Electronics > Beginners
How does a charger balance batteries connected in series?
Prithul0218:
I saw Daves teardown of the Turnigy Accucel 6 LiPo charger that not only charged the LiPo batteries, but also balanced each cell voltage. I saw the schematic of the balance part of the charger and got quite confused. I have attached the balancer schematic down below and here's Dave's video:
So my question is, how does the charger balance the battery voltage for each cell. I have a few assumptions. Which one is correct? if any.
Assuming the charger is charging a 2 cell 8.4V battery. The charger puts 8.4V across the pack. Waits til one of the cells are 4.2V -
1) As soon as one reaches 4.2V, it turns on the transistor to discharge the cell, but "does not" turn off the 8.4V power supply. So, some of the current that was going to the higher cell voltage is now being turned into heat. But still some current might go into the cell and slightly overcharge it maybe?
2) As soon as one cell reaches 4.2V, it turns off the 8.4V power supply and turns on the transistor for the cell. Once both cells are at the same voltage, it turns off the transistor and starts charging at 8.4V again.
Another question I have is, why are R124, R126, R128... resistor values different? Are they not discharging each cell with 20 ohm resistor directly across them? (transistor acting as a switch) In that case, are they discharging different cells at different currents?
SeanB:
Resistor values are different because the voltage across each balance stage rises by 4V per cell, so the higher value is there to keep the current into the control transistor the same. Resistor values in the balance side are going to be able to draw a current equal to the charge current, and as the cells enter balancing the charge current is low anyway, so the resistors will discharge the cell slightly, the voltage will drop down and the charger will attempt to slow charge that cell again to capacity.
Prithul0218:
--- Quote from: SeanB on September 30, 2018, 12:30:36 pm ---Resistor values are different because the voltage across each balance stage rises by 4V per cell, so the higher value is there to keep the current into the control transistor the same.
--- End quote ---
The transistor is connected between each cells positive and negative terminal. Not across the positive terminal of each battery and the negative terminal of the last battery. So all resistor values should be equal right?
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Prithul0218:
--- Quote from: SeanB on September 30, 2018, 12:30:36 pm ---the voltage will drop down and the charger will attempt to slow charge that cell again to capacity.
--- End quote ---
What do you mean by that cell? The charger can not apply voltage across one individual cell, can it?
Prithul0218:
--- Quote from: SeanB on September 30, 2018, 12:30:36 pm ---Resistor values are different because the voltage across each balance stage rises by 4V per cell, so the higher value is there to keep the current into the control transistor the same. Resistor values in the balance side are going to be able to draw a current equal to the charge current, and as the cells enter balancing the charge current is low anyway, so the resistors will discharge the cell slightly, the voltage will drop down and the charger will attempt to slow charge that cell again to capacity.
--- End quote ---
If I have, say a 5Ah battery. The charger will set the cutoff current to 500mA (0.1C, usually used for LiPos). In that case, the discharge current will not be equal to the charge current even at constant voltage phase, because the with the 20ohm resistor, current flow will be equal to 210mA. So the cell will be overcharged, won't it?
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