Lithium-ion charger and questions

Discussion in 'The Projects Forum' started by akis02, Sep 14, 2012.

  1. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    I would like to build a charger to charge two Lithium-Ion battery packs which are rated at 12V/9800mAh. They were bought on ebay and the provided mains chargers are useless - they say 12V/300mA, but I can only get 140mA out of them. After discharging one battery pack to 10.31V (and peculiarly the onboard circuitry did not switch the battery off as I had expected - I wonder how low will it let the voltage drop to?), the mains charger failed to charge the battery over 11.5V.

    I have now charged one battery pack using my bench PSU at 12.6V and 1A in a couple of hours.

    I would like to build my own charger to be able to charge these battery packs. My initial approach would be to use an LM317 to provide exactly 12.8V. Use a constant current sink transistor to set the current to something like 800mA max. Connect the battery in-between.

    Then I would like some indication of charge status and auto-switch off (so I can power it on and go). For that I was thinking of using a reed (or other 1A relay) with an op-amp to compare against some reference voltage.

    I attach a simple schematic as a start. Please let me know what you think.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  3. toffee_pie

    Active Member

    Oct 31, 2009
    162
    7
    why not use a fuel gauge and do a proper BMS, ie: Ti solution.

    what are the individual cell specs? voltage,capacity.?
     
  4. NFA Fabrication

    Member

    Aug 12, 2012
    104
    3
    Lithium cells typically have a 3.7V nominal voltage, and when charging, almost all Lithium batteries are safe to be charged at at least 1C (A 1000ma battery charged at 1A, is 1C). They are typically brought to 4.2/cell and then monitored for current dop off at the end of the charge until next to zero.

    My ? is, how he intends to balance the cells, as this is important while charging lithium, unless you like the occasional fire! And the OP said it was a 12V battery, so i am assuming it is a 3S (3 Cell) battery, nominal voltage 11.1, with a peak 12.6 charge voltage. Also, cells should typically be cut off so they don't get below 3V/cell, which sounds like 9V for this pack.

    It may be easier to get a basic lithium RC model charger. The Hitec X1 is a great highly adjustable AC/DC charger that will charge/balance about any battery for $63: http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXBJBY&P=ML
     
    Last edited: Sep 14, 2012
  5. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    I have no idea what 's inside the battery pack. Here is a link to it : http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=230819766350#ht_4714wt_1397

    All I know is that I must give it 12.6V to charge it and cannot trust the onboard electronics to automatically cut off at precisely 12.6 V, so I have to do it myself.

    Here's a circuit which I think should work. It uses a reference to stop charging when the battery voltage is 12.55V. I realise I will probably need either trimpots or careful selection of physical resistors around R2, R4/R5 and R7/R9 to achieve the correct voltages.
     
  6. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    I was hoping there is some electronics inside the pack that does all this for me, but I am not so sure now. I tested the second battery pack, and its voltage dropped to 8.9V before I saw it and stopped it.

    Regarding my schematic I now realise that once it reaches the 12.55 V and stops charging, it should also "sleep" for at least 6 hours, in case it goes into an oscillation, or cycle, where it keeps charging the battery for a few mV at a time.
     
  7. Veetron

    New Member

    Sep 2, 2012
    2
    0
    I have a couple of these same batteries and one that is 6800 mAh. So I have three chargers. On one of the chargers the yellowish green LED never goes out indicating that the charge is complete but they all charge up to 12.6V. Sometimes it can take 12 hours or so. I use one of the batteries 5 days a week for about four months now and it has been great. I've only depleted it down to about 11.2V and was also wondering about the built in circuitry. For the price and light weight I probably won't be using gel cell batteries anymore. I will be watching this thread with interest.
     
  8. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    I just had some info from the seller. Apparently the green LED on the charger is a direction light - when it is on it means it is charging and when it is off it means it is not. When the green light goes off it means it is done. Considering that sometimes the light is always on, I wonder...

    He did not understand my question about whether there is electronics on these batteries to stop them from draining too much or from charging too much. I will need a Chinese translator...

    For my use I would like to use the battery as long as possible, so I will definitely go down to 10V and below.
     
    Last edited: Sep 15, 2012
  9. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    Latest schematic showing fixed value resistors using standard parts. Battery gets disconnected when it reaches 12.5 V. Green LED comes on when battery gets disconnected and red LED comes on when battery is charging. First LM317 sets current to 1A max, and second LM317 sets voltage to 12.7 V. TL431 creates a voltage reference to sense when the battery has reached precisely 12.5 V. TL431 and op-amp/comparator are powered from the raw/filtered voltage to reduce/eliminate any potential feedback.

    There is a 555 in monostable mode, whose purpose is to keep the battery disconnected for a very long time after it has reached 12.5V, to prevent charging oscillations/cycles when the battery has been left on the charger for too long. Currently the output pin 3 of the 555 shows not connected as I have not decided how to do what I just said :)

    There should also be a disconnection when the battery has been left on the charger for a certain period of time, eg 2-3 hours, come what may.

    Any comments welcome. I will try to build this today but I am lacking the relay.
     
  10. raymcm

    New Member

    Sep 16, 2012
    1
    0
    Just a few comments,
    1 google overcharged LiPo youtube and see what might happen.
    2 Invest in a fireproof LiPo charging bag
    3 Dont ever charge unattended
    4 to be honest I would buy a decent commercial charger, they can be had for next to nothing.
    5 I dont like cheap direct from china LiPo chargers
     
  11. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    You know I have seriously thought about (2). I was thinking of a metal box capable of withstanding an explosion and not catch on fire. Seriously, I was thinking exactly that.

    No commercial charger can "guess" what battery this is. You'd have to tell it and then it is your word against the battery's :)

    Similar LiIon batteries and chargers sell for 100s of pounds ...

    Hopefully, this is a good battery and will last a long time with no hickups as the previous poster said.
     
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