Balance charger doesn't balance

[FlyingTiger]

I don't know if it's the charger or the batteries but I find my flight times getting shorter. I am using a Thunder AC6 charger with Blue Lipo 2s 800mAh batteries. Charging them at 1C the charger says they are balanced at 4.21v per cell, but after 24 hours off the charger they all read around 4.05 on one cell and 4.12 on the other cell, give or take .02v. Should I be blaming the charger or the batteries? Granted, these are not the highest priced batteries but I thought they had pretty good reviews.

[RCModelReviews]

That level of self-discharge would indicate that the batteries are stuffed -- providing your voltage readings are accurate.

How are you measuring the voltage? If you're relying solely on the charger then it may be wildly out of calibration and over-charging the cells because it is under-reading the cell voltages.

[FlyingTiger]

I'm using one of those plug-in voltage monitors from Hobby King that gives a reading of each individual cell, up to a 6 cell pack. These batteries are being used in a Walkera V200D01 heli and they do get pretty warm after every flight. They are 20C batteries and I usually run at around half throttle so I don't think I'm over taxing the discharge rate.

[RCModelReviews]

I wouldn't rely too much on those plug-in battery checkers -- they're notoriously inaccurate.

Find yourself a good digital multimeter and measure the voltages with that. You might be surprised at how far out your voltage tester actually is.

I have a meter here and it measures the first three cells just fine but on a 4-cell pack it always reports the 4th cell as being over-charged, even though it's not. A quick look shows that they've used rather "cheap" components with poor tolerances so it's no wonder it's not very accurate.