I just ordered some 18650 batt and out of curiosity, I want to know the correct capacity of it.

I have an usb meter, which read the following:

  • Charge: 5.25V, 5683mAh
  • Discharge: 5.09V, 2927mAh

The batt was labeled at 3300mAh. The powerbank contains 2 batts.

So I want to ask:

  1. The powerbank rated at 6600(3300x2)mAh @3.7V, mean rated 4884 mAh @5V. Is this correct?
  2. So if 1 is correct then actually the batt is only ~2000 mAh@3.7V?
  3. How to estimate the convertion rate of the powerbank?

Sorry if any noobie question, I'm totally new with this thing.

  • 2
    \$\begingroup\$ If you charged an 18650 to over 5V, you've completely destroyed your battery. You have some sort of charge controller in there that takes 5V in and charges at the appropriate voltage for the battery. \$\endgroup\$
    – Hearth
    Dec 5 '19 at 18:40
  • \$\begingroup\$ Many capacities printed on 18650s are lies, it's quite likely the actual capacity is in the low 2000's of mAH range. It's not really possible to know the efficiency of the converter from the cells' serial or parallel voltage to the powerbank output without either design detail or instrumenting both sides, which is really not something you should try to do. \$\endgroup\$ Dec 5 '19 at 19:10
  • \$\begingroup\$ I mean I use an usb meter connect to a powerbank, which is powered by 2 18650 batts. It had a board to do the convert \$\endgroup\$ Dec 7 '19 at 18:25
  1. Yes, that would be correct if the DC-DC converter was 100% efficient. Maybe it's 80% efficient, so it would be more like 4000mAh @ 5V.

  2. If my 80% number is reasonable, it's more like 2400mAh, which is fairly decent. The number will vary with the current draw (it will be lower at very low and very high currents).

  3. It's not easy to estimate the efficiency of the converter in your power bank. If you use 80% and it's a good quality power bank you'll probably be close. An electronic load with appropriate features for battery testing is the proper way to test battery capacity- you can test with constant current, constant power or resistance type loading and set a safe cutoff voltage at which to terminate the test.

Markings on the batteries, as Chris says, can be exaggerated a bit or they can be extreme lies (like 10% of the claimed capacity- 500 or 600mAh). Especially those procured through dubious channels. If they're willing to lie about the package contents to illegally airmail it, they're more than willing to lie about the battery capacity.

  • \$\begingroup\$ Thank for the detail, my iphone draw an unstable current so this could be an issue. I will grab a tester to have a stable load current \$\endgroup\$ Dec 7 '19 at 18:24

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