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    NIMH Button cell 8.4volt 40mA

    I need help with NIMH Button cell battery I want to Trickle charge this battery for 5 years on my smoke detector alarm stand buy current is 40 mA here is my schematic /circuit I use this is my calculation please tell me if I am right on my calculation 10.4 volt /8.4 volt =2 volt/ 1k8 resistor =1.1 mA
    Please advise me if you can
    Thanking you in advance
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  2. #2
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    Re: NIMH Button cell 8.4volt 40mA

    Your schematic appears to use capacitive drop to reduce current from house voltage, then to feed a simple DC power supply.

    Although your circuit may be suitable, it's taking a chance when we modify a smoke alarm. Does it have an existing 9V battery? Does your circuit charge it too? Alkaline batteries are prone to ooze corrosive liquid when forced to carry reverse current.

    With that note of caution, 1mA of current is small enough so resistive drop could work. In fact it might be possible to maintain a small nimh cell with less than 1mA.
    220k resistor of 1/2W rating could be sufficient.

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    Re: NIMH Button cell 8.4volt 40mA

    Just for clarification, you are talking about 40 mAh NiMH button cells? If so, trickle charge current of 1.1 mA seems appropriate. Standby current is however hardly 40 mA, do you mean 40 µA?

    The circuit looks basically O.K., but the capacitor is unnecessarily large, sourcing about 35 mA DC. Can be reduced to 0.15 or 0.22 µF. Or even lower with slightly reduced LED current.

    NiMH lifetime can be probably increased with lower trickle charge current, e.g. 0.5 mA.


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