SIM300D Charging Circuit - help needed

[Kaktus621]

SIM300D Charging Circuit

Hello!

I have some questions related to the charging possibility of the SIM300D. My aim is to build a device, that can be as well used mobile (power by a LiIon-Battery) or e.g. in a car (powered by Car-Voltage).
What I understand by the datasheet is, that you connect the external supply _only_ to the VCHG pin (well, and common GND, of course) and everything else (SIM300 as well as the other controllers etc. is supplied by the battery.

Now lets look at the situation that the battery is charged to 100% but the external supply is still connected. Is the SIM300 smart enough to supply everything else by the external supply or will the battery still be used (that means it gets discharged a bit, then charged again by the charging circuit and so on and so on - isn't this bad for the battery?)?

If it's not "smart" enough and it's bad for the battery, what would be the best solution to the problem (i.e. not unnecessary (dis-)charging of the battery)? Do you think the attached schematic (well not exactly that, but something in this way) would work?

Thanks in advance,
Kaktus621

 

[GSM Man]

Re: SIM300D Charging Circuit

You need to be aware that the VCHG input must be connected to a current-limited regulator or you may destroy the module. Using a relay as you indicated is a reasonable approach. You may also consider using diodes to supply the other circuits from the car battery when it is connected.

 

[Kaktus621]

Re: SIM300D Charging Circuit

Hi!

Thanks for your answer. I have thought a bit and wanted to get rid of the mechanical components (i.e. the relay). Because of this, I thought about using a transistor. The attachment shows my current approach (Well I didn't thought about the current limiting yet).

Normally, the PNP-Transistor gets pulled down to GND on the base -> Transistor conducts and everything is powered by the battery. If an external supply is connected (through J1), the Base will be pulled to High and the Transistor does not conduct. The Diode makes it possible to power "Everything else" by the external supply thus the battery does not disconnect itself when there is no ext. supply (current can't flow from collector to base).
So at this point the external supply is connected to Vchg (= Vsupply) and to Vcc of the other circuit ( = Vsupply - 0,7V because of diode, but that's okay) and the battery is _only_ connected to the Vbat pin(s).
The 470µ capacitor acts as a bypass supply when disconnecting the external supply.

I've simulated this circuit and it seemed to work. Should also work in real, shouldn't it? Or is there something I've forgot?

EDIT: Do you have any suggestion for a current-limiting circuit? I want to use as few/cheap parts as possible.

 

[Kaktus621]

Re: SIM300D Charging Circuit

Sorry for double-post.

Is a current-limitation using the LM317 appliable to this case? In order to have 625mA, there has to be a 2 Ohm resistor (see attachment).

If I'm right, these 625mA will flow through the resistor. Does this mean that it should be suited for 625mA * 5V = 3,2 W ? That would be a quite large resistor...

 

[GSM Man]

Re: SIM300D Charging Circuit

Yes, the load current will flow through the resistor. You also need to regulate the voltage into the VCHG pin to 5.0 - 5.25 V, so you'll have to place a voltage regulator after the current limiter.

I can tell you from experience that using the charge circuit in the SIM300 requires a good bit of work. Most charger applications use a device specifically designed for this - to supply a fixed voltage with current limiting. They are typically switch-mode devices to eliminate the need for for a high-power resistor. There are a few device available on the market but they tend to be hard to find, and they're usually available in only in very small packages i.e. LT3653.

Do you really need to have the battery charged in the vehicle. If so, it may be easier to charge it using a seperate charger circuit - especially if this is a one-off design.

 

[Kaktus621]

Re: SIM300D Charging Circuit

Hmm if the SIM300 ist that problematic, a seperate chaging circuit might really be a better idea. I already used a MAX1811 in another circuit and it works quite well. Only two capacitors and it works flawlessly.

Using a Charging Circuit would mean that I can connect the whole circuit to the Transistor and not just the "Everything else" part. This would assure that the battery is only connected to the charger and does not get unneccessarily used when there is an external supply available, wouldn't it?

I think this would be the easiest solution to the problem.

 

[ma_ok36]

i working with sim300z and not succeed to charge the Li-Ion battery.
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 hlp me!