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Too Long on the Battery Charger

  1. Jun 14, 2010 #1
    What happens if re-chargable batteries are left too long on their chargers? is there a fire risk? do some of them such as the ones used for moble phones and chordless power tools have some sort of cut-off safety mechanism?
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  3. Jun 14, 2010 #2


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    It depends on the method of charging. At a former employer we charged nicads with a constant current source. As they charged the voltage would rise. The power supplies we used to charge with were set to switch from constant current to constant voltage at a certain voltage level. When the power supply would switch the charging would taper off. This wasn't a method a person would turn their back on overnight but it always insured the batteries were topped off in a relatively short time without the need to watch the clock carefully.
    Most electronics that we use nowadays have smart chargers built in. It is best to look at the manual, it should tell you.
  4. Jun 14, 2010 #3


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    The risk depends on the type of battery and the type of charger, but you can certainly damage most batteries by overcharging them.

    Slow chargers are the least risky, though some batteries can still be damaged by overcharge at a fairly modest rate.

    Many faster chargers these days have auto cut-off as you mention. This can be a simple timer but more advanced ones measure the voltage of the cells to estimate the state of charge. A common method with NiMH batteries is to detect where the battery voltage actually start to fall (normally of course the battery voltage increases as you charge it). This so call "voltage dip" is cause by side reactions in the battery with reagents included to mop up any gases released during overcharge. In other words they actaully detect where the battery is just starting to overcharge and then cut off.
  5. Jun 14, 2010 #4


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    NiCd and NiMH batteries are easily destroyed by overcharging. Sometimes they leak goo out of the end of the battery.
    Some power tools come with a very simple charger which is basically a transformer and a couple of diodes. Then it is up to you to time the charging.
    I wrecked a set of batteries in a cordless drill with a charger like that.
    I left it on overnight.
    The batteries obviously got hot by the effect on the plastic coating on them, so maybe there was a fire risk.

    I built a charger for NiMH batteries. It looks for a constant voltage over a 20 minute period as an indication of full charging, measuring once a minute.
    It also looks for a drop in voltage but has never found one.
    Maybe that is just my type of batteries.
  6. Jun 16, 2010 #5
    Why over charging happens at all?
    when the charger charges a battery, at the permanent phase we will get as if we have 2 batteries in parallel, so no current should flow from the charger to the battery.
    what's the wrong in my analysis?
  7. Jun 16, 2010 #6


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    That applies only if the charger is a constant voltage charger with precisely the correct source voltage. Two batteries in parallel may not be a good scenario, either, if one is old and one is new.
  8. Jun 16, 2010 #7


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    It is usual to charge a battery with a higher voltage than the battery can ever reach.

    A 12 V battery charger might have rectified AC giving DC pulses of up to 20 volts peak.

    Since the battery would never get charged to more than 14 volts or so, charging will continue even after the battery is fully charged.

    To avoid destroying the battery and for safety reasons, overcharging the battery has to be avoided by stopping the charge when the battery is fully charged.

    If the voltage was reduced to the maximum battery voltage, the battery charging would become very slow towards the end of the charging cycle.
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