How does a charger balance batteries connected in series? - Page 1
Forum section: electronics / beginners

author: Prithul0218
author_rank: Contributor
timestamp: September 30, 2018, 11:17:16 am
content: 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?

author: SeanB
author_rank: Super Contributor
timestamp: September 30, 2018, 12:30:36 pm
content: 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.

author: Prithul0218
author_rank: Contributor
timestamp: September 30, 2018, 01:20:15 pm
content: Quote from: SeanB on September 30, 2018, 12:30:36 pmThe 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?Sent from my Redmi 5 using Tapatalk

author: Prithul0218
author_rank: Contributor
timestamp: September 30, 2018, 01:23:08 pm
content: Quote from: SeanB on September 30, 2018, 12:30:36 pmWhat do you mean by that cell? The charger can not apply voltage across one individual cell, can it?

author: Prithul0218
author_rank: Contributor
timestamp: September 30, 2018, 01:28:41 pm
content: Quote from: SeanB on September 30, 2018, 12:30:36 pmIf 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?

author: mikerj
author_rank: Super Contributor
timestamp: September 30, 2018, 04:24:40 pm
content: There are different balancing strategies that could be implemented, I don't know what method the Turnigy uses.1) Start balancing at the terminal cell voltage, when the charger switches from CC to CV. If there is a significant imbalance at this point the either the balancing circuits must be able to sink the entire charging current, or the charging voltage must be reduced to match the charging current with the imbalance current capability.2) Start balancing during the CC stage, well before the terminal voltage is reached, e.g. maybe 3.8v. This gives much longer for imbalances to be corrected even with lowish balancing currents. Charge current could be varied as well to help with large imbalances.

author: Prithul0218
author_rank: Contributor
timestamp: September 30, 2018, 04:36:17 pm
content: Quote from: mikerj on September 30, 2018, 04:24:40 pmGot it. Thank you. What about the resistors?The Turnigy probably uses the second method, cause it's really slow with imbalanced batteries. So I always end up charging each cell separately when they get too imbalanced.

author: mikerj
author_rank: Super Contributor
timestamp: September 30, 2018, 08:50:23 pm
content: Quote from: Prithul0218 on September 30, 2018, 04:36:17 pmThe base of each darlington transistor pair is always pulled to ground via these resistors to switch it on. However the emitter of each darlington pair is tied to different voltages, depending on the cell it's connected to within the battery pack. To get a reasonably consistent base current the resistors have to be different. The values will have been chosen to ensure the balance switching transistors are fully saturated under worst case conditions.

author: Bratster
author_rank: Frequent Contributor
timestamp: September 30, 2018, 09:15:36 pm
content: Quote from: Prithul0218 on September 30, 2018, 04:36:17 pmThe groups of resistors on the right side of that schematic are the balance load. Those are all identical as shown by the schematic, looks like 120 ohms, although I'm not sure if that's the value of each resistor or the final value after they're all put in parallel.The resistors that you're noticing are different for each group are there to control the current going into the transistors, as already stated by another member, the voltage on the different balance circuit sections is different so it needs a different resistor value to keep the transistor current the same.That is separate from the balance current.Also side note, if your batteries are getting that unbalanced that's not good.Sent from my Fi Moto x4 using Tapatalk

author: Prithul0218
author_rank: Contributor
timestamp: October 01, 2018, 02:17:29 am
content: Quote from: Bratster on September 30, 2018, 09:15:36 pmI finally understood it. The emitters of the transistors are not going to the same voltage potential, they are going to separate terminals of the battery. That's why they need to use different value resistor accordingly.I was wondering if an N channel logic level MOSFET with the discharge resistors on drain pin would work. In that case, they would still need a drive transistor for the MOSFET in order to get the required VGS, right?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 01, 2018, 02:37:01 am
content: I don't know what this charger does, but the chargers I have all generally use the following strategy, which involves a sequence of steps. The charging/balancing is not continuous.First the battery string is charged to near full charge. Then charging current is turned off and cell voltages are evaluated. High cells have a light load applied and charge current is restored. Repeat until time limit is reached.This process will generally balance a battery, but if it is too unbalanced to start the time limit will be reached before balance is complete. I then either separately charge the low cells, or use an external balancers.