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Device that knows when a battery is fully charged?

Discussion in 'Power Electronics' started by Shoester, Apr 22, 2011.

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  1. Shoester

    Shoester

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    Jan 23, 2011
    Alright, quick question for you guys....

    Does such a device exist that knows when a battery is fully charged, and can then do "work" when that happens?

    IE, a device that closes a circuit when it detects that the battery has reached full capacity?
    Say, a regulator charges a battery, and once the battery has been fully charged up, this "device" closes the circuit and the regulator can no longer send power to the battery.

    Does such a device exist?

    Thanks guys!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Different battery technologies require different means of measuring the end point of charge. Occasionally there are multiple ways of determining this.

    For lead-acid and Lithium-Ion cells, the end point of charge is noted by voltage. However the voltage is different for each cell type, and in the case of lead-acid, it differs between battery construction (flooded or gel electrolyte) and also strongly with temperature.

    For cells like NiMh and NiCad, the voltage is not a reliable determinant, and you look for changes in cell temperature, or perhaps even a slight dip in voltage.

    Some battery types are better overcharged than undercharged (see "equalisation" of flooded lead acid batteries and explosions of Lithium Ion cells).

    Whilst it is conceivable that a universal charger could be designed to charge anything on the market, it is unlikely that it would be a commercial success.

    Oh, and also note that many devices called "chargers" (think of mobile phones) are not chargers at all, they are power supplies, with the charger circuit in the phone itself.
     
  3. trobbins

    trobbins

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    Jun 15, 2010
    steve, lead acid batteries are considered to reach full charge when current level reduces below a 'nominal' value (close to their float current) when they are at their manufaturer recommended float voltage. This could be quite some time after reaching the float voltage level. Some charge schemes initially have a higher equalise/boost voltage level, which is maintained for a certain period, before reverting to float voltage, after which the current will taper down to the actual float current.

    Wrt to the question posed, the end point would be a mix of voltage and current conditions.

    Ciao, Tim
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    That is true, but the state of charge at a constant temperature can be determined by the unloaded cell voltage. Monitoring the current at a given charge voltage is really a way of indirectly measuring the unloaded cell voltage. In fact many graphs you get for lead acid batteries are a variation on this theme.

    Incidentally the same is true for Li-ion cells too. They can be charged to a "higher" voltage when rapidly charged due to the effect of internal resistance. The cell voltage will be below the voltage read during charge.
     
  5. trobbins

    trobbins

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    Jun 15, 2010
    Measuring the OCV is probably not part of the charging process that Shoester had in mind. Using OCV requires additional temperature info, as well as a reasonable amount of settling time. Chargers that change mode when they nominally get to full SOC typically use a 'final current level' detection.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yes, I get your point.

    In any case, the main point is that there are many different methods of determining end of charge as there are battery chemistries (and probably more).
     
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