Connect with us

Two phone batteries in parallel?

Discussion in 'Power Electronics' started by seanspotatobusiness, Sep 7, 2014.

  1. seanspotatobusiness

    seanspotatobusiness

    193
    4
    Sep 11, 2012
    I bought a relatively unpopular phone and therefore there are very few accessories available for it. I'd like to have a battery that can last the entire day on a single charge and so I was wondering whether I can wire in a second battery in parallel to the first. Would they both have to be of the same capacity or is this unnecessary?

    I made a schematic which is pretty pointless I guess because it's not a very complicated concept!
    [​IMG]

    keywords: diy extended battery
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,214
    2,695
    Jan 21, 2010
    If they are the same voltage and both equally charged then there will likely be no problem.

    If one is charged and the other one not, high currents can flow between them until the charge levels are similar.

    If one gets damaged, the other can discharge into it, damaging it too.

    You can't easily protect them using diodes because:
    1. The voltage drop may be significant
    2. They would prevent charging
    If you could simply replace the existing battery with a larger one, that would be better.

    Another option is a higher capacity battery in a similar form factor.

    A third option is to have a DC to DC converter produce 5V from the additional power and connect this to the (presumably) USB charging port. This will manage the power flow into the internal battery and essentially power the phone from your larger battery until it goes flat, then use the internal battery. This is a fairly common arrangement with external power packs.

    It would be wise to incorporate voltage sensing on the external battery to prevent it from over-discharging as this will damage the external battery in very short order.
     
  3. Colin Mitchell

    Colin Mitchell

    1,418
    314
    Aug 31, 2014
    "until the charge levels are similar."

    This is incorrect. It should be: Until the voltage of both batteries is the SAME.

    We have put cells in parallel for the past 80 years.

    The 1.5v battery in a "portable" valve radio consisted of 10 x 1.5v cells in parallel for the filaments.
    This was ok for 40 years, so why not now?


    .
     
  4. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Colin,
    Good catch on the typo, the charge level may not directly relate to voltage level if the two cells are dissimilar, but the problem is still present that the extra battery must be at or close to the same voltage as the cell it is being paired with to prevent excessive current flow between the batteries.
    You may notice that unlike batteries from 80 years ago, the chemistries and charging methods have changed. Not by a lot mind you, but enough to warrant different methods to charge the devices and to require additional components for protection.
    If a battery is to be put in parallel with the original factory battery, care should be taken in selecting an appropriate battery.
     
  5. Colin Mitchell

    Colin Mitchell

    1,418
    314
    Aug 31, 2014
    "extra battery must be at or close to the same voltage as the cell it is being paired with to prevent excessive current flow between the batteries."
    The point you are missing is this:
    We are talking about two batteries for a cell phone. If one battery has a slightly lower voltage than the other, the flow of current will from one to the other will only last for a short period of time because the weaker battery will create a "floating charge." This voltage will prevent the weaker battery being charged by any more than a few milliamps (or less) and the charging will only take a minute or so.
    "the chemistries and charging methods have changed. Not by a lot mind you, but enough to warrant different methods to charge the devices and to require additional components for protection."
    This has NOTHING to do with putting two batteries in parallel.
    "If a battery is to be put in parallel with the original factory battery, care should be taken in selecting an appropriate battery."
    Of course the two batteries must be same "type" because no two batteries of different construction will have an output voltage that is close enough for you to parallel them.
     
  6. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Thank you for agreeing with me. As stated they must have the same or close to the same voltage. If you attach a dead or low battery to a fully charged battery what happens?
    You may not be aware, but many batteries are using feedback such as temperature as a method to control charging, this is relevant to the topic.
    Again, thank you for agreeing with me.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,214
    2,695
    Jan 21, 2010
    Because even 40 years ago we knew we should not mix types of batteries, or old and new batteries when we placed them in series or parallel.

    Whilst I don't know, I allow for the possibility that the OP wants to be able to connect/disconnect the auxiliary battery independent of the main battery. I would prefer to limit the opportunity for uncontrolled currents flowing between the batteries.

    A fairly important consideration that has not been specifically raised above is that the difference between "standard" LiPo and LiFePO4 may not be obvious and placing these in parallel will likely destroy the LiFePO4 battery, albeit with smoke and not flame.
     
  8. Arouse1973

    Arouse1973 Adam

    5,164
    1,078
    Dec 18, 2013
    Ah yes your right Steve LiFeP04, but that's not as bad as LiMn2O2. Some battery technologies when placed in parallel exceed both the maximum charge and discharge rate as you put them together. This is why it is best to connect them when the state of charge is say 50%, this limits the charge and discharge currents. Then let them balance for a time then charge them. The batteries we use are 50% charge before being connected together.
     
  9. Arouse1973

    Arouse1973 Adam

    5,164
    1,078
    Dec 18, 2013
    "We are talking about two batteries for a cell phone. If one battery has a slightly lower voltage than the other, the flow of current will from one to the other will only last for a short period of time because the weaker battery will create a "floating charge." This voltage will prevent the weaker battery being charged by any more than a few milliamps (or less) and the charging will only take a minute or so"

    Hi Colin
    I don't know what battery technology your referring to here. But many lithium batteries will produce huge charge and discharge currents that can last several minutes. Take a standard LiFePO4, connecting two nearly charged cells in Parallel could produce currents in region of 10C for a couple of minutes. It all depends of the level of battery charge who's initial current (exponential decay) is limited initially by the internal resistance of the cells which is related to the batteries state of charge.
    Adam
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-