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Thread: Question about charging and calculations.

  1. #1

    Default Question about charging and calculations.

    Hey, could not find dedicated sub forum for power banks and cells so decided to post it here, hope you don't mind. I decided to update my outdoor electricity pack to unify the battery cell standard between different devices like flashlight, headlamp, e-cig and power bank. Decided to go for 18560 cell "DIY" power bank, so I was looking for a long time for PB case that support Q3, 18560 cell and USB-C type of input/output and certified by CE or FCC. I found XTAR PB2S charger/PB (I'm aware of XTAR sub forum, but the problem of this topic is not related strictly to XTAR) that more or less cover my requirements. I've read some revives of XTART PB2S and was aware of problems like inaccurate measurement of voltage/amps and not fully supporting even Quick Charge 2, regardless manufacturer advertising. So I bought XTAR PB2S and confirmed product's originality on the XTAR webpage. Also I bought few 18560 cells from trusted source and decided to do some testing while charging my phone. I don't have any good equipment like multimeter nor professional charger to do proper testing of cells and PB, neither I'm not an expert in the matter so I'm asking you for opinion if I do something wrong or I'm cheated.

    For the test I used, XTAR PB2S and original USB to USB-C cable, one and two 18560 cells (LGEBMJ11865) and my phone Redmi Note 7. As for the phone batter is 4000mAh and its voltage is 3.7V I guess. As I understand, for battery capacity comparison I need to convert it watt hours. So for the single 18560 cell its 3.6v x 3500 mAh / 1000 = 12.6 Wh and for phone battery its 3.7v x 4000 mAh / 1000 = 14.8 Wh. So while charging my phone from a single cell I should be able to charge it at least to 85% without including power waste on a transfer and PB/phone electricity consumption during the process? In both tests I started to charge my phone as soon as battery level reached to 0% and system turned off and in both tests the 18560 cells from which I charged my phone were 100% charged.

    So in first test I was charging my phone from a single 18560 cell. It took me 58 minutes to charge my phone to 58% before 18560 cell dried. The battery at finish was unpleasant hot in my hand, but I could hold it tight without worrying about skin damage. I left the cell to cool down for a moment and then putted it back to PB, the display has showed me 20% more of power in battery but I was only able to charge my phone for 2% more. I expected to charge my phone to at least 70% including power loss on transfer. From the PB display at the peak I was able to charge my phone with 8W top so I was a little disappointment as my USB charger that came with phone has 10W (5V/2A).

    In second test I used two cells to charge my phone. I was able to get the phone fully charged in 2h and 11m. The phone at peak was charged with 10W from the V/A readings and my phone had indicator of Quick Charging, which is weird as while I charge my phone from 10W USB plug its only "normal" charging mode. After finish, both cells were insignificant warm and the charge level was around 20% on each cell.

    Does such huge loss of power, during power transfer is normal or I'm wrong on some point? Thanks for the answers and opinions.

  2. #2
    Join Date
    Jan 2004
    Pleasanton (Bay Area), CA, USA

    Default Re: Question about charging and calculations.

    You are running into a few assumptions that are slightly imperfect, but it is a good test for learning.

    a) It is a lot better to charge / discharge a battery like this at C/2 than in one hour like you are doing.
    - In other words operate them so that it takes at least 2 hours to do a charge or discharge.
    - Yes, it is possible to operate them at faster charge / discharge rates, but this is for once a person is fairly experienced.
    - The reason is exactly what you are experiencing - things get hot and this heat needs to be managed.
    - This heating effect is part of the power loss that you have experienced.

    b) USB "charging" is really happening at closer to 4.2 volts
    - Part of what you are experiencing is that the "discharge" is happening at a lower voltage than the charging voltage
    - When you re-calculate the results using 3..6 volts for discharging and 4.2 volts for charging, then the watt-hrs of power will be closer

    c) Usable capacity
    - The rated capacity of a battery is not the same as its usable capacity
    - Think of it like a car's fuel tank where you really are only using perhaps 75% of it on a typical "charge / discharge" cycle.

    d) Quick charge
    - Just because USB C is rated for higher charge currents, doesn't necessarily mean that it is a good idea for the batteries

    Others will point out that I am a bit conservative about pushing batteries to their limits and I am ok with that.

    In the past I have built some "hot rod flashlights" for fun that discharged the cells in ~ 10 minutes. I also tossed the batteries every 5 cycles to avoid issues.
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  3. #3
    Flashaholic* turbodog's Avatar
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    Jun 2003
    Southern USA

    Default Re: Question about charging and calculations.

    Concerning the temp. You are feeling the temp through 1 or more layers of cell shrink wrap. And even then, you are feeling the outer cell temp. Inner temps are usually higher.

    I don't know the allowable temp for your cells, but after a lifetime of r/c model experience... if it's too hot to hold, it's too hot.
    This is your life, and it's ending one minute at a time.
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  4. #4

    Default Re: Question about charging and calculations.

    Thank you for reply. I have no way to control the C-rate, the problem is related to specified devices as I'm feeling that something is wrong and should get more power from a single cell. In this case the time is dependent on V/A which is dependent on cell, cable, PB and phone lowest/highest rate. Yes I understand the heating effect, I mentioned it as a detail that might help as after cooling down, the PB showed the cell has 20% more power left. So not sure if its cell related problem or the extreme inaccurate XTAR PB2S measurement.

    The rated capacity of a battery is not the same as its usable capacity
    So what is the usable capacity for LGEBMJ11865? I understand, the cell can not be over-discharged for battery life protection, but I thought that 3500mAh is what you can get from it as no one is really interested in capacity that cant be reached. As I understand for LGEBMJ11865 the limit of discharge is 2.5V but from the charger display I never seen that cell volts ever dropped below 3.6V.

    Just because USB C is rated for higher charge currents, doesn't necessarily mean that it is a good idea for the batteries
    Right, because the heat, that's why I don't use QC while in home. I bought this charger for the outdoor reasons and as far I can tell it does not work in XTAR PB2S charger I'm using. But the point is, the charger should support QC3 and I cant pass 8W on single cell and 10W on double cell while charging device that fully supports QC3. Something is wrong, not sure what...

  5. #5

    Default Re: Question about charging and calculations.

    I have a phone that uses QC2.0 and works fine off all chargers I have including a few Anker power banks that support fast charging and a 12vdc car adapter and a charging module I bought for $3 off ebay.
    First off let me clue you in about power banks, charging and QC fast charging.
    Here are the facts:
    1)18650 batteries are about 3.7v nominal (sagging under load) and from 4-4.2V fully charged
    2)USB output is around 5v it can vary a little less or more often sags a little when charging.
    3)QC2 & 3 have several modes of charging most of the time however it used the 9V or 12V charging rate at about 18watts
    4)most power banks that are DIY or chargers operate them at the single cell voltage if more than one cell they are in parallel so the input voltage is about 4.2v max
    5)power banks have a boost circuit to increase the voltage to 5V for USB, these circuits vary in efficiency and output and quality and a few other things and you usually lose some power when you boost it up to 5V, the more current required off USB 5V the more power from the battery and the increased power losses is amplified at increased power levels.
    6) Power banks that support 9/12v for QC charging have to boost the voltage over 2-3 times as high than 5V so the power loss is increased substantially.
    7)Most likely power banks have shutoff circuitry to keep from damaging the batteries and they probably measure voltage under load, the heavier the load the more voltage drop is encountered thus a battery under load may be unable to sustain voltage when it is fairly depleted as 18650s do drop in voltage as they are discharged so this would explain the 20% left over.
    8)Battery gauges aren't exact at all for lithium batteries so if it says 20% it may actually be less than that or possibly even a little more but unable to sustain proper voltage at the load applied.
    9)Batteries in parallel add capacity and the load is shared between them making for less power loss due to high current demands (less internal resistance) thus you can get more power out of 2 cells in parallel than both of them alone added together.
    10)power banks range all over the place in efficiency some of the cheaper ones are around 50% efficient others can be close to 90% I've read it is likely the charger/power bank setup is not 90% probably running QC output closer to 65% I would "guess" due to high current losses.
    11)batteries under light loads typically have their rated capacity, as the load increases the capacity goes down so at a 1/2A load a battery rated at 2000mah may get close to that but at 2A it may only get 1800mah and at 5A maybe 1200mah. These numbers are nowhere near exact just an example as batteries differ in their internal resistance which can greatly affect their performance under load.
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  6. #6
    Flashaholic* turbodog's Avatar
    Join Date
    Jun 2003
    Southern USA

    Default Re: Question about charging and calculations.

    Here's just what you need.


    USB voltage & current meter.

    Or this one. It apparently has a capacity counter feature.

    This is your life, and it's ending one minute at a time.
    Be prepared for the truth.

  7. #7
    Flashaholic* RetroTechie's Avatar
    Join Date
    Oct 2013
    Hengelo, NL

    Default Re: Question about charging and calculations.

    Welcome to this forum, Bienbobry!

    A more comprehensive review for this charger/power bank can be found here: https://lygte-info.dk/review/Review%...PB2S%20UK.html (also posted right here on this forum).
    Manufacturer's page is here.

    Nominal voltage for the Li-ion in your phone, and the Li-ion(s) inserted into the power bank is the same (3.6 or 3.7V, difference depends on which marketing department you ask ). So in this case you can directly compare mAh's. Which shows that a single 18650 can't do a full charge of your phone, 2 of them might if their capacity is high enough, and maybe a bigger cell type like 21700 might do it with only 1 inserted.

    That's only relevant for a ballpark figure. Usually either phone or power bank will start from >0% and <100% charge.

    When transferring charge, power is up-converted to USB voltage (5V, and perhaps higher if USB-C is used), and down-converted again in the phone. Each step has losses. A quick calculation suggests you got about 2/3 efficiency (~66%), meaning if losses were distributed equally between each step you saw ~80% efficiency for each conversion. Which isn't that bad for low-voltage conversions like this. Especially considering that power bank itself uses power (for the display, and control electronics), and so does the phone.

    Also when using one 18650, it will have to 'work hard', meaning you won't get 100% of its rated capacity. Highest capacity number for a cell is obtained at moderate or low loads, at high load useable capacity (mAh's) is less.

    Summarized: I'd say both cell(s) and power bank seem to perform as could be expected.

    Quick charging: note that support for quick charging (as mention on manufacturer's page) may refer to the power bank's ability to be charged quickly itself from a mains adapter. Possibly not to the ability to have the phone QC from the power bank. Perhaps both may work, perhaps 1 of both, perhaps neither. This can also depend on cabling used, the cell(s) inserted in the power bank and their State-of-Charge, or the mains adapter used for charging the power bank (read: USB is only a standard in theory Quick charging support is hit-or-miss). Also repeated quick charging degrades your phone battery faster than when charged slowly. So not having your phone quick charging may actually be preferred...

    As you found
    Quote Originally Posted by Bienbobry View Post
    The battery at finish was unpleasant hot in my hand, but I could hold it tight without worrying about skin damage. I left the cell to cool down for a moment and then putted it back to PB, the display has showed me 20% more of power in battery but I was only able to charge my phone for 2% more.
    using a single cell, means pushing that cell hard(er). Especially if quick charging the phone works in that situation. May I recommend:

    a) Make it a habit to have 2 cells in the power bank when using it as such. When using as a Li-ion charger 1 cell is fine.
    b) If a single charge of your phone is enough, charge the power bank as short as practical before you leave home. That way you avoid having the cells sit at full charge voltage (~4.2V) for extended period, which isn't good for cells' longevity.
    c) Whenever slow charging of the phone is acceptable, opt for that over quick charging. For example by using a cable with which QC doesn't work. Both the cell(s) in the power bank, and the Li-ion in your phone, will thank you.
    d) If you need multiple phone charges between plugging the power bank (AC adapter) in a mains outlet, consider buying a pair of bigger cell types (like high capacity 21700) to leave home with. If that's still not enough, charge up a bunch of 18650's while at home (which are cheap and plentiful these days), and carry those as spares in addition to those in the power bank. If the latter are bigger cells like 21700 size, make sure storage box(es) for carrying spares will also hold that type. As you don't want cells like 21700 loose in a bag or pocket even if they're mostly drained. Same goes for 18650 (or any battery, especially high capacity Li-ions). Batteries, keys & pockets don't mix well.
    e) If d) occurs regularly, consider buying a power bank that holds more cells. When you carry those cells anyway, that is more convenient that swapping in/out cells between power bank & storage boxes while on the go.

  8. #8

    Default Re: Question about charging and calculations.

    Hey turbodog, Lynx_Arc, HarryN, RetroTechie thank you very much for the replies, you clarified me a lot in the matter of cells and power. I gave the charger for a few days to my friend who plays with electronics a lot and he also could not pass the 9W so the specification of XTAR PB2S charger is fake or my charger came broken as brand new. Anyway again, thank you guys for your help, cheers!
    Last edited by Bienbobry; 07-14-2020 at 04:53 AM.

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