[McRat]

I've got both LiFe and LiPo batteries I'm testing. I'm starting with LiPo.

The R/C automatic charger brings the charged battery cells up to 4.2 volts each when it completes. Is this too high? The charger has a storage setting that keeps them at 3.7.

I've read a lot of different thoughts on what the voltage should be of a charged battery. What's the latest scoop?

[onegreenev]

I would charge them up then pull them off the charger. I would not let them stay on the charger. The LiFePO4 should be only charged to around 3.6 volts. Beyond that there is little usable capacity from 3.6 to 4.2 volts. This will also give your cells a longer life. Be sure you don't over discharge your cells during flight. If you run packs in series you really need to bottom balance even the small cells. As for the LiPO cells I would still bottom balance and charge to just below the upper limit the manufacturer states. Some LiPO cells are like 3.7 volts instead of the 3.2 nominal of the LiFePO4 cells. So, yes the LiPO can be charged higher but also you can't discharge them low like the LiFePO4 cells. Each has its limits and if you stay off the top and off the bottom no matter what kind of cell you use you can't go wrong. Monitor your charger to be sure it trips off when the nominal pack voltage reaches the desired voltage. Some smart chargers aren't so smart. RC chargers may actually charge way to high. Shortening the life of the cell which will drive you to buy more cells. See how that all works? In the RC world there may be a way to cut off the power once the pack reaches a specified voltage. That way you don't ruin your battery but you best know how to fly under no power to get your beast to the ground safe. If there is a way to reduce the power to force you to land under limited power but not under no power.

Is there such a controller for the RC vehicle with the ability to cut back power once a low voltage is reached?

[major]

Originally Posted by McRat

I've got both LiFe and LiPo batteries I'm testing. I'm starting with LiPo.

The R/C automatic charger brings the charged battery cells up to 4.2 volts each when it completes. Is this too high? The charger has a storage setting that keeps them at 3.7.

I've read a lot of different thoughts on what the voltage should be of a charged battery. What's the latest scoop?

Hi Rat,

You really should get the cell manufacturer's specification for the exact cell you have and follow that to a T. In general though, LiFePO4 charges to 3.7 V/c and LiPoly charges to 4.2 V/c. These are maximum figures. And is the voltage at the end of charge at the minimum charge current for that cell's charge profile. Once the cell is taken off the charger and allowed to sit a while, that OC (open circuit) cell voltage may go down. This depends on the exact cell as to how much it will relax.

Charging incorrectly can damage Li cells. LiPoly cells can have serious safety issues. When I charge them and have the least doubt the cell health, I use a LiPo bag, which is a fireproof envelope made to contain unpleasant events. LiFePO4 cells do get hot and can swell but rarely ignite (if ever) on overcharge or when charging a bad cell.

Have fun

major

[McRat]

Thanks guys!!!

[dougingraham]

Originally Posted by McRat

I've got both LiFe and LiPo batteries I'm testing. I'm starting with LiPo.

The R/C automatic charger brings the charged battery cells up to 4.2 volts each when it completes. Is this too high? The charger has a storage setting that keeps them at 3.7.

I've read a lot of different thoughts on what the voltage should be of a charged battery. What's the latest scoop?

The RC LiPo batteries are fully charged at 4.2 volts. The nominal voltage is 3.7v which is not a bad place to set them for long term storage. You can float these cells at 4.2volts without issue. What you dont want to do is go above 4.2 volts. The organic solvents in the electrolyte start to break down at 4.35 volts and the cell will puff up. If you want long life I would probably not charge that close to the maximum. Maybe stop at 4.1 volts.

LiFe cells are different. The nominal voltage is considered to be 3.2V. Typically they are charged at a constant current up to 3.6 volts or so and then held at that voltage until the current drops to C/5 or maybe C/10. A123 systems on the 26650 cell data sheet indicates you can float them at 3.45 volts. I've left a cell hooked to a lab supply for several months without any issues. The tested capacity of the cell was unchanged. It is probably safe to float any LiFe cell at 3.45 volts. I have only seen A123 systems spec a float voltage.

[danh818]

Originally Posted by onegreenev

Is there such a controller for the RC vehicle with the ability to cut back power once a low voltage is reached?

Yes, most of the speed controls that I use to power the brushless motors in my R/C planes have this feature. Some are programmable to reduce power at X% of pack voltage from the time the controller was energized, others are preset. The only drawback to this is that usually the controllers do this by % of voltage at the start. This means the system can be fooled by plugging in a pack that is already discharged. The reason it is done this way is because most speed controls can handel packs of varying voltages, eg. 3s, 4s or even more LiPo without having to program them. Just plug in a battery and fly. I am unaware of any R/C hardware that does this by means of another method, such counting mAh.

[dougingraham]

Originally Posted by onegreenev

Is there such a controller for the RC vehicle with the ability to cut back power once a low voltage is reached?

I would like to elaborate on the answer already given. I manufactured and sold the first digital speed control in the US in 1992. I was told that someone in Switzerland made one in 1991. My first model did not have a low voltage cutoff but at that time there was no point. The NiCd batteries that people were using then don't mind being run flat. This was a 0-24 volt and 60 amp continuous current brushed motor control. When I got my Warp9 I used one of the prototypes I still have to spin it up. My second controller was intended to be used with a class of racer called Speed 400. These planes weighed between 9 and 14 ounces in race trim and their straight and level speed was a little over 100mph on about 100watts. This second control was capable of operation up to about 15 volts and 15 amps and weighed about half an ounce. It also had a circuit that could power the radio receiver and servos from the flight pack instead of their own battery. (Think of this as a DC-DC converter.) This circuit ended up being called a BEC (Battery Eliminator Circuit). The problem with BECs is that if the flight pack voltage drops below about 5.5v the radio stops working and the plane will crash. So I incorporated a low voltage cutoff that would stop the motor when the flight pack reached a preset voltage. You would then have a few minutes in order to land before the radio dropped out.

This particular feature has been incorporated in one form or another in pretty much all the motor controllers since then. It was a no brainer to modify this feature to protect battery from over discharge instead of protecting the radio from failure.

There is one exception to the use of a low voltage cutoff and that is with the RC helicopters. Almost nobody uses the low voltage cutoff because if you cutoff the power to the rotor the heli will crash unless the pilot is very skilled in auto rotations and the heli is in an attitude with some altitude to allow recovery.

I stopped manufacturing my controls in 1994 when I started consulting with AstroFlight and designing circuits and writing code for them. The main reason I did this was the business was ruining the hobby for me and I was at the point where I needed to hire some people or do something else. I chose to continue to enjoy the hobby and let someone else do the stuff I didn't enjoy.

[danh818]

That's awesome info Doug. It is so cool to know that some of us EV'ers have roots in R/C.

In some of my 'foamie' planes I run Eflight brand speed controls that when low voltage is detected, limit the output power instead of just outright cutting it. I also have a mini quad copter that has this feature. But in my larger electrics I use a stand alone BEC and do not enable low voltage cutoff as they can become a handful to land under this reduced power condition. In the event that I forget to set my flight timer I would rather ruin a $40-$100 battery in favor of dumping a $800 airplane.