Power Bank Question + Any Recommendations?

• 

  CheapAzChips on 16/12/2022 - 11:59

  +5

  Battery pack capacity will always be less than advertised, and actually pushing the mAh numbers higher as they do is marketing at its very best: consumers largely have no idea what it means, nor any way to verify it, all they know is "bigger is better"!

  There's a lot to consider, but first thing is that the advertised capacity in mAh is for the_actual lithium cell inside the pack_, not the USB output that you get by plugging your phone in. That voltage is nominally between 3.8-4.2V. You have conversion losses when it's boosted to 5V USB output, let's say an optimistic 10% loss there, then conversion losses again (10%?) when it gets reduced back to ~4.2V to charge the cell in your phone. There are resistive losses in the cable: cheaper cables will have higher resistance causing more loss, let's say another 5%. Then there's the fact that neither your phone nor the power bank will allow the lithium cells to be fully discharged (doing so causes significantly shorter life), so even though your phone and bank advertise a capacity ("higher is better"), you're likely to only see perhaps 80% of that in real life (though these do cancel out somewhat in your case, as reduced capacity potential in the bank is balanced by an approximately equal % reduction in consumed capacity).

  In summary, a very rough calculation would be to take your current reported battery capacity, multiply by the number of charges you need and divide by about 0.7-0.6 to be on the safe side for conversion losses: ie. you can almost double what you calculated above and that's likely to meet your expectations.

  Good luck and enjoy your holiday!

  • 

    sirhelenclark on 16/12/2022 - 14:48

    That explains a lot! Thank you very much

    Am I correct to assume then that with those car battery packs is that they are quite inefficient and there is a lot of loss with those, as generally they are made to deliver a lot of power in a short time so they are not built for efficiency unlike a phone battery pack which would be considering efficiency more?

    Either way very helpful reply, cheers

    • 

      CheapAzChips on 16/12/2022 - 15:46

      +1

      No, on the contrary they may be a tad more efficient, at least in the jump starting side. They will operate at a higher voltage which means lower resistive loss at the same power output. For USB output it will probably be much the same. If you're teetering on buying one of those then I'd say go for it. The added benefit of using it for jump starting is probably worth it, especially if you're in an RV, or other large vehicle, far from assistance.

      • 

        sirhelenclark on 16/12/2022 - 17:08

        Gotcha, thank you :)

        • 

          CheapAzChips on 16/12/2022 - 19:11

          +1

          @sirhelenclark: Anything for the Rt Hon Sir Helen Clark!

  • 

    kiwijunglist on 17/12/2022 - 07:04

    +1

    You mean multiply by 0.7 to 0.6 (not divide)*

    • 

      CheapAzChips on 17/12/2022 - 18:57

      No, I mean divide.

      Example: phone is 4000mAh, and I want to charge it 4 times: 4000 x 4 gives 16000mAh.

      Multiply by 0.6 gives you 9600mAh, which is clearly not enough to charge the phone 4 times.

      Divide by 0.6 gives you ~27000mAh. This allows for total losses of up to 40% and still allows you to charge 4 times.

      HTH

      • 

        Bill on 17/12/2022 - 19:22

        +1

        I think kiwijunglist means multiply the powerbank capacity which would be the same thing. In either case, you are doing whatever makes the required powerbank size larger.

        • 

          CheapAzChips on 18/12/2022 - 07:14

          +1

          @Bill: Yes I get that, but the point was to go from phone battery capacity and number of charges to power bank capacity, not the other way around. My calculation was correct.

          • 

            kiwijunglist on 19/12/2022 - 10:41

            +1

            @CheapAzChips: You are correct, I skimmed through the comment and made a mistake as you're doing your calculations correctly (albeit backwards from how most people would do this).

• 

  CJ on 17/12/2022 - 11:07

  +1

  Using xiaomi 10000mah, fast charging. Pb tech has special from time to time

• 

  scott on 17/12/2022 - 12:04

  +1

  Charging isn't perfectly efficient, and the phone continues to consumer some power while charging. For me (rated 3300mah phone), a 5000mah power bank gives roughly one charge, and a 10,000mah one gives roughly two charges.

  A key question is if you are going to be happy with 5v, 2A charging. Prob OK if you are going to charge while you sleep, but a bit slow if you want to top up during the day. If you want faster charging, you will need to seek out a powerbank that is compatible with whatever fast charge standard your phone supports. i.e. Qualcomm QC3.0 (Samsung fast charge), or USB PD.

  On that car jump starter, I have similar products from anouther brand (for car jump starting), but have used them as a phone power bank in a pinch a couple of times. Note that (as you would expect with the capacity), they are fairly big and heavy compared to the 10,000mah powerbanks you are likely used to. My ones charge of USB, but the unit you linked seems to charge exclusively from a car cigarette lighter (4 - 5 hours), which could be less convenient

  If you ever want to take the powerbank on a plane, they need to be less than 100Wh (27,000mAh each)

  Unless you want the ability to jump start cars, I would likely go with one or two normal power banks in the 10,000 - 25,000 mAh range.

• 

  BlackHT on 17/12/2022 - 14:12

  +1

  I got a used xiaomi 20000mah off Trade Me for $20. It works mint but theres always a certain amount of risk when buying used.

• 

  toomanywises on 17/12/2022 - 14:58

  +1

  Got this one (https://www.pbtech.co.nz/product/BAPMIX34108/Xiaomi-Mi-20000…) as a freebie when I bought my latest phone and I honestly can't live without it. Husband used it for his Google Pixel and it is faster than usual power banks. It charges mine in less than 30 minutes from flat (if it gets that bad). Works a beauty on a Redmi Note 10.

  Of course I don't know what it does to other devices aside from Pixel and Xiaomi but I am so glad to get it as a freebie!

• 

  Bill on 17/12/2022 - 19:30

  +2

  Also for OP's question:

  But I have read somewhere that I cannot take the mah value and divide it by my battery capacity to get the amount of full charges it will do. Why not?

  P=VI
  I pulled some stuff from the internet.

  A milliamp-hour is not a unit of power, but a unit of charge. Power is an instantaneous measure (voltage x current). What you're looking for is the energy being stored, which is voltage x charge (which is Wh). Obviously, a 300V battery with 1Ah charge stores much more energy than a 3.6V battery with a 1Ah charge, the reason why is watts = voltage x amp hours.

  For example, a 12-volt battery with a capacity of 100Ah has a capacity of 12 x 100 = 1200Wh.

  Basically, Wh takes into account the voltage to get a comparable measurement of storage. Ah is not enough to determine what the stored energy is as watts need voltage.

  If the voltage changes, the storage (ah) changes

  • 

    CheapAzChips on 19/12/2022 - 11:02

    Yeah, this is all correct. I was going to add this to my comment above but decided it can be ignored for OP's question as the voltage of the source battery and destination battery are (approximately) the same, so mAh (which is what all the marketing material uses) is a good enough approximation. All you're left with then is approximating the conversion losses, which are indeed very approximate, but good enough for our purposes.

    • 

      Bill on 19/12/2022 - 12:27

      +1

      Just reading through it again, I made some mistakes where I should have said watt hours rather than watts.

      The basic understanding is one is a measure of current, whereas another is a measure of power/energy.

      If you use the water analogy, litres is the amount of power, and you can't determine how much water is flowing if you only know the diameter, you also need the pressure.

      While this isn't the most important or useful thing for op to consider (as you would need to know the voltages which aren't constant) it is important to know basic physics.

      One way this would be applicable for this situation is the power tool battery USB adapters. Those things you put on a power tool battery to get USB ports.

      The difference in voltage would be important to consider as a 10000mah(10ah) power bank does not store twice the energy as a 5ah power tool battery pack as one is running at ~5v and another is 18v

      The power tool battery stores ~80% more energy assuming voltage is proportionally constant.

• 

  AyyTso on 18/12/2022 - 17:02

  I quite like the idea of a GaN powerbank charger. Acts as a charger and a powerbank. Unsure if they have any made for the Au/Nz market yet.