Backup power supplies for USB devices are very common now. I bought the ones shown below for personal use, and for the most part, don’t regret buying any of them. But I wouldn’t say they are the best deal. You should always shop around and compare product claims and reviews. I also bought some USB portable battery packs on Ebay. Most of those are DIY, where you put the batteries in. Many of them I haven’t tested yet. But so far I have been disappointed compared buying well reviewed packs from Amazon.
- My preferred pack that I own (shown below)
- Some of my tips of which ones to buy
- Common portable power bank differences
- Milliamp hour basics
- Milliamp hour (mAh)/Amp hour (Ah) explained in more detail
Read all product information and reviews for more accurate information than I provide here.
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Power bank differences:
- Voltage: Primarily 5V, but many have a different voltage or multiple voltages.
- Outputs. Mostly different types of USB, but may have female barrel plug or other connectors.
- Capacity: Amount of stored energy (see below)
- Weight: Heavier battery packs typically store more energy. Light packs that claim lots of capacity are likely ripping you off.
Some tip of which pack to buy.
I usually buy a portable power bank when it has
- Good reviews
- Not much more than $1 per mAh (about $30 for a 30,000mAh pack). Of course, improved packs may be more. For example, if you want one that is able to jump a vehicle, then you will probably have to pay a lot more. A really low price will likely be a rip off. I find some nice sales from time to time though.
- The more capacity it has, the heavier it is. Be careful though, some ripoff products will add weight to simulate a larger battery. You may prefer one or 2 lighter ones that are good enough to cover what you need, over a heavier one with more capacity.
My preferred pack shown above
Of the packs above, I easily like the Romoss pack the best for capacity, price and the fact that it has USB C for both charge and discharge (same slot apparently). I mostly wanted the USB C in case my daughter forgets her phone charger, but it looks like USB C is slowly taking over other products, so might as well start using it now. I actually made a mistake ordering that particular unit, because currently (11/25/2019) it has the same unit but with a nicer display, at a cheaper price, available to Amazon prime members.
Improved pack of what I have. Lower price for prime members as of writing this.
Milliamp hour (mAh)s/Amp hour Ah basics
The most common property of a battery based product that you will see in the description is the mAh. This lets you know how much total current (current over time). Thus Amps per hour. Some math is involved (explained more below), but the main take away is that when battery (packs) have the same voltage and claim a higher mAh (Ah) than another battery (pack), then you can expect it to power the same load longer.
Some sellers outright lie, or use specific test procedures/calculations, that over state real world conditions. Thus, all I can recommend is to read product reviewed and see how much other people like how long they can power things.
More detailed: Milliamp hour (mAh)/Amp hour (Ah)
The amount of stored power in a battery is given in milliamp hours (mAh) or Amp hours (Ah). a milliamp is one thousandths of an Amp. 1A = 1000mA. So a 10,000 mAh might be referred to as a 10Ah battery pack.
If the unit’s 5V output (not the battery itself, see below) provides 1,000 mAh, then you can power a 5V load that needs 1A of current for an hour. Or you could power a 0.5A load for 2 hours. Etc.
I believe that most, if not all, sellers use the mAh that the battery provides and not the mAh that will be provided by the 5V output of the portable battery pack. This is important because the battery(ies) has/have a lower voltage than the output. A chip steps up the voltage by converting some current into voltage.
Lithium batteries usually have a voltage that ranges from 4.2V while charged to about 3V while discharged. So an average of 3.6V.
To make the math easy, I will use an example of a fictional 2.5V battery being boosted to output 5 volts. Current from the 2.5V battery goes to a booster that steps up the voltage to 5 volts. The 5V is then used to power a load. If the load demands 1A of current from that 5V, then the battery has to provide 2A of current to the voltage booster. Twice the voltage at the output needs twice the current from the battery.
So, in that case. If a 2.5V battery provides an exact 20,000mAh, which is boosted to 5V with no loses (there’s always loses), then the output will actually provide 5V @ 10,000mAh).