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I have two large commercial USB batteries I need to combine to produce power for a 10hr period. Each alone can power my load for ~6-8 hours.

Reading a few other articles:

I believe I can safely plug Battery-A to the charge port of Battery-B, and then connect the load to Battery-B output.

Diagram:

Battery-A-output -> Battery-B-charge; Battery-B-output -> Load

Assuming this setup is reasonable (please correct me if it is not), then which of the following do I expect to happen?

  • Battery-A and Battery-B drain at roughly the same rate.
  • Battery-A drains fully keeping Battery-B charged until Battery-A shuts down.
  • Battery-B drains at a higher rate than Battery-A.

Based on what I've read in the aforementioned articles, I believe they will drain at roughly the same rate in terms of % charge remaining as variances in voltage cause one or the other to take over the bulk of the work in turn. Please correct me if this is wrong, or confirm my understanding.

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    \$\begingroup\$ I've never seen a USB battery pack that will output power while it's plugged in to charge. Allowing that would allow you to plug the power bank into itself, which would probably cause Problems. \$\endgroup\$
    – Hearth
    Commented Dec 21, 2022 at 20:54
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    \$\begingroup\$ I can do this on my common household USB battery, just tested it now, it seems to work fine, I've also used this configuration overnight without incident when I had only one charge port and needed to charge the battery and a device. Also this question pops up on amazon batteries quite often with suppliers reporting that it works, but never really explaining the underlying mechanics of why with confidence. Also there are conflicting reports on those forums, so answers are somewhat untrustworthy in my opinion, which is why I'm posting here. \$\endgroup\$ Commented Dec 21, 2022 at 20:58
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    \$\begingroup\$ @DavidParks - Hi, (a) I guess what you call "USB batteries" are also known as "USB powerbanks". Correct? Personally I've never heard the term "USB battery" before :) (b) Thinking about this briefly, I get different results depending on the exact specification of the two powerbanks (e.g. max charge current of powerbank B, max discharge current of powerbank A & the actual load current). Writing a general, hypothetical answer to try to cover all cases, would take a while. Can you give the specifications of the batteries/powerbanks & load? (As Hearth said, not all powerbanks do "passthrough".) \$\endgroup\$
    – SamGibson
    Commented Dec 21, 2022 at 21:21
  • \$\begingroup\$ There's no definitive answer to this without knowing details of the internals of these "USB batteries". \$\endgroup\$
    – brhans
    Commented Dec 21, 2022 at 22:46

1 Answer 1

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It depends on the charge circuitry they are using. If, for example, they are using a LTC4155, your assumption should be correct: bank "B" should pass along the power from bank "A" until bank "A" is fully discharged and bank "B" starts supplying power (assuming that the load does not exceed the input current limit of power bank "B" in which case it will draw power from the battery to supplement the current-limited input). From the LTC4155 datasheet:

The LTC4155 will partition the available [input] power between the external load on VOUT and the battery charger. Priority is given to the external load and any extra power is used to charge the battery.

The LTC4155 is a fairly expensive and full-featured device however, and I would be surprised if many consumer-grade power banks use it. I can't see any design preferentially discharging the battery when external power is present but who knows.

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