Can i charge a 7.2v Li-ion cell from a 7.5v 1200mA supply?
This supply also runs a GPRS application drawing 300mA with 1A pulses.
What i would like is a circuit that trickle charges the Li-ion cell (dont care how long it takes) but also runs the application, then on mains failure the battery needs to take over.
Will a FET so the neccessary switching between battery power & apadter power ?
I wouldn't have thought so?, I'm not particularly familiar with Li-Ion batteries, but ohms law tells you that you need a higher voltage for current to flow.
I'm presuming the 7.5V is regulated?, otherwise it will probably be considerably higher than that anyway.
how can a li-ion cell be 7.2 volts my friend?? its electrode voltage is 3.6 and it shows uoto 4.2 volts.
now as regard to charging, li-ion battery is a bomb!! handle it with care. dont charge them serially as you are saying and individual cell should not be charged above 5 volts. there are special ics for this algorithm. moover charging the cell above 4.2v is equally bad for cell life. moreover you need a temperature sensor in your charging system. why do you want to use FDC638P? you have ics specially for li-ion batteries cheaply available. go for them.
charge balancing is a troublesome work. if you want to apply and forget then you must use the ic's recommended for this these are MAX1737 , LP3947 , LP3921. and many more. just search the web. these set the algorithm for charging the batteries. one of these ic's support 2 cell charging. but charge balancing i dont recommend.
I made many sets of two 2200mAh Lithium-ion cells in series. I charge them from an LM317 set to exactly 8.4V so each cell gets 4.2V max. The wall-wart that feeds the LM317 limits the charging current.
I don't use and don't need a "charge balancing circuit". After 20 charges and discharges the cells are well balanced.
After about 4 hours the cells are fully charged and can be removed from my charger.
No.
My wall-wart is listed as 9VDC at 500mA. When the battery is discharged to 6.4V (its load shuts off when the battery voltage drops to 6.4V) then the charging current is limited to about 500mA because if it tried to go higher then the wall-wart's voltage will sag causing the LM317 to dropout which limits the current. The battery gets warm during the first hour and a half of charging then it cools for the remaining two and a half hours of charging.