I have come across the following circuit in a datasheet for a battery charger:
I have also seen a similar use of the charging circuit for use as a lithium battery charger, e.g http://shdesigns.org/pdf/lionchg.pdf
But is this circuit implementation actually suitable for a lithium battery? (I know the 6V circuit would require the voltage output of the LM317 (R1 and R2) to be set dependent on the attached lithium battery voltage.)
I have read that lithium batteries need the following order to charge safely and correctly (Assuming the battery is not below its discharge condition, in which case trickle charging is required first):
- Constant Current (CC)
- Constant Voltage (CV)
The constant current stage requires the battery to be charged at its max (fully charged) voltage with a fixed current. The max charge voltage is set by R1 and R2 in the top picture (Although would R3 affect the output voltage as it is in series with R2?). R3 and the BJT act as a current control.
The constant voltage stage requires the battery to be maintained at its max (fully charged) voltage, whilst the output current is steadily dropped. The battery is deemed "charged" when the output current is below a very small current limit, which is specific to the lithium battery. This stage initiates when the voltage across R3 gets close to around 0.6V the BJT starts the turn on, which shorts the adjust pin and drops the LM317 output voltage until it is stable at around 1.25V, which then reduces the output current to the battery?
Does R3 affect the LM317 output voltage as it's in series with R2?
How does the BJT and resistor precisely work in this setup? Won't the battery initially try and draw as much current as it can, meaning 0.6V would develop across R3 before the battery is sufficiently charged to transition into the CV stage?
Wouldn't the 0.6V developed across R3 mean that the potential "seen" by the battery is approx 6.3V? (6.9V LM317 output subtract 0.6V). Wouldn't this be insufficient to charge the battery fully?
When the BJT turns on, the voltage output of the LM317 will drop to 1.25V as the adjust pin is shorted (which bypasses the resistors). Surely this is not suitable for the CV stage of charging, as the potential drops to way below the battery charging voltage? How is current output to the battery affected during this voltage drop?
Is the BJT even required for the CV stage? If just the resistor was used to limit the current, wouldn't the current to the battery decrease as it reaches full charge anyway?