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battery charging / testing multimeter question

Discussion in 'Power Electronics' started by pauld, Jan 23, 2015.

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  1. pauld


    Jan 23, 2015

    I have limited electronics knowledge but know how not to kill myself around them, I have done a lot of research on the subject but just need someone with the know how to connect the dots and give a solid answer. I am looking at buying an intelligent battery charger however it only fits AA/AAA. I have a 18650 type battery and would like to charge it intelligently.

    The charger I am looking for is similar to this

    So my three questions are

    1) Can I just put a wire from both terminals of the charger to the terminals on the 18650 3.6v battery. Will this just trickle the juice in until full or will the charger realise 3.6v is way too high for the AA / AAA battery that it is expecting and either not work or cause a disaster

    2) If the above is bad then is there a way to test a dumb charger while its charging as to when it is finished. I have a basic multimeter and wondering when the battery is taking a charge does it show a different current compared to when its fully charged. Does it behave differently and if so what is an example of the difference between taking a charge and full. And where should I put the multimeter terminals. I seen a Russian video where he put a piece of copper between the bottom of the battery and the bottom clip of the charger. Then he touched one multimeter terminal on the copper and the other just on the back of the bottom clip of the charger. I have absolutely no Russian so wanted to ask if this makes sense to you guys.

    6:15 minutes into the video

    3) If the above isn’t possible then finally I believe you can test a battery with a multimeter but need to put it under a load. Can anyone send me a link of the resistor that is needed to do this.

    Thank you so much for your time

  2. Gryd3


    Jun 25, 2014
    Paul, you cannot use a simple (or smart) AA or AAA charger to charge a 3.6V Lithium cell.
    There are a couple reasons:
    Reason 1. Chargers works by charging the battery by connecting a reverse polarity voltage that causes current to flow in the opposite direction back into the battery. This current is what should be regulated. Different battery chemistries have different limitations as to how much current they can handle.
    Reason 2. When attempting to charge a 3.6V cell with a charger meant to charge a 1.2V cell, you could very well cause the charger to fault causing damage to the battery, charger, or both.
    If you wan't to rig something together to charge your battery, you should google around for 'charging characteristics' for the specific chemistry of cell you will be working with. From there, you will need to keep an eye on the current and voltage through and across the cell to determine when the cell is charged.
  3. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    No your battery is a lithium battery. The charger you linked to is for Nicd and Nimh. You must not do this.
  4. BobK


    Jan 5, 2010
    There are plenty of inexpensive chargers for 18650 batteries out there. Buy one.

  5. pauld


    Jan 23, 2015
    thanks for the replies, so option 1 is definitely a no go. i am glad i asked

    As for question 2 and 3, I do have a basic charger for the lithium battery, i am not trying to build one. The problem is that it just charges for how ever long it is plugged in. if it was plugged in for 2 days then the charge light would just stay on. And the other way around I don't want to under charge it and only get half the use time out of it when camping.

    So question 2 and 3 are about using a multimeter to find out when either the battery is not accepting the charge anymore (or not allowing current to flow through with the reverse polarity that i just learned about today from gryd3, thanks), or if that is not possible then what resistor value should be used to put a load on it to get a true voltage reading.
  6. Gryd3


    Jun 25, 2014
    Well Paul..
    The tricky part is that a battery simply wont 'stop' holding a charge.
    If I could over simplify it, replace the battery with a resistor... when the battery is low/dead, the resistance is low. The charger will therefore put a lower voltage across it to maintain a how many Amps or milliAmps are acceptable or desired to charge the battery.
    As the battery charges, it's resistance increases. The charger will compensate and increase it's output voltage (again to maintain how many Amps or milliAmps are acceptable or desired.
    At some point in time, the batteries resistance will stop climbing, the charge will also stop increasing it's voltage (because it no longer needs to in order to keep pushing current backwards through it).
    Now... This charger is a 'constant current' charger, and you need to pull the battery out when it's fully charged. You 'know' this based on two things: The time it was in the charger, or the voltage across the terminals of the battery.
    Some chargers are 'smart' and will monitor the voltage they are applying to a battery, and when it reaches a certain level, the charger assumes the cell is full and stops charging, or will drop down to a 'trickle' charge. A trickle charge is a much smaller rate of charge for a battery... the idea here is that the trickle charge is 'just' large enough to keep charging the battery for an excessive amount of time and won't cause damage. This trickle charge also tops up the battery.

    Now for the 'gotcha' .
    All batteries are different.
    You could have a lithium battery from one manufacturer, and charge it up to 3.7V, and when you put a resistor across it, it could drop to 3.5V... now a different brand could drop to 3.2V... both batteries are fully charged, but one battery is manufactured with the capability to output more current at a time than the other.
    So the best you can do is make a judgement call based on the voltage of the battery, and how long it has been charging.
    Some chargers actually implement temperature sensors as well to help determine a full charge.

    I think the best thing for you to do is do a little reading ;)

    There are much more details here about charging characteristics for Lithium Ion cells.
    There will always be variation based on the chemistry you choose, but the general rules are all pretty much the same with the exception of a few numbers.
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