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Can I join the two usb outs from the batteries in series to make 10 volts? This is not running from a computer and I will charge each pack separately. The device draws very little power but needs 9-16 volts to run. I will be using a hirose connector at the device end and a fabricated y cable with dual usb (male) plugs that are wired in series? Possible?

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    \$\begingroup\$ If two of the sources are floating (and it sounds like they are), it should be possible. \$\endgroup\$ – Eugene Sh. Aug 3 '16 at 17:13
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    \$\begingroup\$ Just to clarify, the battery packs are lithium packs used to charge usb devices. I have two laying around that i never use and would love to put them to work with this audio recorder if it's possible. \$\endgroup\$ – Carl Geers Aug 3 '16 at 17:13
  • \$\begingroup\$ Not sure what floating means? I have a very basic understanding of electronics. They are independent battery packs not running on a board together. \$\endgroup\$ – Carl Geers Aug 3 '16 at 17:14
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    \$\begingroup\$ "Lithium packs used to charge USB" means these are batteries with some kind of regulators (to get the right output voltage). That means you might have problems if/when one pack goes dead and the other pack tries to drive it to negative output voltages. \$\endgroup\$ – Whit3rd Aug 4 '16 at 0:19
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    \$\begingroup\$ Whatever you do, don't charge them while their outputs are in series. Doing so would short out the internal battery of one of the two "power banks". \$\endgroup\$ – jms Oct 3 '16 at 22:58
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Technically speaking, two floating packs of batteries should work with no problem.

However, these charging packs use buck or boost converters (depending how many Li-Ion cells the pack is constructed from) to make 5V output on each port.

The problem will arise because these packs are somewhat intelligent, so they will shut down the port power if they sense that the current taken from them is too low. At least this was my experience.

Practically, if, as you say, "the device draws little power", your arrangement will not work.

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I agree with the other answer about possibly needing a minimum load (easy to fix: add more load!)

But the bigger concern is safety. The unsafe part comes when the power packs are nearly depleted. One pack will shut off before the other once since they are not coordinated.

Let's say PACK1 shuts down and PACK2 is still running. Then it looks like this:

schematic

simulate this circuit – Schematic created using CircuitLab

PACK1 now has 0V but PACK2 is still trying to shove current through it. The bad part is that PACK1 is applying current into the - side and out the + side of PACK2. This means PACK2 is applying a reverse voltage to to PACK1!

If enough current flows (e.g. the load is low enough), this can damage PACK1 and may cause some dangerous internal condition.

If your load is a smart electronic device such that it is guaranteed to disconnect when the voltage is too low (e.g. < 8 or 9V) with no residual current, then it might work fine. The shutdown pack would see only a small reverse voltage and little to no current flow.

If the load is some light bulb, motor, or something where 5V will continue to flow, then you have a high potential for a safety or damage problem.

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  • \$\begingroup\$ The device (Zoom F8 Field Recorder) allows you to set a min voltage to shut off or switch to another battery or power device. I think it's pretty smart but not sure. You can have up to three power sources connected to it and it will cycle through them as they hit the minimum voltage for each source. \$\endgroup\$ – Carl Geers Jan 12 '17 at 16:11
  • \$\begingroup\$ Their text reads as follows: "Three power sources A 4-pin Hirose connector allows the use of external 9-16 volt DC battery packs. Internal power is provided by 8 AA batteries. The F8 can switch power sources from a DC battery pack to the AA batteries at a user-defined voltage level for uninterrupted recording. You can also use the supplied 12 volt AC adapter when not on the go." Here is the link to the device's manufacturer page: zoom-na.com/products/field-video-recording/field-recording/… \$\endgroup\$ – Carl Geers Jan 12 '17 at 16:12
  • \$\begingroup\$ Ok, good it has min cutoff V. It would just be easier if you just use a 5V to 12V boost module. A lot of small manufacturers make them in China, websites just dedicated to SMPS boards, LED/LCD voltage and current displays, etc. Put off-the-shelf boards in a box with nice display and looks semi-pro. Here's a boost: amazon.com/RioRand-Adjustable-Automatic-Buck-Boost-Converter/dp/… I don't see an input current spec. Worrisome that it uses a 2A 12V adapter but says 10+hours on 2450 mAh NiMh so 245 mA average or 3 Watts. I just hope peaks don't shutdown your 5V pack. \$\endgroup\$ – Vince Patron Jan 12 '17 at 18:12

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