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LiFe Battery Discharge Characteristics

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Old 07-12-2016, 04:17 PM
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All Day Dan
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Default LiFe Battery Discharge Characteristics

Has anyone seen a plot of a LiFe battery discharge? Is it a sudden drop off near the end? What is the recommended percent capacity discharge to avoid getting near this drop? Thanks, Dan.
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Old 07-12-2016, 07:23 PM
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Hi Dan,

Yes, LiFe batteries maintain a very flat discharge curve (fairly constant voltage) until they are depleted then the voltage drops off very quickly. Probably a flatter discharge curve and faster drop than any other type of battery I have been exposed to. Not really an issue though as it is very easy to keep track of power used as the recharge power put back in the battery is very close to what is taken out .... within just a few %. LiFe batteries can also be cycled to measure true capacity just like the NiMh and NiCd batteries you are so use to using. You just have to set the correct cut off voltage.
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Old 07-13-2016, 05:28 AM
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Truckracer,

What cutoff voltage do you recommend?
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Old 07-13-2016, 07:56 AM
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Thanks Truck,
According to the Tritan manual the cutoff should be 2.5 volts per cell. According to Hitec, it should be 2 volts per cell. Neither one recommends deep discharging the cells. However it would be a good safety factor to monitor a packs capacity on a regular basis. Dan.
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Old 07-13-2016, 09:03 AM
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Quote:
Originally Posted by All Day Dan View Post
Thanks Truck,
According to the Tritan manual the cutoff should be 2.5 volts per cell. According to Hitec, it should be 2 volts per cell. Neither one recommends deep discharging the cells. However it would be a good safety factor to monitor a packs capacity on a regular basis. Dan.
Dan and Lifer, according to various sources and spec sheets, the most common recommended cut off voltage for LiFe cells is 2.5 to 2.6 volts though I have seen sources recommend voltages up to 2.8 volts per cell. I wouldn't have a problem with the Hitec recommended 2.0V / cell. Reason being, the voltage is dropping so quickly when LiFe cells discharge to this level, the discharge will terminate very quickly anywhere in this voltage region. In this voltage range, you'll be seeing flashing digits on the display as the voltage will be changing that fast! I usually set discharge to terminate at 5.0V for a typical 2 cell airborne receiver pack.

If using A123 cells, I wouldn't worry at all about deep discharging them as they were designed for rugged, high discharge environments. If using the Poly Pack type LiFe batteries, there are few real specifications available for these types of batteries but I wouldn't worry at all about doing discharge tests on them. If they won't withstand a discharge capacity test, they're not worth using as an airborne battery.

Dan, are you using some of the LiFe batteries these days?
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Old 07-13-2016, 11:25 AM
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Thank you for the info!
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Old 07-13-2016, 03:06 PM
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Truck, I’m still using the good ole four cell 2400mah NiCads from NOBS Batteries. They are performing exceptionally well in my 50cc 20 pound planes. With a little math I come up with a current consumption of 500ma to 600ma as the steady state average. Using the higher value, I need 900mah for the 90 minutes I fly each time at the field. With a two to one safety I use the 2400’s from NOBS. They are easy to charge, discharge and monitor. However, sometimes when I grab my aileron servos at the end of the day their torque is reduced. I’m sure it’s caused by the connector drops and line loss although I have 20 gauge extensions. If I up the cell count I’ll be increasing the current draw. That’s where the LiFe batteries idea came from. Right now, the easiest thing to do is to use aileron servos that will operate at the lower voltage and I have them. Dan.
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Old 07-13-2016, 04:10 PM
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Dan, you know that I was once as conservative as you are about batteries and resisted the switch to Li based airborne packs. But A123 batteries changed all that and I've been using them for about 6 or 7 seasons now. I just replaced several packs this past winter that had 6 flying seasons on them and they still had well over 90% of new capacity and still exhibited nearly zero self discharge. Incredible batteries and nearly the same price as quality Ni based batteries. They're much easier to charge and monitor and in general they're just easier to live with than any Ni based battery I've ever used. I'm not overly fond of the poly pack LiFe batteries though as they do degrade much faster than the metal cased A123 cells ..... at least the poly batteries I've tried and that includes several different brands. I don't worry about any Li danger with the metal cased A123 batteries.

I'm still pretty much in your camp when it comes to LiPo batteries. I use them in several smaller electric powered airplanes but I don't trust them at all. Frankly, I hate having them anywhere around the house. I just accept them if I plan to continue to fly any electric stuff. I'm not sure that is a good long term plan though!
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Old 07-14-2016, 07:52 AM
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Thanks again Truck. For now, I'll stick with the NiCads and explore the alternatives as I have started here. For some reason I do not get any of the posts in this thread in my email. It only shows up in the active section of RCU's home page. Dan.
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Old 07-15-2016, 05:40 AM
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Dan, all the info you need is available on the Hangtimes.No BS site. Steve covers LiFe in great detail. I use his batteries too and they are top quality. Check your settings and make sure you have selected to receive emails when someone posts in a subscribed thread.
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Old 07-15-2016, 09:52 AM
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Old 07-15-2016, 02:32 PM
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For another reason, I am now getting the posts. Thanks.
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Old 07-18-2016, 06:53 PM
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I've finally seen the light and ordered a LiFe pack from NOBS. I'll do some bench testing before I install it in a plane. By the way, I'll be using a Hitec multi charger X4. They recommend a LiFe minimum discharge voltage of two volts or more but the charger is fixed at two volts. Am I doing something wrong with the charger? I thought 2.5 volts was the minimum. Dan.
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Old 07-19-2016, 04:19 AM
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Dan

I have the X4 Charger and to discharge/load test my A123 LIFE batteries use the NIMH program and set the end point voltage to +5 or +5.5 volts.

Cheers

Ray
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Old 07-19-2016, 06:56 AM
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Ray, Good information. I just checked with Hitec and they confirmed that the cut off voltage is set at 2 volts. I may use my Triton which has it adjustable and I'll set it at 2.5 to 2.6 volts. Dan.
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Old 07-19-2016, 09:13 AM
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Is it then reasonable to assume that 2.0 volts per cell is too low a cut off point for discharging Life batteries.
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Old 07-19-2016, 02:02 PM
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For A123 cells, 2volts might be a bit low but it won't hurt them. Voltage drops so quickly in this region, the discharger will terminate and the cell voltages will return to a normal range almost instantly. I wouldn't discharge the poly pack batteries to this level.
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Old 07-19-2016, 02:04 PM
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Dan, you will be amazed at the performance A123 offers. You'll really appreciate the almost zero self discharge. Welcome aboard!
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Old 07-21-2016, 04:45 PM
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Quote:
Originally Posted by Truckracer View Post
For A123 cells, 2volts might be a bit low but it won't hurt them. Voltage drops so quickly in this region, the discharger will terminate and the cell voltages will return to a normal range almost instantly. I wouldn't discharge the poly pack batteries to this level.
Thanks Truckracer
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Old 07-21-2016, 04:55 PM
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I just received the 2500 2S pack from NOBS today. I'll be doing a good deal of testing to help me become familiar with it's characteristics before I install it in a plane. I also got a regulator and a harness for the Hitec charger. The three "right" pins on the balancer connector at the charger are compatible with a Futaba connector. The same for the Triton. You just have to make sure that you get the polarity right. Dan.
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Old 07-21-2016, 08:06 PM
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Dan, what is the purpose for the regulator for the Hitec charger?
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Old 07-22-2016, 06:48 PM
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Truck, Oops. My second grade English teacher will never forgive me. Your right. It does read that way. The regulator is for the airborne pack. I'm still using the 4.8 to 6.0 volt receivers and servos. I am concerned about stressing them with a steady state 6.6 volts. It also keeps the current draw lower. I don't need anymore torque from my servos. I get enough as they are. Dan.
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Old 07-25-2016, 06:48 AM
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I completed some tests with the pack. The pack was discharged to 5.5 volts. It was then balanced charged at 1 amp. The Capacity Cut-off terminated the charge at 2750 mah. It was discharged at 1 amp down to 5.5 volts with a capacity of 2500mah. It was charged at 2 amps and was cut off again at 2750 mah. This time it was discharged to 6.0 volts for a capacity of 1962 mah. This was good since I only need 900 mah for a day of flying. I do not charge at the field, now for a different subject. How do I charge a partially depleted battery? Should I let the charger possibly over charge the pack as it searches for a peak or should I use the capacity cut-off function? I’m using the Hitec X4. Dan.
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Old 07-25-2016, 05:52 PM
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You can recharge from any state of discharge. The charger will do all the work. Most all LiFe / LiPo chargers are constant current / constant voltage (CCCV) chargers. They start the charge cycle at the constant current rate you enter into the charger during setup. For a 2 cell LiFe battery the charger will charge to 7.2 volts then the charger will change to constant voltage mode where it will hold the voltage at 7.2 volts and the charge current will taper off until the charge terminates. If the battery just needs just a few MA of charge, the charger will handle that. Likewise if it needs a full charge or anything in between a full and partial charge, it will also handle that.

By the way, other than the applicable voltages and currents, this charge routine is the same for LiFe, LiPo, LiOn and PB batteries. A balance charge will take a bit longer than a non balance charge as the charger analyzes the cells and a bit of charge energy is wasted in the process.

So in short, set up the charger correctly, start the charge cycle and forget it.
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Old 07-26-2016, 04:55 PM
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Once again Truck, you’re right on. I discharged the fully charged pack at 600ma for 90 minutes. That’s a days worth of flying for me. It was a discharge of 900mah. The pack measured 6.42 volts with a 500ma load. I then balanced charged the pack at two amps. The charger terminated the charge at 945 mah in 33 minutes. The pack measured 6.72 volts with a 500ma load. All amazingly close and good data. Thanks again, Dan.
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