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Few Questions about Batteries.

Discussion in 'Power Electronics' started by gogetax1, Aug 6, 2012.

  1. gogetax1

    gogetax1

    4
    0
    Aug 6, 2012
    Sup all,
    i was been thinking.
    there are lots of batteries in their kind.
    low Voltage, high AMP or high Voltage and low AMP.

    and the higher voltage + high amps are quite expencive batteries. specially the Lithium ones.
    i found a battery on ebay 48V and 12AMP. this cost about 450$\

    but i also found a battery 3.7V and 3AMP wich you can connect 4 in paralel to get 12AMP and then multiply it by 13 (in series) to get to the 48 Volt.
    the Final Result is 48 volt, 12AMP with 52 Batteries. correct me if im wrong in this formula.
    but if im right, x52 batteries of this kind is about 110$. im saving here about x3 times less money.

    and another question about Lithium batteries.
    i understand that each Lithium battery has their own charger, so it wont blow up, cuz you dont want it to happen when you around.. it could hurt >_<
    whats the formula on that? does the charger has to be same Voltage as the battery, and pulls out a limited amount of AMP or what?
     
  2. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    You are comparing 'parts' to a complete 'unit'...

    It's like saying you can purchase a house hit at the local lumberyard for $60K and than asking why the house coast $100K to purchase an already built house... There is a 'labor' , 'tooling' and 'equipment' that needs to be factored in to a complete assembled unit vs just the parts to make it...

    In your case how much is the spot welder going to cost you to properly and reliably gain the individual cells together? How many minutes/hours is this going to take you to complete?

    And a word of advice, NEVER trust any claimed battery ratings unless it's from a highly reliable battery company that has a proven track record of not exaggerating the numbers...

    And yes there are charging concerns and they have to be addressed on an individual basis as there is no one proper way to do it... Especially when you start ganging Lipo cells in parallel and series configurations... Lipo chargers are 'smart' chargers so they will be controlled with a circuit that is specifically designed for the purpose...
     
  3. GreenGiant

    GreenGiant

    830
    5
    Feb 9, 2012
    Theoretically this will work, but they need to be spaced enough for the heat generated from charging/discharging can dissipate, and they need to have a good solid connection to transfer all the current etc

    There is not a specific formula, its all about how you want the battery to charge.
    Lithium batteries are (typically) able to handle quite a lot of current, but not a lot of overcharge voltage, a typical charge pattern will be to charge at a 1C (this is a current rating, basically the amount of current it would take for the battery to go from fully depleted to fully charged in 1 hour, in your case it would be 12A, assuming that when you say 12 amps you mean 12 amp hours) rate until it reaches full voltage then trickle the current off until it gets down to .1C or .05C or lower (depending on how fully you want the battery charged, and how long you don't mind waiting)
    The faster you charge a battery the less fully charged it will be, and the more damage you do to it internally

    Overcharging the voltage of the cell (charging with a non voltage limited charger) can lead to venting (leaking of noxious gasses from inside the cell), fire, or even explosion

    You would be better off, like CocaCola said, to get the more expensive one, as setting all this up is very complicated and dangerous
     
  4. gogetax1

    gogetax1

    4
    0
    Aug 6, 2012
    so to charge a battery, i will have to do 1 second of charge, 1 second of voltage reading. repeat this process untill the reading will be 48v or whatever the battery capable of.. right?
     
  5. GreenGiant

    GreenGiant

    830
    5
    Feb 9, 2012
    Not exactly, you could do that, but it will take forever, plus you would harm the internals of the battery, typically you charge at a specific voltage/current and have a voltage sense the reads from the tabs of the batteries, so that way you have constant charge/readings
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,191
    2,692
    Jan 21, 2010
    One of the big gotchas of charging batteries in series is that not all batteries have the same capacity.

    Assume that you have 2 batteries in series with different capacities (or states of charge). As you charge them, one gets fully charged first. If you stop here, only one battery is fully charged. If you continue until the second one is charged then the first will be overcharged.

    Some battery chemistries are very tolerant of overcharge. Lead acid (wet) cells are particularly tolerant, and it is quite standard to routinely overcharge them to "level" the cells. All you need to do is ensure that they're topped up with water.

    LiPo cells are at the other end of the spectrum. They are very intolerant of overcharge. Whilst a small amount of overcharge won't make them explode, it will reduce their capacity. This leads to a situation where the lowest capacity cell deteriorates faster than the rest, and is eventually the cause of failure (sometimes pyrotechnically).

    LiPo batteries almost always have internal circuitry to prevent any single cell from being overcharged (or indeed over dis-charged). This is not entirely trivial and is one of the hidden reasons why a LiPo battery costs more than the sum of its cells.

    Another reason is that you would like to make a battery from cells that have as close as possible to the same capacity, since often the total capacity of the battery is determined by the weakest cell.
     
  7. gogetax1

    gogetax1

    4
    0
    Aug 6, 2012
    Thanks.

    thanks guys for all the responses. Im amazed to see so much info about batteries just from few responses.

    i guess ill drop off from the idea of making batteries in series. :D
     
  8. gogetax1

    gogetax1

    4
    0
    Aug 6, 2012
    one more thing,
    could anyone give me a small circuit sample of Lithium Battery charger? so i will understand more of how it charging and when he knows when to stop the charge..
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,191
    2,692
    Jan 21, 2010
    Here is something very simple for charging a LiPo battery.

    The circuit comprises a current regulator (to provice a maximum current), then a voltage regulator (to limit the maximum voltage), then (across each cell) a circuit to limit the maximum voltage per cell.
     
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