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SIM300 charging question (VCHG and VBAT)

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ShahbazAli

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sim300 vbat

I am building a module with SIM300 (supports both SIM300C and SIM300D). This contains micrcontroller and some other circuitry also besides SIM300. I intend to use SIM300 charging circuit to charge Li-ion battery. But I need to use the backup battery for all of the circuit and not only SIM300. VCHG is 5V coming from an LM317T, whereas VBAT is 4.2V coming from MC34063A. VBAT is VCC for all the circuit. So that when the 12V supply to regulator is gone, battery takes over the whole circuit.

Will this work? How will SIM300 gauge the charge on the battery when VBAT is coming from power supply?

Thanks.
 

GSM Man

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sim300 battary charge current

ShahbazAli said:
VCHG is 5V coming from an LM317T...
You have to limit the current into the VCHG pin, which an LM317 will not. Look carefully at the battery charger requirement for those devices.

ShahbazAli said:
How will SIM300 gauge the charge on the battery when VBAT is coming from power supply?
The Lithium Ion chemistry is very sensitive to charge current and voltage. If not properly controlled you will effect the life of the battery. You'll need to design your circuit to isolate the other circuitry from the VBAT pin when the SIM300 is charging the battery. Otherwise the SIM300 will 'think' that it is supplying more current to the battery than it actually is.
 

    ShahbazAli

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ShahbazAli

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sim300 vbat

Thanks a lot GSM Man!

So what would you suggest for VBAT separation while charging? A miniature relay or some semiconductor?

P.S. I want to buy GSM Book from GSMMadeEasy but can't. You have any idea how to contact them?
 

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ShahbazAli said:
So what would you suggest for VBAT separation while charging? A miniature relay or some semiconductor?
If your other circuitry can handle a lower voltage you could use a Schottky diode to seperate the VBAT fro the 12V line. A relay would also do.

ShahbazAli said:
P.S. I want to buy GSM Book from GSMMadeEasy but can't. You have any idea how to contact them?
We're still working on the eCommerce part of that web site. Send me a PM with your contact info and I'll arrange for you to get a copy.
 

ShahbazAli

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I am using SIM300D and it proved to be very good at Li-ion charging. Just connect 5V to VCHG pin and connect VBAT to battery's positive terminal. No current limiting is needed now with SIM300D. Charging circuit draws a maximum of 650mA.

Just ensure that Li-ion battery is over-voltage protected(or put a fuse) as I destroyed one board because of an accidental short.
 

ma_ok36

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I doing this work:
connect "CHG_IN" pin to 5v & "VBAT" pin to battery pack(3.6v)
however when i sending "AT+CBC" message to sim300, the response of the module is no charging.
is other component necessary to charge battery or the method of charging is mistake?
plz jelp me
 

ShahbazAli

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This should work. Please post your schematic. How much is current usage? SIM300 is not turned on unless you turn on using PWRKEY. It will start charging though.
 

ma_ok36

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the module is working but by battery and not by 5v

41_1265286653.jpg
 

ShahbazAli

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I am afraid your short posts will help not to understand what exactly is the problem. When looking for help in the forum once needs to write as much as he can.
 

ma_ok36

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Hi all, i'm working on designing a circuit, including a GSM module and a microcontroller. as i told, i've connected VBAT pin to a 3.6 Li-ion battery. but; i dont know how to charge this battery, using sim 300 module.
can i connect VCHG pin directly to a 5v supply? how can i be aware of charging completion? if some additional circuits or elements are needed, plz make me aware of that.
tnx in advance...
 

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Charging will start in this scenario after a few seconds. Are you using a DC power supply? You should see current usage increasing to 650mA or above. This use will go on decreasing over period of time, depending upon the battery size.

Added after 2 minutes:

I don't think SIM300Z has battery charger. SIM300C and SIM300D do have.
 

ma_ok36

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i'm using a sim300cz module, and according to its datasheet, i'm sure that i can charge the battery(connected to VBAT), just by connecting the VCHG pin (of module) to an external supply(1.1VBAT<VCHG<5.2). So, i used a 5v ac supply(trans, like a mobile charger) (cause VBAT=3.7v).
the problem is that: the charging process never started, as whenever i checked it by AT+CBC command, i received: "+CBC 0,..." (which 0 means battery isn't charging). and after couple of hours, the battery level was decreased, so that my module turned off!
whats the reason :!::?:
 

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I have used both SIM300C and SIM300D for charging. Have you tried some other unit? There might be a fault with the unit.
 

GSM Man

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Review Section 3.5.5 "Charger Requirements"in the SIM300C hardware manual and you will see that it states "Charging Current Liitation:650mA".If you are not using a current-limited charger you may have destroyed the circuit inside the modue.
 

rongo024

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Built in charger capacity of gsm module is not used in professional design. i dont know what is the reason. but it is very sophisticated.
 

susano

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hi every body. i've the same problem as ma_ok36. i'm charging a Li=ion battery via VCHG pin(of sim300cz), connected to an ordinary 300mA cell phone charger. is it still needed to limit the current? if so, how can i do that? i've opened(!) my out of use cell phone, but i couldn't find any special limiting regulator inside it!...
dont u have any suggestion 4 us(like a simple recommended circuit or part)? i've spent too much time 4 this & i'll appreciate if some one help me.
tnx
 

susano

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which ic do u recommend 4 this reason?
 

GSM Man

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susano said:
i'm charging a Li=ion battery via VCHG pin(of sim300cz), connected to an ordinary 300mA cell phone charger.
Is this a current-limited charger, or just a 300mA transformer with filter/regulator. If it's a transformer, its output voltage may drop when the battery voltage is low and it draws a lot of current.

What you need is a voltage regulator that has a current-limited output. There are several available from various manufacturers. Here's one from Linear Technology: https://cds.linear.com/docs/Datasheet/3663fb.pdf
 

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