SIM300 charging question (VCHG and VBAT)

[susano]

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: http://cds.linear.com/docs/Datasheet/3663fb.pdf

tnx a lot, that's great! but unfortunately i couldn't find this regulator in my country!!
can i use a simple 7805 regulator which it's output current is limited to 300mA instead?
Besides, is there any easy way for finding an equivalent (replacement) for some electronic parts? i'm using some books or softwares, like "vrt-disk". but i couldn't find anything there too.
tnx alot in advance

 

[GSM Man]

You can build a 2-stage regulator using two LM317's or equivilant. First stage regulates voltage to 5.0V, and second stage limits current to 650mA. If you search the web sites of voltage regulator manufacturers you can find schematics for current limiters (TI, National, etc.).

Unless you have a special need, you shouldn't limit the current to 300mA; limit it to 650mA. Otherwise, it will take longer for your battery to charge.

 

[susano]

1) the 2-stage LM317 regulators need at least 25V minimum input voltage, thats too much 4 me. cuz i have to charge the battery using a vehicle battery(max 12V)

2) i've a more important problem: i'm using a nokia cellphone battery to supply my system, which includes a GSM module, an atmega16 micro controller and a GPS module mainly; and there r only 1 fixed and 1 blinker LED. Now;without connecting the charger circuit, the rate of battery discharge is so high, that its voltage decreases from 3.8 to 2.8 just in less than 20min!!

3) after 30min that i connected an standard LM317 structure to the VCHG pin, the level of battery, not only increased, but also decreased from 2.8 to 2.6!! while because of low level input voltage, both GPS and GSM modules are off!!
can any one help me with these problems plz??

 

[GSM Man]

1) the 2-stage LM317 regulators need at least 25V minimum input voltage, thats too much 4 me. cuz i have to charge the battery using a vehicle battery(max 12V)

I don't know where you came up with the 25V! The regulators only need a couple of volts overhead to regulate.

2) i've a more important problem: i'm using a nokia cellphone battery to supply my system, which includes a GSM module, an atmega16 micro controller and a GPS module mainly; and there r only 1 fixed and 1 blinker LED. Now;without connecting the charger circuit, the rate of battery discharge is so high, that its voltage decreases from 3.8 to 2.8 just in less than 20min!!

It's easy to compute the amount of time that the battery should last. Either measure, or calculate, the current draw of your circuit and then divide that into the mAH rating of the battery. Also, your battery should be charging to 4.1V (assuming it is a LiOn battery). We routinely build systems like yours that run continuously for days on a fully charged, properly sized battery. Your battery may be undersized for the application.

3) after 30min that i connected an standard LM317 structure to the VCHG pin, the level of battery, not only increased, but also decreased from 2.8 to 2.6!! while because of low level input voltage, both GPS and GSM modules are off!!
can any one help me with these problems plz??

If the current into the SIM300 charger pin is not limited you may have damaged the circuit and the battery.

 

[susano]

I don't know where you came up with the 25V!

tnx GSM Man for useful replies! i've studied the LM317 datasheet from ON semiconductor there is an option for a 2stage regulatorfigure 1)Vin=32-40 and one for current limiterfigure 2)Vin=25 do i need lots of diodes and MOSs just for charging a battery, by means of the GSM internal charging circuit?!!

We routinely build systems like yours that run continuously for days on a fully charged, properly sized battery. Your battery may be undersized for the application.

im using a 860mAH 3.7v Li Ion battery, and as i told none of my system's parts(sim300cz GSM, LEA5H GPS, and mega16 av0r); dint draw so much current.are there some problems with LEDs? have u used some LEDs in your systems? i think the problem is that; even a 1stage regulator couldn't charge the battery?! dont u think that maybe there is something wrong with GSM internal charger?! how can i charge the battery directly?

 

[GSM Man]

The schematic you show is for a laboratory supply with adjustable voltage and current - those input voltages are not required for all applications. Look at this page for a good description of using the LM317 http://www.techlib.com/electronics/regulators.html.

Most LED's used as 'indicators' draw 20mA or less - check the spec on yours. You need to calculate or measure the current draw of your circuit, to figure out what the problem is.

There are plenty of IC's for charging liOn batteries - you can do a web search to find them.

 

[susano]

tnx for the link; it was very helpful. i tried to measure the current drawn by the whole circuit; using an ampere meter in series with the battery line. but the amount of current is strangely high!(about 300mA for mainly a gsm, a gps, and a micro controller) should i try any other way for measuring the current, or there's something wrong with the circuit?

 

[GSM Man]

300mA is high for those components - try to measure the current draw of each one individually to see what is drawing ther most current.

 

[susano]

hi again!!
i measured the current drawn by different parts of system. the whole system draws about 170mA (gsm:30mA, gps:110mA, the rest:30mA) as u can see, the gps module consumes a very high current(either while sending NMEA messages or doing nothing!!) as a result, i'm not able to charge the battery through GSM module, unless i disconnect the GPS from the system!! (i found out that the problem is with gps high current consumption, not the charging circuit(2 LM317s))
what should i do to overcome this problem? dont u think that this high current is unusual?(in the data sheet of gps(LEA 5H) only a 150mA peack current is mentioned and nothing else!)