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I am using a 3S (series) lithium polymer batteries (the ones we get from hobby king). I have designed 2 boards .
1) charging board - using the BQ24170.
2) balancing board using - S8204B.

I am charging the battery via the charging board and it takes about 1.5 hours to charge at a current of 3 Amps or so.

Now, the connection is as stated - bench power supply/wall adaptor -> BQ24170 charger -> battery.

I put the balancing circuit as stated - bench power supply/wall adaptor -> BQ24170 charger -> S8204B balancer -> battery.

Now, as is shown in fig 1 -balancer circuit- the battery takes about 4-5 hours and the current does not exceed 1.2 Amps, How it is possible ?

I checked if the MOSFETS is turning on/off making the charging slow, but they were not behaving so, then i isolate the charging MOSFET (charge FET Q3). I did it by constantly pulling the gate to EB+ and never shutting the charging FET. I hope that the charging would fasten up, but still doesn't work.

Any suggestions please ?

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The S8204B is a battery protection IC, not a balancer. It will disconnect the battery if it thinks any cell is getting over charged or discharged, but it cannot equalize the cells.

With Q3 forced on the charging current still has to go through Q4. If the protection IC doesn't turn this FET on then about 0.6V will be lost through its body diode. You could force both FETs on, but then the protection circuit is useless!

Another factor that may be reducing your charging current is the resistance of components between the charger and battery (FETs, sense resistor, connectors, wiring). As the battery approaches peak voltage (12.6V for a 3S Lipo) any extra resistance will make the charger throttle back to keep the voltage at its output constant. at this point even a small amount of resistance can have a dramatic effect.

If your battery is out of balance then the protection circuit may keep switching it off and on to prevent overcharging. Measure the voltages on each cell of the battery when charged - are they equal? If not then charge or discharge each cell individually until they all have the same resting voltage.

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  • \$\begingroup\$ Thank u sir. Can u recommend any balancing chips pls. \$\endgroup\$ – Board-Man Jan 12 '15 at 19:29
  • \$\begingroup\$ Battery protection ICs with built-in balancers are hard to find. You could build a separate balancing circuit like this using standard parts. However so long as you ensure that the battery is well balanced initially it should not go out of balance in use (if it does then it has a weak cell and should be discarded). \$\endgroup\$ – Bruce Abbott Jan 12 '15 at 22:45

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