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Need 2A to get through a 5v/12v converter

  1. Aug 31, 2014 #1
    So I need 2A to get through a 5v/12v converter, so from what I understand I'll need around 5-6A, if I take a portable battery pack (the ones with multiple USB ports for charging devices like phones and such) and put a plug in 2 if them, then strip the other end of both the cables and connect them in parallel, would that get me what I need without being unsafe? I feel like it would be alright but I just want an ok on it. 12v batteries are so expensive and won't power my project for very long with just 5,000 mAh or such, you can get a USB battery pack for pretty cheap with around 20,000 mAh inside
  2. jcsd
  3. Aug 31, 2014 #2
    Connecting converters in parallel is usually not recommended. It's likely that the converters will slightly disagree about what the output voltage should be, such that you'll get asymmetrical loading. If one gets loaded down and drops out, this will cause another to get overloaded and drop out, and you'll get a cascade of converters powering up and dropping out. Lots of oscillation at the output.

    Is it really that much more expensive to just get a 12 V regulated battery pack?
  4. Aug 31, 2014 #3
    No I'm not connecting the converters in parallel I only have 1 converter, what I'm doing is I'm taking 2 USB cables (just normal USB cables with the standard 2 power wires and 2 data wires) and plugging each if them into seperate USB ports on the battery pack. Then I will take the ends of both wires and strip them. Then I will connect the USB wires in parallel so the power from both of them is connected in parallel, then that goes into the converter to get the 12v 2A output, I'm asking if doing the parallel thing with the wires is ok to do or not because both ports are powered from the same battery pack
  5. Aug 31, 2014 #4


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    desperately trying to visualise what you are trying to do .... this doesn't make much sense

    how about a drawing of what you are trying to achieve ... show battery, converter, voltages in and out, where you are connecting the USB cables

  6. Aug 31, 2014 #5


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    What you want already exists. They are plugged into two USB ports of a laptop then into a phone. This gives a current flow (and therefore charge rate) similar to a wall charger.
  7. Aug 31, 2014 #6
    Ah, I see. If the USB-ports all source from the same regulated output, then joining their power lines is usually fine. You'll need to make sure the battery pack (the USB-connectors especially) is actually rated for your load.

    It's a bit of an oddball configuration, though.
  8. Aug 31, 2014 #7
    The battery packs you're talking about are intended to charge a device, so to push that current into the charging device, the voltage could well be above the 5V that you desire (not sure what your goal is since USB chargers are intended to charge 5V batteries, but you also mentioned 12V).

    You can parallel these devices to get more current, but if one pack is more charged than another, it may supply more current. In summary, it probably isn't unsafe, but it just isn't a good way to go about getting what you're after.

    If you're trying to get 5-6A (and you also mention 2A??), USB chargers might be over-taxed and some may have a fuse which could open rendering them non-functional.

    What are you trying to power (what voltage), and what current does it draw? There might be some simple alternatives. One is a switch mode wall adapter. These are available with fairly high current ratings now and have regulated outputs.



    If the supply needs to be a battery (you don't have access to an AC outlet), that's another issue for discussion.
    Last edited: Aug 31, 2014
  9. Aug 31, 2014 #8
    It has to be battery and as I said earlier, I need the 5v to go through a 5v/12v dc/dc converter, so 5v goes in and 12v comes out but I also need the converter to output 2A so I need around 5-6A to go into the converter for it to output the required 2A, the power from the USB ports will be on the same battery pack, not different packs
  10. Aug 31, 2014 #9
    Now I understand what you're after. You either need a 5V battery equivalent delivering about 5A, the output of which is converted to 12V, or you need a 12V battery capable of 2A.

    I don't think you'll find a USB charger suitable since it isn't going to source 5 Volts. If it did, no current would flow to a 5V device to charge it. Personally, I'd invest in a small 12V rechargeable battery (lead-acid) and do away with the 5 to 12 Volt conversion. The Ah rating of the battery will determine the length of time it will power your device.

    If you're committed to the USB fast charger approach, I'd use a meter to verify that it's delivering 5V to your converter. It could be risky to the converter if a higher input is applied.
  11. Sep 1, 2014 #10
    I think you have the wrong idea about USB specifications and how battery charging works. No sane device manufacturer would ever willingly allow the voltage of its USB device ports to exceed the specification of 5.25 V without clearly marking it as such (they would still be constant voltage sources) - imagine the liability they could face. If the ports are rated for 'charging', then that implies something about their current rating, which is typically below 2.5 A per port for the 5 V specification (with the rating of the connectors usually being the limiting factor).

    A battery with a charge voltage (voltage at full capacity) of 5 V (as an example) isn't going to stay at that voltage as it discharges. Most battery chemistries will sink a significant amount of current when slightly discharged if you apply their charge voltage to their terminals. For some battery chemistries it is also extremely important that you do not exceed their per cell charge voltage, which is the case for lithium-ion batteries.

    Edit: I assume you know this, but it's worth mentioning that most modern battery-powered devices have a lot of circuitry between their power terminals and the terminals of their battery.

    It's a good suggestion. I don't know what price the OP had in mind, but you can also get 12 V regulated lithium-ion battery packs fairly cheaply nowadays.
    Last edited: Sep 1, 2014
  12. Sep 1, 2014 #11
    I fully understand USB specifications, but the OP was talking about using a USB quick charger battery pack, which is a different case. One approach used in chargers is to employ a higher voltage source, and current limit it to the charge current value (50 mA or whatever). This could potentially result in the presence of a voltage higher than 5 (or 5.25) Volts at the charger output. His original question was "is this safe". I just want the OP to be aware of all the risks involved. Measuring the charger output would be wise.

    Battery charging can only be accomplished if the source voltage is higher than the battery to be charged. In the case of 12 Volt batteries, the source can be 20 Volts or higher depending on the charge current desired. That current is then limited or controlled by the charge circuitry to desirable values. Try charging a dead 12V battery from a fully charged one (identical otherwise) and see how long it takes to raise the voltage of the dead battery (and you'll never completely charge it). It takes a significantly higher voltage to push current into the cell under charge, and usually an "overcharge voltage" is used (somewhat higher than the cell rating) to deliver the final charge to the battery before a charger enters its "float" mode.

    Now in terms of a USB supply which plugs into a wall socket and provides USB power, I completely agree that the output must comply with USB standards.
  13. Sep 1, 2014 #12


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    I agree that a USB source needs to meet the USB spec for 5V power sourcing. If folks could post links to datasheets to "USB Battery Chargers" that put out higher voltages, we could take a look at that.
  14. Sep 1, 2014 #13
    Let's just clarify something: When you say quick charger, do you mean the type of charger which you insert a battery into when it needs charging?
    Last edited: Sep 1, 2014
  15. Sep 1, 2014 #14
    The device the OP described is sometimes called a "mobile battery pack". Rather than substituting for the depleted battery in a cell phone or device it's being used with, it connects, and fast charges the battery in that device, using a micro USB connector which is the standard for charging many mobile devices. It's not just a battery, but a battery with charging circuitry that substitutes for a charger that would otherwise plug into an AC outlet. Here's an example of one such product:


    In the OP's case, he has such a device with several outlets, and is wondering if they can be wired in parallel (to facilitate higher current for the 5V converter). The short answer is yes, but there could be several complications when using this device that way (one being, he may not get 5V). If he wants to battery power his 12V project, a better approach would be to use a 12V battery to begin with. He would be pulling 2A from the 12V supply, and if he needs to convert the 5V to 12V, then he would be pulling around 5A from the 5V supply (his rationale for paralleling the outputs of his mobile battery pack). This is a rather large current for these packs to deliver (even with paralleled outputs), so that's another reason this might not work. I think the main reason the OP wants to do this might be he already has the mobile battery pack, but if this isn't the case, the OP would be wiser to just invest in a 12V battery from the gitgo.
  16. Sep 1, 2014 #15
    No, it's simply just a constant voltage source with a relatively high current capability. Its function is to take the place of a wall wart - nothing more, nothing less.

    No, he'll get 5 V out because that's what the USB specification of the devices it connects to dictate.

    We can agree on that.
  17. Sep 1, 2014 #16
    Look at its specifications:
  18. Sep 1, 2014 #17
    The only clever thing that might happen is that the smartphone, MP3 player etc. will probe the data lines to figure out what current the charger can source.

    Apple, for instance, lets its products determine the current capability of the connected charger using a combination of the data line voltages.

    The charging circuitry for smartphones etc. is in the phone itself. It's tailored to the chemistry and configuration of the battery in the device. "Chargers" don't actually do any of the conditioning. If you open one up, you'll typically find nothing more than a rectifier and a tiny, fixed-output switch-mode converter.
    Last edited: Sep 1, 2014
  19. Sep 1, 2014 #18


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    good responses and info milesyoung :smile:

    I suspect RBTO doesn't understand that you will NEVER find more than 5V (+- 0.25V) on a USB lead unless there is a fault

  20. Sep 1, 2014 #19
    I stand corrected. My understanding of USB charging was incomplete. Thank's to milesyoung for clarifying this.

    Now I can understand how this would work if the charge circuity were in the phone and the battery pack simply acts as a 5V source. I would question though, that if the battery in a USB fast charge mobile pack directly connects to the output, how would they get 5V since no typical battery has that terminal voltage? There must be some kind of regulator in the battery pack to set the output to 5V. If that's the case, and each output of a mobile pack has its own regulator, there can still be the problem mentioned in post #2 by milesyoung.
  21. Sep 2, 2014 #20
    The battery doesn't feed the USB connectors directly. Even with a battery chemistry with a 5 V per cell charge voltage, it won't stay at that voltage as it discharges, and the USB connectors must output 5 ± 0.25 V to remain within specification.

    If you open up one of those chargers, you'll typically find that the battery takes the place of the AC input + rectifier stage, such that it just feeds a switch-mode converter directly. The input range of this converter will cover the full voltage profile of the battery as it discharges. Often it will also be bidirectional and capable of constant current/constant voltage (CC/CV) operation so that you can charge the internal battery pack using the same converter.

    I doubt you'll have more than a single converter in there responsible for regulating the USB power lines (they'll be fed from the same rail), since one should be plenty to source whatever power is necessary. Any more would just add needless complexity and cost.

    If in doubt, you can take a 'true RMS' voltmeter, AC-couple it (edit: DMM on AC voltage setting) and use it to measure the ripple on the output after you've joined the power lines. This should be well below the 0.25 V of the USB specification.
    Last edited: Sep 2, 2014
  22. Sep 2, 2014 #21
    That leads to one last question: If there is a switchmode regulator feeding the output of the mobile pack, how much current is it capable of? The OP needs about 5A of current (at 5 Volts), and that could damage the regulator in the mobile pack or cause it to shut down if it's not rated for that much current.
  23. Sep 2, 2014 #22
    Yeah. That's why I wrote this in post #6 :wink:
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