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    Charging Voltage for a 1.2V NiMH Battery

    Hello everyone. I am designing an energy harvesting system using indoor light as my source and a rechargeable 1.2V NiMH battery as my storage element. I would like to ask, what should be the range of my charging voltage for the 1.2V NiMH battery? Thank you so much.

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    Re: Charging Voltage for a 1.2V NiMH Battery

    Hi,

    There are many informations in the internet. Just do a search on your own.
    Wikipedia, every NiMH battery manufacturer, every charger manufacturer, other sources (like www.batteryuniversity.com) will give reliable informations.

    Not only voltage, but also charging scheme, temperature and expected lifetime plays a role.


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    Re: Charging Voltage for a 1.2V NiMH Battery

    Thank you Klaus.

    Yes I already searched it in the internet many times and all I was able to find regarding the charging voltage with given values is from Wikipedia which states that "Charging voltage is in the range of 1.4–1.6 V per cell." I also checked the reference of this data which is from Panasonic NiMH handbook, but I can't find that the document mentions specifically about the charging voltage. I also visited the website you mentioned, the www.batteryuniversity.com, and it also didn't mention anything about charging voltage except for one comment in the comment section which says, "Either way, your charging voltage needs to be higher than the pack/cell voltage. For a 1.2v cell, 1.8V-2.8V works nicely." And I don't know which of this two is more reliable. By the way, I am going to use a Varta rechargeable NiMH battery, V450HR.

    From this reference from wikipedia, https://eu.industrial.panasonic.com/...nteractive.pdf, I think, if I am right, the basis for the 1.4V to 1.6V range of charging voltage is because of this graph.

    Click image for larger version. 

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    It is the range of the voltage of the NiMH battery when charging until fully charged. Therefore my charging voltage should not exceed this range, is my understanding correct? Thanks for the response is advance, Klaus.


    Littlerock



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    Re: Charging Voltage for a 1.2V NiMH Battery

    During charging a battery shows a rise in voltage. Even a healthy nicad or NIMH might go to 1.6 or 1.7 V. (When taken off charge it drops to 1.45 or so, settling down to 1.35 or 1.4 after a few hours.)

    Aging rechargeables tend to develop high internal impedance. Charging might cause a reading high as 2V.

    Whatever your source voltage, it's a good idea to install an inline resistor to limit current going into the battery. My old AAA rechargeables are 700 mA-Hrs, with instructions to charge at 70mA for 10 hours. (Notice a 10 percent ratio, which is generally accepted as a safe charge rate.)
    I have recent AA NIMH rated 2400 mAHrs. For those 240 mA is a reasonable charge rate.



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    Re: Charging Voltage for a 1.2V NiMH Battery

    Thanks BradtheRad, yeah I agree regarding the drop in battery voltage when the battery is taken off charge. Currently, I am trying to observe the characteristic of the battery by using a power supply to charge the battery. The upper limit current is set to 100mA and charging voltage is set to 1.6V, but from the reading in the measurement, the battery is only drawing about 14mA of current. Is it right that since I set my charging voltage to 1.6V, the battery voltage will reach 1.6V when almost fully charged?



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    Re: Charging Voltage for a 1.2V NiMH Battery

    Hi,

    Pleae clarify:
    You said you "found about charging voltage" but then later you said you "did not find about charging voltage".

    It's confusing to me.

    Even the given chart shows the charging voltage. What else do you need?

    Klaus
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    Re: Charging Voltage for a 1.2V NiMH Battery

    V450HR has a datasheet, it's strongly suggested that you observe the specifications when designing your charging circuit.

    https://products.varta-microbattery....ts/DS55945.pdf
    https://www.varta-microbattery.com/w...ls_NiMH_en.pdf

    Fast charging of NiMH cells above 0.1C must use timer, recognition of voltage drop or temperature rise, it doesn't work with simple constant current/ constant voltage supply. What's a realistic maximum charge current in your energy harvesting application?



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    Re: Charging Voltage for a 1.2V NiMH Battery

    Quote Originally Posted by KlausST View Post
    Hi,

    Pleae clarify:
    You said you "found about charging voltage" but then later you said you "did not find about charging voltage".

    It's confusing to me.

    Even the given chart shows the charging voltage. What else do you need?

    Klaus
    Oh, I'm sorry. I found the charging voltage range 1.4V to 1.6V in Wikipedia in reference to Panasonic NiMH manual and handbook. But upon checking the handbook, it did not mention specifically about the charging voltage. The chart shows the battery voltage when it is charging at specific state of charge (not charging voltage which is supposed to be the voltage that supplies for us to charge our battery). I'm very sorry for the confusion. Because I am really confuse to what will happen if my charging voltage on the battery exceeds 1.6V, say 2V, is it still permissible to charge the 1.2V battery with this charging voltage? Will it damage the battery?

    - - - Updated - - -

    Quote Originally Posted by FvM View Post
    V450HR has a datasheet, it's strongly suggested that you observe the specifications when designing your charging circuit.

    https://products.varta-microbattery....ts/DS55945.pdf
    https://www.varta-microbattery.com/w...ls_NiMH_en.pdf

    Fast charging of NiMH cells above 0.1C must use timer, recognition of voltage drop or temperature rise, it doesn't work with simple constant current/ constant voltage supply. What's a realistic maximum charge current in your energy harvesting application?
    Hi Fvm. Thanks for the link. Yeah, I already checked the datasheet of the battery to be used in our application. The truth is our circuit is only capable of charging the battery at about 20mA and below which is below the normal charging current stated in the datasheet (45mA). But my problem is that, given this condition, what if my charging voltage reaches above 1.6V, like say, 2V, will it damage the battery?



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    Re: Charging Voltage for a 1.2V NiMH Battery

    If you make the battery rise to 2V, then expect it to send such great current through your battery that it heats up alarmingly. It's likely to shorten its useful life.



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    Re: Charging Voltage for a 1.2V NiMH Battery

    Hi,

    To me it seems you did not read how to charge a NiMH battery or you did not understand it.
    Some basics (details are already given as link above)
    * don't push voltage into a battery
    * push current into a battery
    * observe voltage during charging (maybe temperature, too) to find whether battery is full.
    * There are upper limits for voltage where you need to switch OFF charging.
    (This is simple and safe method. There are other charging methods).
    If you don't fully understand NiMH charging, then use a dedicated NiMH charger IC. That's why they are produced.
    They include all circuits and logic that is needed for charging, high lifetime and (last but not least) safety.

    Klaus
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    Re: Charging Voltage for a 1.2V NiMH Battery

    Charging VR450 continuously with 20 mA will not quickly damage it but probably reduce its lifetime by overcharging it. Continuous current without voltage limiting shouldn't be higher than the specified 4.5 mA trickle charge current.



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    Re: Charging Voltage for a 1.2V NiMH Battery

    Ni-xx batteries can be safely charged even with 0.5C without any damage. BUT, not continuously. You have to "fiddle" a little bit with chemistry (ions buildup on electrodes during charging) and to observe battery voltage. Charging must stop if any of the following conditions are met:
    * negative dV/dt
    * excess temperature rise
    * time

    All Ni-xx chemistry batteries, as well as Pb batteries exhibit -dV/dt when they are fully charged

    Even charging with 0.5C will not significantly heat up the battery(ies) if you fiddle with chemistry by discharging them. All my batteries are charged this way with my home made charger and are still well (some of them are at the 80% of capacity, but with over 700 charge/discharge cycles).
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