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Old 04-12-2014, 03:59 AM   #2
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Dennis V
 
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Originally Posted by FlyWheel View Post
I have an old HiTec Flash5X (72MHz)) transmitter that currently uses a 1000mAH 8cell nickel metal hydride pack. it works OK, but if I want it to last for more than a few flights I have to charge it the morning of or at least the night before or it will lose it's charge sitting. I know a 3 cell LiFe pack is close to the same voltage and lithium batteries can hold a charge for a very long time (lithium-iron can better tolerate full charge storage also). They also last a lot longer and I have even seen them made in an 8 AA battery size for transmitter use.

HOWEVER, I am concerned by the voltage differences and discharge curve. Also the low voltage warning in my older TX which is calibrated for 8 NiCd or NiMH. Can I use a LiFe pack in my transmitter without harming it or the battery pack?
H'mmm
Lets see.

A typical Nih 8 cell battery pack will charge to 1.35 Volts per cell, times 8 cells or 10.8 volts, right off of the charger. While in use, that Nih battery will operate at about 1.2 Volts per cell, or 9.6 Volts DC. And, when that Nih battery is pretty flat at 1.1 Volts per cell, the Nih pack will read about 8.8 volts DC.

Now, a LiFe 3 cell pack will measure 3.6 Volts DC per cell times 3 cells or 10.8 Volts right off the charger. While in use, that LiFe battery will operate at about 3.3 Volts per cell, or 9.9 Volts DC. And, when the LiFe battery hits 3.0 Volts per cell, or 9.0 volts for the pack, it is pretty much flat.

One difference on these LiFe's, their voltage during a discharge curve is pretty flat, so when it is nearly flat, that voltage will drop like a rock.

So, just top it off before a flying day. They hold their charge for many months, if you don't fly. And, storing them at full charge doesn't hurt them.

Just be aware that you must use a LiFe balancing charger to charge these type cells. Not using balancing can damage them over a long period of time.

DennyV
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