SIM548 / SIM508 charging battery

[tasosmi]

hello,

i am using a sim548 module (is almost the same as sim508 ). So if anyone has used one of these modules, please answer!!!

Take a look at sim548 picture,
when i do NOT give 5 V in the VCHG pin, the battery does NOT charge and gives to the sim548 module 3.7 V (i saw that from the battery characteristics). So the VBAT pin is an input. Also, the battery gives 3.7 V to the GPS part of sim548 and to microcontroller.

When i give 5 V in the VCHG pin, then the battery is charging with maximum voltage: 4.2 V ,
so my question is: Now the VBAT pin is an output? How much voltage is going to the GPS part of sim548 and to microcontroller? I want always the supply input of all to be 3.7 V. How can i do this?

Thank you in advance!!!!!:-D

 

[buiducthang]

I think SIM548 have inside regulator. When we use 5V or 3.7V Battery, it convert into lower reference voltage (ie 3.3V). When have no 5V Power, it works with 3.7V Battery, when have 5V power, its charge for Battery and works with 5V Power.

 

[tasosmi]

First of all thank you for your answer buiducthang!

The SIM548 has an inside regulator for charging the battery. This regulator has as input the 5V from the VCHG pin and convert it to 4.2V to charge the battery.
As you said, when we have no 5V power, all the subsystems (GSM part, GPS part and the microcontroller) works with 3.7V from the battery
And as you said again, when we have 5V power, its charge the battery and all the subsystems are working with 5V power.

BUT , i don't want to work with 5V. I want to work with 3.7V all the time! How is that possible?

 

[wayneft]

I want to work with 3.7V all the time! How is that possible?

It's not possible because even with just the battery connected you are not working with 3.7 volts. You are working at anywhere from 4.2 volts down to 3.4 volts (The GSM I believe shuts off if the voltage drops below this level) depending on the charge of the battery.