I'm currently trying to build a battery charging circuit and decided to reverse engineer the battery management circuit of BeagleBone Blue to understand how it works. Some of my questions are pretty basic/shallow, so please bear with me.
1) The circuit is using a MP2615 for charging 2S Lipo/Li-ion batteries.
As I go through the datasheet, I wonder what happens if you try to charge a over-discharged (unhealthy) lipo/li-ion battery (lets say 1V per cell).
1.1.Q) Will MP2615 still try to charge it in trickle charge mode even the cells are at 1V (I couldn't find anything related in the datasheet with the minimum necessary battery voltage to start charging)?
1.2.Q) It says that Recharge Threshold at Vbatt is 3.0-4.0 V/Cell. Does this mean that it will not charge the batteries at 4.00-4.20 V/Cell? So you cannot charge a slightly discharged battery unless it is below 4.00 V?
2) The circuit has a S8261 for extra protection.
And the model they are using is S-8261AAJMD-G2JT2x which has an overdischarge voltage protection at 2.50 V/Cell.
2.1.Q) As I know that a healthy cutoff voltage for a Li-ion/Lipo is around 3.00-3.30V. So it is not a wise idea to use the batteries until the protection kicks in (2.5 V/Cell)? Why is undervoltage limit so low in BeagleBone Blue board?
Best Answer
Most of the answer comes from the Operation Flow Chart (Figure 4):
1.1.A) trickle charge will start if the Vbatt Vtc=3 Volts/Cell. So yes trickle charge will start with the cells < 1 Volt.
Note that there is a safety timer that is set using a cap (C10 1uf) on the TMR pin (14). This sets the period of the internal oscillator period to .46 seconds, The trickle charge will go for 8192 cycles or about 3768 seconds , about 62 minutes. If the Vbatt doesn't get above Vtc (3 V/Cell) then it will be terminated and the CHGOK becomes open drain to indicate the timer-out fault.
1.2.A) The recharge threshold only applies when the the charger already charged the battery (Vbatt=VBatt_full and the current came down to Ichg
If you plug the BBB into the charger and the battery is at 4.1 volts, the charger will go right into constant current charge (CCC).
2.1.Q) I agree with you. This seems way to low of a voltage to drop out.