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Series AND parallel?

Discussion in 'General Electronics Discussion' started by h4xs4w, May 25, 2012.

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  1. h4xs4w

    h4xs4w

    6
    0
    May 25, 2012
    I'm building a project that has 8x 5mw lasers, each requiring 3 volts. To power the lasers, I have 4 AAA rechargeable batteries. They are currently wired in parallel, as that is how they are wired when in the charger. I would like to wire the batteries in series to achieve the 3 volts total output, yet still be able to recharge them WITHOUT removing them from the device.

    Is this safe/possible/advisable?

    Do the batteries HAVE to be in parallel to be charged? I'm concerned about mixing the polarity, but not sure if its a valid concern.

    The attached schematic below is how I think it should work, but I'm not sure its safe or will work.

    NOTE: Battery 5 is actually the wall charger for the batteries. Additionally, S2 and S3 cannot be closed at the same time because of how the switches are built.
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
    2,826
    Jan 21, 2010
    If you wire them in series you'll get 4.5 volts nominally (I presume).

    To be able to charge them without removing them you will need a charger that can charge three cells in series.

    This is OK as long as the cells maintain a very similar charge.

    Your schematic will result in shorting the batteries as soon as you close switch 2. The rest of it is such a rat's nest that I can't easily figure what you're trying to do.

    However it is clear that there are pairs of batteries in parallel and another one that gets switched in. It is highly likely you're also doing something wrong here as the batteries in parallel will have more capacity that the single cell on its own. If they are in series, the single cell will discharge faster and may be damaged.

    Charging 5 batteries arranged to give 3V is never going to work.

    If you want 3V you need 2 cells (or a multiple of 2)

    If you want 4.5V you need 3 cells (or a multiple of 3)
     
  3. donkey

    donkey

    1,293
    56
    Feb 26, 2011
    wiring 4 AAA rechargeable batteries in series wil give you approximately 4.8v
    do the HAVE to be wired or can you just buy a battery case and swap them back and forth?
    yes the batteries do have to be in parrallel to charge as its try to charge 1.2v items, if you put them in series it'll be outputting enoung to charge 1.2v but the batteries in series will be 2.4v
    however an alternatavie would be a 3.6v lipo battery pack... they are a little expensive but they just clip in ready to go
    a cr123a battery offers 3v and there are rechargable variations
     
  4. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    On that subject you can find cheap cell phone lipo flat packs all over Ebay, and even cheap chargers that can be kit bashed to work...

    This is by no means a recommendation just an example...

    http://www.ebay.com/itm/NEW-1800mah...582539?pt=PDA_Accessories&hash=item3372df820b
     
  5. h4xs4w

    h4xs4w

    6
    0
    May 25, 2012
    There are only 4 batteries, the 5th battery on the schematic is actually the wall charger to charge the batteries, I just couldn't find a wall charger icon in the schematics program. Additionally, the two switches can never be on at the same time, either one will be closed and the other open, or vice versa. Either way, I'm thinking you're right about a short (sorry for the rats nest, I'm new to this, if you couldn't tell.... :eek: )

    Let me try and simplify the problem:

    I need 4 batteries to produce 3 volts. I can wire a set of 2 batteries in parallel, then wire the two sets in series to produce the 3 volts. But, they can't be charged in series (as far as I know...), because the charger charges them in parallel. So, I either need to produce 3 volts with them wired in parallel (which I don't think is possible), or I need to be able to safely charge them in series.

    Info on the charger:

    The charger itself has 4 slots. The charger requires at least 2 batteries inserted to charge, and they must be adjacent. This tells me that the first two slots are wired in parallel, and the second two slots are as well, with no connection between the two sets of slots.

    I apologize for my ignorance, and thank you all for your time and effort helping to educate me.

    Thank you!
     
    Last edited: May 26, 2012
  6. donkey

    donkey

    1,293
    56
    Feb 26, 2011
    ok maybe I can help you out this way
    batteries you have are 1.5v (actually 1.2v for rechargeable) if you solder them together you get 2.4v not the 3v you require that is issue 1
    issue 2 isn't about CAN you charge them in series, the answer to that simply put is yes. but not with the charger you have as it is trying to charge a 1.2v battery, when they are wirred together you get 2.4 volts, effectively you are only half charging them.

    would I solder 2 batteries together myself then find a charger to suit, nope not when so many options are available out there
     
  7. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    I agree, is there a reason you are steadfast in using the batteries and charger you have? It's only complicating things trying to make it work as you desire... If you must use AAA then be it you can certainly do that and make it work, but IMO it's not cost effective unless necessary...

    If you look at the link I posted for $5 delivered to your front door you can get a 3.6v 1500-1800mAh lipo battery pack that should be a comparable (or at least in the same neighborhood) run time as you AAA batteries in a series/parallel configuration, and you get the appropriate charger that just needs to be hooked up... With a single 1n4001 diode to lock in polarity of the battery your output will be about 2.9-3.0 volts right where you desire... Total cost just over $5...

    There is also the option of using 10440 lipo cells close to AAA sized but 3.7v output, but if you wire them parallel (to get more mAh) you get into the same issue as you have needing to possibly modify the charger...
     
  8. h4xs4w

    h4xs4w

    6
    0
    May 25, 2012


    True, the batteries do output 1.2v (2.4v combined), but this is sufficient to power one the lasers. I popped them into one of the assembled units and it works, without a detectable difference between using regular alkalines, so this may not be an issue. But the charger issue is spot on, so that's just semantics at this point.

    I may need to explore more options, but I believe using AAA's will be required, as I'm working within a space of about 2 hockey pucks stacked (3" diameter, 4" height). Picture the hockey pucks stacked with a 3/4" hole drilled all the way through center. I have room for a pair AAA's stacked, on each side of center. Could one of you kind experts provide any suggestions? I tried looking up the size of the lipo pack you suggested (which looks like a REALLY good solution, btw, with the exception of a possible size issue. Thank you for that!), but I can't find any real dimensions, so I'm not sure it would work, from a size perspective.
     
    Last edited: May 26, 2012
  9. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    The lipo pack I linked to is for the HTC Evo phone, it's

    44 x 65 x 5.3 mm or about 1.75 x 2.55 x 0.20 inches, if you have the height of 1.75" it should nest fine on it's side...

    There are of course several dozen other sizes out there used in other phones, as well as those used in higher end RC toys...

    Some phone batteries like the iPhone have wired leads and don't have their own stand alone chargers... But, there are dirt cheap 'universal' lipo phone chargers that will work with batteries like the iPhone one...

    http://www.ebay.com/itm/New-Univers...040?pt=LH_DefaultDomain_0&hash=item3375c68ad8
     
  10. timothy48342

    timothy48342

    218
    1
    Nov 28, 2011
    I think I know what the OP was originally looking for, because I have though a lot about this. In my case I want a weak power source, like a salvaged solar cell to charge batteries that will power somthing that takes a little more voltage that the soler cell puts out.
    One strategy is a complex switch that puts the batteries (1.2V to 1.5V) in parallel during charging from the weak source, but then in series for powering the load.(which might need 2.0V to 2.5V, like an LED)
    So here's a (?)schematic(?) that sort of describes the OP's desires.
    [​IMG]
    The upper red bar represents 4 connections that need to be made to put the battery pairs in parallel with the charger. The bottom red bar would make 3 connections to put the 2 parallel battery pairs in series with each other and the load.

    I know it can be done with switches. Umm... Seven single-throw-single-pole would do the trick, but I imagine there is something better than that.

    --tim
     
  11. h4xs4w

    h4xs4w

    6
    0
    May 25, 2012
    Ok, so assuming I go with a lipo pack (which seems inevitable after the wisdom and advice provided), how do I wire it? I've never worked with either lipo packs or diodes before....

    You guys have been great, I can't thank you enough!
     
  12. donkey

    donkey

    1,293
    56
    Feb 26, 2011
    just buy a rated charger and connect it. li-po batteries need a specific charger. DO NOT USE A CHARGER FOR NI-MH OR NICD. shows an example of what happens when you use the wrong charger.

    using a battery pack for arduino will provide 3.6v and a zener diode will drop that for you. or you can buy the battery i was talking about before and then find a charger that plugs in.
    if you have ever had an rc car you will know the batteries come as plug packs so finding a charger to suit will be easier.

    you can save money and find a 3v nicd battery pack and charger that just plugs in to charge the device too.
     
  13. h4xs4w

    h4xs4w

    6
    0
    May 25, 2012
    I ended up going with this batt. pack + charger:

    http://www.overstock.com/Electronic...y-and-Charger/4384372/product.html?CID=208011

    Its the closest one I could find that met the size requirements, in addition to being dirt cheap. So I think the charging aspect is covered, but I'm now concerned about the required use of a diode. I know what a diode does in theory (control the direction of electron flow) but I'm not sure where it would need to go in the circuit. I'm assuming between the positive lead of the battery and the output devices? And is the diode recommended previously by Coca Cola the exact one I need?
     
  14. h4xs4w

    h4xs4w

    6
    0
    May 25, 2012
    Just hit me...

    To drop the voltage down, I got it.

    Thank you!
     
  15. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    The diode I recommended will work and will be supper easy to find as is pretty much the standard of power rectifiers, and yes on the positive lead before the device (polarity must be noted)... The diode will drop about .7 volts as well as rectify the direction of the current... The charger should be interfaced directly to the battery...
     
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