Real Time Clock (RTC) Breakout Boards WARNING

Discussion in 'General Electronics Chat' started by MrAl, Dec 5, 2015.

  1. MrAl

    Thread Starter Well-Known Member

    Jun 17, 2014
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    Hello fellow members and visitors,

    This is not about time keeping but of the possible danger with some of these boards.

    Recently i have been testing one of the RTC boards obtainable on the web for very low cost. The part this board uses is the DS3231 RTC chip, but the other affected board is the type that uses the RS1307 RTC chip, and there may be others.

    The problem is with the boards that contain a back up battery. This can be either the LIR2032 or the CR2032.

    The main problem is with the charging circuit used to charge the battery on the ones that contain the LIR2032 and of course if it has a charging circuit then it should NOT have the CR2032 unless the charging circuit has been disabled.

    For now i will concentrate on explaining the problem with the LIR2032 battery and charging circuit.
    The problem is that the charging circuit is completely inadequate for the type of battery. An Li-ion battery requires a specific charge method and parts of that specification must be met very accurately or else damage to the battery can occur or even fire.

    The problem is that the boards have a charging circuit that is made up of just a silicon diode and 200 ohm resistor. If the board is being run on a +5v power supply (very typical for Aruduino type boards and others too) then the battery WILL overcharge. There is no question about this because i've tested one now and even without testing it is apparent from the schematic that a diode and resistor will cause overcharging at some point.

    The best case here is that the battery overcharges, is damaged, and simply does not act as a battery backup when main power source is lost so the unit looses the real time, something that it is supposed to be able to do easily. The worst case is that it starts a fire. I think it is less likely to cause a fire, but after all who wants a back up battery that does not work when it is needed. So i am posting this in an effort to warn the general hobby based public about the defect and possible danger.

    The solutions are many, and the one i used was to remove the 200 ohm resistor. That allows the battery to still act as a battery backup because now being charged it will do that. It just wont recharge again, but we dont need that in a normal application anyway where the +5v power is usually available.
    Another solution is to remove the 200 ohm resistor and replace the battery with a CR2032 battery, which may be better.
    A solution which comes naturally in some applications is to run it at 3.3v (such as with the Due board) as that will prevent battery charging because 3.3v is too low to get the battery to charge.

    I used hot air to remove the 200 ohm resistor by directing the heat AWAY from the main IC chip so it would not subject it to an sudden extreme temperature shift.

    So PLEASE pass the word around the web so other people wont have this problem. People will thank you :)
    Also, if you have experience with any of these boards please reply with some information about that. Note also that the smart designs found on the web dont include that charging circuit.
     
    Hypatia's Protege and cmartinez like this.
  2. DickCappels

    Moderator

    Aug 21, 2008
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    :bump:
     
  3. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Strictly speaking Mr Al, the warning should say "do not buy it" because the design is seriously flawed, implying a risk. I think that the practical part would be to identify the boards that shouldn't be bought from the possible offers you could find in the Web.

    Any specific vendor you could (not) recommend?
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Photo
    Schematic
    Vendor
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    anything.

    ak
     
  5. MrAl

    Thread Starter Well-Known Member

    Jun 17, 2014
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    Hi there,

    That's a good idea, but i didnt really want to try to stop people from buying them, i just wanted them to know that they should remove the resistor for safety and long battery life. They work really well for keeping time, it's just the battery charge system that is bad.

    Here is a pic of two of the boards in question...
    Note the diode is the red colored component in both pics.
    Also note one board was damaged due to rework of the header but that has nothing to do with the charge problem.
     
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