Battery power back-uped circuit


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  1. #1
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    Default Battery power back-uped circuit

    Hello All,

    I'm making GSM alarm controller using a SIM800L GSM chip from SIMCOM.

    Since the project is about "monitoring" and "alarming", it is important ton ensure a redundant power feeding (= LiPo battery).

    The module supports 3.4 to 4.4 VDC and I would imagine to go for 4VDC (my imagination - will depend on circuit design). For info, this chip draws up to 2 Amps when bursts occur (!) but draws around 40mA in idle state.

    I've read some threats about "oring" DC power sources (with Schottky diodes or so) but here I don't know how to achieve a correct circuit since I'll be using a rechargeable battery (not like a battery for an RTC).

    My points are:
    - is the schema here under or something looking like "possible" at all?
    - the LiPo 3.7V cell will need to be charged (or not!) after a +/- long main voltage supply (i.e. 12VDC) supply but how is it possible since it is connected in parallel with another circuit
    - if there is already something "out there", can anyone point me to a link please?

    Thanks for any info

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    Roger

  2. #2
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    Default Re: Battery power back-uped circuit

    if you charge lipo batteries incorrectly there is a substantial fire risk, drawing power from the charging cct can lead to improper charging.
    you might like this
    http://ww1.microchip.com/downloads/e...tes/01149c.pdf
    This is more entertaining than Free to Air TV

  3. #3
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    Default Re: Battery power back-uped circuit

    Thanks Richard,

    That was really quick!!!

    Yup, I think this is exactly what I'm looking for.

    I should have thought about these chips since I sometimes use MCP73831 ones.

    Thanks a lot again
    Roger

  4. #4
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    Default Re: Battery power back-uped circuit

    Quote Originally Posted by richard View Post
    if you charge lipo batteries incorrectly there is a substantial fire risk, drawing power from the charging cct can lead to improper charging.
    you might like this....

    As Richard pointed out risk of fire, you may want to consider Li-fepo4 batteries.
    Almost no risk of fire under high heat, short circuit or over charging ...

    But they are 3.2 Volts.
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

  5. #5
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    Default Re: Battery power back-uped circuit

    Thanks Sayser,

    Unfortunately, 3.2V will be too low - anyway, thanks for the Li-FePo4 advice

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    BTW, going back to AN1149 and the choice of the P-Channel MOSFET, as I don't know if a FDN360P will do the job?
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    Any clue?
    Roger

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    Default Re: Battery power back-uped circuit

    Consider this one: DMP3099L-7

    I have my own charging circuit controlled by PIC with FVR.
    Measure batt voltage by adc read on FVR to calculate the batt voltage.
    If over 4.25V then cut the charge; if lower than 2.75V then suspend all operations.
    All done within the pic.

    You need an ppnTR-P-ch charging circuit; very simple.
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

  7. #7
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    Default Re: Battery power back-uped circuit

    FVR is not at all accurate, so that 4.25... I doubt you are safe.

    Ioannis
    Last edited by Ioannis; - 19th May 2020 at 09:58.

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    Default Re: Battery power back-uped circuit

    Roger, the minus Vgth is correct. Consider it a different visual angle!

    In P-channel or PNP transistor the voltages are the opposite of the N-channel or NPN transistors. Means that the voltage og the gate in respect to the source is negative. The key to understand here is the term "in respect".

    Ioannis

  9. #9
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    Default Re: Battery power back-uped circuit

    If I may ask further,

    The "most important" value to take care off is the "Gate Threshold Voltage"; right?

    It has to be as low as possible...but how low and why?

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    I thought RDSon value is very important too. But in this case, where the current is somehow small, is this less important in choosing the "right" MOSFET?
    Roger

  10. #10
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    Default Re: Battery power back-uped circuit

    Quote Originally Posted by Ioannis View Post
    FVR is not at accurate, so that 4.25... I doubt you are safe.

    Ioannis


    Why is not not accurate?
    I get 1.024V with 1x output.
    And if the Vdd is 2,5V I still have 1.024V at my FVR output. I never encountered an accuracy issue.

    But, if you have a high freq circuity on the same pcb, or even if your PWM module is running, then I agree, FVR can be disrupted.
    Also, here is a simple circuit diagram for NPN-PCH charging. Assuming you are not charging the batt with high amps, LM317 would do the job. If not, change LM317 to a switching circuit to have higher efficiency and lower heat.


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    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

  11. #11
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    Default Re: Battery power back-uped circuit

    Because according to Electrical Specs, the FVR may be from -8% to +6% for example on 16F1827.

    I would not trust this to charge a LiPo battery at all!

    Ioannis

  12. #12
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    Default Re: Battery power back-uped circuit

    Roger,

    The important specs is dependent on the application.

    For example, if you drive the MosFet from a logic level chip, say a MCU, then you want the Vgs to be as low as possible for the MosFet to be fully closed and have minimum Rdson

    Other spec like the circuit on the above Note, is the Rds itself, so you don't have thermal losses.

    Also the Vds max should be more than the worst case on your circuit voltage levels.

    Ioannis

  13. #13
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    Default Re: Battery power back-uped circuit

    Quote Originally Posted by Ioannis View Post
    Roger,
    The important specs is dependent on the application.....
    Ioannis
    Thanks Ioannis.

    I'll give it a try with a few different MOSFETs and see what happens.

    For any reason, until I haven't tested it with my "hands" (and the voltmeter ), it doesn't get inside my head that easy.

    Maybe I'm getting too old to play with all these fantastic toys
    Roger

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    Default Re: Battery power back-uped circuit

    Experience is gained only in action!

    Ioannis

  15. #15
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    Default Re: Battery power back-uped circuit

    Quote Originally Posted by Ioannis View Post
    Experience is gained only in action!

    Ioannis
    This statement is giving me naughty thoughts
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

  16. #16
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    Default Re: Battery power back-uped circuit

    You got yourself 2 points so far with naughty thoughts!

    Ioannis

  17. #17
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    Default Re: Battery power back-uped circuit

    Hello, Flotul,

    LiPo batts are somewhat tricky to use ...

    I suggest you to use something called " BMS " with your battery ... it's a device that cares with every problem that could occur : Overchage, under voltage, load shorted ...

    that can avoid fire or exploding the battery ... not negligible !

    https://www.nkon.nl/fr/accessories/1...2mos-5530.html for example ...

    Alain
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  18. #18
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    Default Battery power back-uped circuit

    Hi Alain,

    I thought these BMS circuits were already interated in the batteries?

    When I look to one of my LiPo battery, I can see some kind of a circuit at the top of it.

    I might be wrong, but I think this is a protection circuit (difficult to see okay, but believe me, there is a small circuit here ) or isn't it?

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    Roger

  19. #19
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    Default Re: Battery power back-uped circuit

    Hi, Flotul,

    Those BMS are not always installed ...

    I have dozens of LiPo / LiFe batts home and only one has it included ... so, consider it's not mandatory !!!

    sometimes, there's only a fuse !!!

    Alain
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  20. #20
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    Default Re: Battery power back-uped circuit

    my experience to date is that the bms is far more likely to fail before the battery and that it will then do its best to destroy the battery utterly.
    i have had good success in removing failed bms modules to discover perfectly good batteries . better bms modules are cheap and plentiful.
    just because they won't take a charge is not always a dead battery
    Last edited by richard; - 2nd June 2020 at 11:43.
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  21. #21
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    Default Re: Battery power back-uped circuit

    Quote Originally Posted by Acetronics2 View Post
    Hi, Flotul,

    Those BMS are not always installed ...

    I have dozens of LiPo / LiFe batts home and only one has it included ... so, consider it's not mandatory !!!

    sometimes, there's only a fuse !!!

    Alain

    I agree with this.

    When we order batts, there is option having it with the protection; As it is an option and if you have a smart charging circuit, then it just additional cost which I think is unnecessary.

    The ones with the protectioın we received have higher failure return rate than those without the protection. Circuit burns out eventhough there is nothing wrong with the charging.
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

  22. #22
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    Default Re: Battery power back-uped circuit

    Lithium batts should be under surveillance either charging or discharging.

    The little circuits are supposed to work in either case, not allowing overcharge or flat discharge the batteries. The second is important as many devices continue to drain the batteries to very low levels, finally destroying them.

    It is really annoying that the protection circuits fail...

    Ioannis

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