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Charging Li-ion cells in series

Discussion in 'Electronic Design' started by Mark Jones, Nov 30, 2004.

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  1. Mark Jones

    Mark Jones Guest

    Hi. I'm looking at the LM3622 datasheet
    (http://www.national.com/ds/LM/LM3622.pdf). It's a simple Li-ion
    linear charger. Comes in 4v/8v Li-ion versions. In Li-ion topology, a
    lot of fuss is made about the cell's final voltage accuracy and how
    overshooting this will destroy the cell. But what would happen if two
    cells in series were chargeed with one unequally depleted? Wouldn't
    the other cell shoot way over its rated voltage, destroying it? It
    makes no sense that nowhere has this been mentioned. Is this a real
    concern? Should the two Li-ion cells in series be equalized when
    charging or charged separately?

    TIA.
     
  2. Joerg

    Joerg Guest

    Hi Mark,

    For a brief but concise summary see Atmel's app note AVR450. It's about
    a uC based charger but explains charging methods of the different
    chemistry batteries.

    Be careful since Li Ion batteries can explode when mistreated. As far as
    I know it is not a good idea to charge in a series configuration.

    Regards, Joerg
     
  3. Ian Stirling

    Ian Stirling Guest

    Yes.
    There are various ways to get round this.
    For example, when on discharge you detect an imabalance, you connect
    a small load to the unbalanced cell.
    Or, do the same on charge, to shunt some of the charge current away.
     
  4. Tim Wescott

    Tim Wescott Guest

    Li-Ion batteries have been taken up by the electric-powered model
    airplane crowd, because of their good energy density characteristics.
    They are usually pushed _hard_; discharged fast, charged in series, etc.

    So now there's a little ceramic vault you can buy to charge and store
    your Li-Ion e-flight batteries in, to contain the fire...
     
  5. Joop

    Joop Guest

    You should take it into account. In one of my hobby project I use 2
    cells taken from a used laptop pack in series. These are (slow)
    charged with a simple 78L08. I put a 1K resistor parallel to each cell
    during charging.
    Cell are thus not fully charged (4 instead of 4.2V) which gives me a
    margin of inbalance that will also be corrected up to 4mA by the
    shunt.
    There is some lost capacity due to the 4V, but it is minimal.
    Worked for me, simple and cheap.

    Joop
     
  6. Mark Jones

    Mark Jones Guest


    Thanks everyone for the responses. It's amazing that no datasheet
    mentions this posibility. Then again, we are supposed to know this,
    right? :)

    Here's an interesting equalization technique, wonder if it will work:


    Mark Jones .-------------+--------+--> Vdd

    Helios Studios ||--' EM MOSFET | |
    .--------. ||<-. x4 | |
    | Q |-----+---||--+ | |
    | | | | | 10uF | +4.1v
    | Flip | | +-----. --- -----
    | Flop | | | | --- ---
    | 10kHz | | ||--' | | | -
    | _ | | ||<-. | | |
    | Q |--+--|---||--+ | | |
    '--------' | | | | | |
    | | +-----|-------+--------+
    | | | --- | |
    | | ||--' --- | |
    | | ||<-. | 470uF | |
    | +---||--+ | | 10uF | +4.1v
    | | | --- -----
    | +-----' --- ---
    | | | | -
    | ||--' | |
    | ||<-. | |
    '------||--+ | |
    '-------------+--------+--> Vee


    -M
     
  7. mike

    mike Guest

    I've thought about doing this, but couldn't figger out how to make it
    happen automatically. Cells are in a PDA, so while I have access to the
    center point, don't have access to much else to control it.
    What method did you use to switch in the resistors?

    Thanks, mike

    --
    Return address is VALID.
    500MHz Tek DSOscilloscope TDS540 $2200
    http://nm7u.tripod.com/homepage/te.html
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  8. budgie

    budgie Guest

    Most commercial multi-cell packs have a pack protection module. Among its
    functions are detecting under/over voltage (of the series string as a whole),
    over-current on discharge, and cell_voltage_imbalance.

    The module intervenes by effectively turning off a FET in series with the cells,
    thus preventing any significant current flow in or out. Cell imbalance is one
    of the causes of pack "failure" in laptops.
     
  9. Joop

    Joop Guest

    With a switch (dual pole/make). The GND is always connected to the
    cells. The cells center tap is switched in to the center between the
    resistors. The V+ is switched to the +8V of the 78L08 and the actual
    circuit.

    The 78L08 seems to handle the 8V without noticable current draw. The
    shunt takes 4mA of course. But my circuit uses about 50mA so I found
    the added 4 mA neglectable.

    Charging only happens when I plug in the transformer. It could all be
    done a bit nicer on all fronts but it works with affordable
    of-the-shelf components.
    My device is an LC meter which I do not use a very often. The li-ion
    makes sure that it always works because the self discharge is
    extremely low. A potential reduced capacity of the old cells, the sub
    optimal charge (4V) and the waist current (4mA) are all unimportant
    compared to full enough cells whenever I need the meter for a couple
    of minutes.

    As for your PDA, do you really have multiple cells in there? Most
    mobile phones e.g. just have a single cell for simplicity. The are
    already enough rumours out there about exploded non-brand cells. Any
    explosion due to unsafe charge circuits with aging cells put in series
    is not what manufacturers would like to be linked to. They just use a
    dc/dc converter if needed at all to avoid the issue.
    For laptops the high power draw makes it more efficient to use a
    higher pack voltage. But then you see 3 of 4 cells in series and
    complex circuitry to safeguard the pack as well as provide SMB
    communication with the computer.

    Joop
     
  10. Joop

    Joop Guest

    Sorry, I got carried away. To come back to your question, why not use
    a small relay to switch in the top and center cell connection whenever
    the mains power is available?

    Joop
     
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