Question about Hilti Cordless Hammer Drill Sfh 151 A 2.0Ah

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Battery pack failure

Wickes 18 volt battery pack, would not charge. Have investigated the cells and found what looks like a disintegrated resistor in the wire connectors, taped to the adjacent cell. Does anyone know the identity and rating of this component as a replacement would solve my problem.

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  • stephanie773 Apr 20, 2009

    Thanks, but I don't think it is a thermal fuse;there are no 'remains' which I would expect to find from this type of component. It seems to relate to the charger which has two lights; Red when charging and green when charged. If I link the wires across where the component has failed I get the green 'charged' light so I guess what it should do is modify the current flow as the charge cycle completes.

  • stephanie773 Apr 21, 2009

    Thanks for the info, I shall try charging with the wires linked just to see if it takes the charge OK . The only concern I have is the green 'charged' indicator light which is permanently lit on the charger unit.

  • stephanie773 Apr 22, 2009

    Charger is fine; I borrowed a battery from a colleague to check it out. Will get around to checking individual cells after I return from a trip to Spain. Thanks for all the good ideas and assistance.



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The device you found isn't likely a resistor but a thermal fuse. These are incorporated to open at a fixed temperature to prevent cell explosions if the charger fails or cells are shorted exposing the remaining good cells to either too high of a current or overcharging. These fuses do not heal, once open, they are finished.  

If you are in the US, you can buy a low-cost digital multimeter that will let you check the individual cells; nominal for NiCad is ~ 1.25-1.4  volts after charge or about the same for NiMh cells. Any cell 10% more or less after charging is not normal. It will also let you check the continuity of the thermal fuse and this should show up as a nearly dead short - 0 Ohms.
You can buy such meters at Radio Shack or auto parts stores for under $20 and under $10 if you have a Harbor Freight Tool store in your area.  

Posted on Apr 19, 2009

  • Steve Allison
    Steve Allison Apr 21, 2009

    If I link the wires across where the component has failed I get the green 'charged' light so I guess what it should do is modify the current flow as the charge cycle completes.

    I have to disagree; the thermal fuses are generally in a 'bullet-shaped' metal tube with one end common to the case, the other insulated.
    I don't think there are any chargers made anymore that have simple current limiting to protect batteries since that doesn't work well. NiCads, NiMh both have a measureable charge characteristic that is used to drop the charge current down to a low level once a certain point on their particular voltage rise is reached.

    Shorting across that part, regardless of its nature is probably quite safe but I wouldn't leave it charging while absent; without the protection, a failure of the charger could cause cells to burst.    

  • Steve Allison
    Steve Allison Apr 21, 2009

    It would help if you had a meter at hand to verify the condition of both the battery pack and the charger for this reason:
    - The charger itself may be faulty if the green LED is permanently on. The circuit in the charger monitors the voltage rise and then switches from red to green once the terminal voltage of the battery pack drops off a minute amount; all rechargeables with which I am familiar react much the same way.
    They receive a controlled charge current (which can be constant or pulsed) and this causes the voltage of the pack (or cell) to rise but eventually 'fold back' once they have reached full charge. 
    The fast-charge type of charger actually slaps a heavy current into the battery, then reverses the pulse polarity and discharges at a rate of 10-15% of the charge current. For reasons of physics I am not cognizant of, this prevents the buildup of heat and gases that normally accompanies a high current charge.

    Having a meter available would also allow you to locate cells that are defective (they will show little or no voltage at all) and even replace them if you have some manual skills and a good quality soldering iron or station.
    I've retrieved many of my packs from the dead this way.
    Unfortunately, a failed charger can destroy a good battery and the reverse is true too.



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