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Batteries Won't Get to Peak Voltage

Old 09-20-2015, 10:59 PM
  #1  
oldtyme
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Default Batteries Won't Get to Peak Voltage

I have a lot of LiPo batteries. We're talking about some Turnigy 3S 2200mah batteries. When I put them on the charger (Hitec X1Pro) they charge up to around 12.57v and just stay there. Most all of my other 3S batteries reach the 12.6v mark and reach the end of charge. I've measured the IR and they all seem to be fine and the cells are pretty evenly matched. When I measure the battery packs on my Futaba battery checker it says that they are 99%.............so why won't the packs charge up to 12.6v????

Thanks,
Andy
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Old 09-21-2015, 10:42 AM
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chuckk2
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When a battery is charging, it must have voltage applied that is slightly more than the desired end of charge voltage. Besides that, A lipo at rest after charging will drop slightly in voltage.
Then think about how accurate is the voltage measurement in reality. +/- one least significant digit +/- whatever the accuracy of the measuring device.
You are concerned about 3 hundredths of a volt, or 30millivolts. I'd be more inclined to have a bit of concern if the difference was due to only one cell in a three cell pack.
What may be happening has to do with the batteries and the balance current settings as well. Batteries with mismatched cells can exhibit similar behavior.

Saying much beyond this gets into the exact charge settings, as well as the battery and cell internal resistance.
Things like C/10, C/20, balance current, settings for offsets, the chargers use of a setting that specifies the battery capacity, and so forth.
On my chargers, an easy way to experiment with the end voltage is to reduce the desired charge current. This, in turn, reduces the current used to determine the end of charge.
Lower charge currents can allow the balance circuitry to do a better job, and cause the end voltage to be closer to the desired 4.2v per cell.
Naturally, the cost is an increase in the total charge time. Charge termination at C/20 instead of C/10 will usually improve end of charge results, again, at the expense of charge time.

While I can set a charger to come very close to charging a lipo such that it ends up at exactly 4.2v per cell, I seldom do so. Simply because it takes more time than I like to spend charging.
My usual result is about 4.18v per cell after charging and a rest.
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