Battery backup design for SIM584C Gsm Module based Circuit

We (me and my friend) have designed a circuit that includes SIM548C Gsm Module. This circuit is powered by 4.24 V DC. Actually this voltage is acquired by using LM2576 regulator IC and input of it is 12V DC. So while 12V dc is available, my purpose is served. But in absence of 12V, i have nothing to do.
So, i want to design a circuit that includes a Li-ion battery-3.7V ( Not Battery Pack). In presence of 12V it will be charged and after fully charged it stops charging. As a result, while 12V is not available, battery can rum the whole circuit.
SIMCOM company (SIM548C Manufacturer) suggest a circuit (picture attached) for that in which 5v DC is given to the VCHG Pin and Li-Ion battery is attached to the VBAT Pin . At this case battery is charged through SIMCOM module . But it is for only Battery Pack that senses its own charge condition. Moreover it is not good for the Module.
So, i want to use an additional circuitary to serve my purpose in which battery is charged externally.
Can anybody suggest me such a circuitary?[/img]

Actually an external IC is required to serve this purpose. I know one of them. It is ZJ74. But i cannot find any datasheet for this IC. Can any body please help to find this.

Added after 58 minutes:

Actually i have found one...BQ25040

Hello, I have the same problem with battery charge.. I was wondering If you solved this problem please help !!! thank you

Re: Battery backup design for SIM584C Gsm Module based Circu

MCP73871
Microchip
Simultaneously charge and run the system

I've made a battery charger using MAX1811 for 1 Li-Ion cell.It is working perfectly. My question is can i use the same charger to charge 3 cell in parallel.Charging time is not a matter of concern.
thanks

First my standard disclaimer, that I am electronics n00b, and just started fiddling a while back, so plz validate with gurus here.

I was in the market for a battery backup solution for my project, in fact, I still am - just that it got pushed down in the list of priorities. In my research I found that the power-management IC's came accross, which assume LiION as the battery-power source, put down strict limit on no. of cells that can be charged with it, at a time. Apparently, charging LiION batteries is a complex or non-trivial algorithmic exercise, where adjustments are required and caution needs to be exercised so as not the damage the battery or in worst case, case an overheating & explosion. I would presume the power-management IC then to be limited by it's computational resources (i.e. how many computations it can do, to keep check that charging process is in control) is limited. And the result is, you need multiple such IC's to charge multiple LiION cells. IIRC, there are clear application-notes for specific Power-Management IC's (I think I checked TI ones), where such details are mentioned.

Li-ion batteries can be connected in parallel.But the current sharing between the batteries wont be even during the constant current charge mode due to the difference in impedance,SOC algorithm put in etc.So its always better to choose some standard ic's......

where can i get max1811?

hellow,
iam in india,where can i get max1811?...i tried in bangalore,no result..if you know the particular shop...pls let the adress.
thank you...

why dont you try to get some samples if the quantity s fine....
i can see its available in the maxim.....

hai arun,...pls let me know the particular shop name in bangalore/bombay/chennai...for this part...i dont have any paypal account and dont know the procedure to order thru internet..and it may take more time also...so pls help me..

@bikashh: you need to consider time multiplexing