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Thread: Charging of RTC battery

  1. #1

    Charging of RTC battery

    Due to some fairly tight physical constraints, I need to use a fairly small battery for my RTC. 20mm, 12mm and even 10mm are all too large and I'm looking at using a 6.8mm ML621 which is a rechargeable 5mah. Rough calculations suggest this should last around 100 days for the RTC, which is fine as this will rarely go 100 days without use, but means I need to ensure that I can charge it when the unit powers on.

    Panasonics recommended charging circuit is fairly straight forward, just a Schottky to drop the voltage (Max charge voltage on the battery is 3.2v) and a resistor of specified value, so I was hoping to use something like this
    Click image for larger version. 

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    I know there is a diode already on the 3v3 -> VBat line though and just wanted to make this wasn't going to interfere with anything that's already happening on that circuit. Does this setup seem reasonable?

  2. #2
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    Following the manufacturer recommendation sounds reasonable. Is there something that prevents the battery from being discharged to an unsafe voltage?

  3. #3
    Quote Originally Posted by jonr View Post
    Following the manufacturer recommendation sounds reasonable. Is there something that prevents the battery from being discharged to an unsafe voltage?
    Unsafe for the VBat input or unsafe for the battery?

    These ML batteries should be safe to fully discharge is my understanding, with the proviso that they 'only' get around 100 full charge cycles, which is fine for this use case.

  4. #4
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    You mention an existing diode, but don't explain...

  5. #5
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    Quote Originally Posted by MarkT View Post
    You mention an existing diode, but don't explain...
    have a look onto schematic: VBAT-pin and 3.3V are combined with diodes to go to Teensy

  6. #6
    Quote Originally Posted by MarkT View Post
    You mention an existing diode, but don't explain...
    There's a pair of diodes on the VBat circuit of the Teensy 3.5 from what I can see (https://www.pjrc.com/teensy/schematic36.png). One is feeding 3v3 onto the controller pin, the other is on the VBat line itself preventing this 3v3 connection back feeding out the pin on the board. I'm assuming they're both schottky's

    This makes me think that it's fine for there to be around 3.2v on the VBat MCU pin (Higher than the typical 3v battery provides) and so should be OK to essentially bypass one of the existing diodes to allow charging, but I wasn't sure if I was missing anything.

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