communication mechanism is (sometimes signalled by a specified resistance between the data lines), but it would have to be pretty foolproof and may vary manufacturer to manufacturer (you can be sure Apple don't want you plugging in a Samsung).
Two cells acting as one battery.nick8967 wrote: ↑Tue May 29, 2018 11:18 amAs far as this is concerned:
https://www.xiaomitoday.com/xiaomi-mi-p ... price-tag/
as I can figure out, inside the case there are 2 separate batteries, right? So I can use the one battery to supply the Raspberry and a few sensors, and the other battery to supply the other sensors, right?
Because as I have calculated, for the Raspberry and the other sensors, I need more than 2.5 Amperes, so I will need two powerbanks.
Also If I have a load that needs 12 Volts, I just plug it to the usb input of the powerbank, and the powerbank detects it, right?