Now LiPo's batteries have a higher Specific Energy Density than NiMHs. Meaning that they can store more energy by mass. Ususally 50-100% more energy.
NiMH can store 60-120 Wh/kg, and LiPo can store 100-265 Wh/kg.
Example:
Turnigy Sub-C 5000MAh 7.2V NiMH (10C/15C Burst) (137mm*47mm*24mm = 154.5cc volume @ 436g)
Turnigy 5000MAh 2S 7.4V Lipo (20C/30C Burst) (148mm*49mm*16mm = 116.0cc volume @ 282g)
Lipo has 2x the constant discharge capability. 75% of the physical space, and 65% of the mass for basically the "same" voltage and MAh capacity... Hence why most electric powered aviation RC craft use LiPos (Planes, Heli's, multi-rotor).
Now however, with Lipo comes a trade off. They require more care and limits for discharge levels... Why? A poorly cared for Lipo can catch fire. And it may not be the batteries fault...
And yes the fire/heat that a burning lipo can cause can be home threatening if not properly stored/charged/etc... So now that we have scared you off already from using LiPo's…
Probably the best description regarding Lipo danger of fire here.
Most modern ESC's in RC planes/cars/trucks/boats that are Lipo "allowed" have built in Low Voltage Cutouts. Which usually cuts performance in 1/2 as a warning level and then completely shutting off when at the set low level. Newer boards like the Clark TK-60 have programmable LVCs. For tanks, because we never need a high rate of energy discharge. (Ultra low gearing means mechanical torque generation which actually reduces motor load) a discharge of 10C is usually plenty. Using a higher C rating in a tank will not change your performance at all.Roger's Hobby Center wrote:LiPo batteries offer plenty of power and runtime for us radio control enthusiasts. But that power and runtime comes at a price. LiPo batteries are capable of catching fire if not used properly - they are much more delicate than the older NiMH/NiCd batteries. The problem comes from the chemistry of the battery itself.
Lithium-Polymer batteries contain, quite obviously, lithium. Lithium is an alkali metal, meaning it reacts with water and combusts. Lithium also combusts when reacting with oxygen, but only when heated. The process of using the battery, in the sometimes extreme ways that we do in the R/C world, causes there to be excess atoms of Oxygen and excess atoms of Lithium on either end (be it the cathode or anode) of the battery. This can and does cause Lithium Oxide (Li2O) to build up on the anode or cathode. Lithium Oxide is basically corrosion, albeit of the lithium kind; not iron oxide, which is otherwise known as "rust". The Li2O causes the internal resistance of the battery to increase. Internal resistance is best described as the measure of opposition that a circuit presents to the passage of current. The practical result of higher internal resistance is that the battery will heat up more during use.
Higher Internal Resistance = Higher Operating Temperature
As we touched on earlier, some modern chargers can read the internal resistance of the battery in milliohms (m?). If you have one of these chargers, you can get a sense of how your LiPos are performing, and how their internal resistance increases as they age. Simply keep track of the internal resistance reading each time you charge your battery, and chart the increase over time. You will see how just the process of using the LiPo battery begins to wear it out.
Heat causes the excess oxygen to build up more and more. Eventually the LiPo pack begins to swell (due to the oxygen gas build up). This is a good time to stop using the battery - its trying to tell you that it has come (prematurely or not) to the end of its life. Further use can, and probably will, be dangerous. After the pack has swollen, continued use can cause even more heat to be generated. At this point, a process called Thermal Runaway occurs.
Thermal Runaway is a self-sustaining reaction that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Basically, when this reaction starts, it creates heat. This heat leads to a product that increases resistance (more Li2O), which causes more heat, and the process continues until the battery bursts open from the pressure. At this point, the combination of heat, oxygen, and the humidity in the air all react with the lithium, resulting in a very hot and dangerous fire.
However, even if you stop using the battery when it swells, you still have to render it safe (a process I'll get into later on in the LiPo Disposal section). If you puncture a LiPo that has swollen and still has a charge, it can still catch fire. This is because the unstable bonds that exist in a charged battery are in search of a more stable state of existence. That's how a battery works; you destroy a stable chemical bond to create an unstable chemical bond. Unstable bonds are more apt to release their energy in the pursuit of a more stable bond.
When a LiPo is punctured, the lithium reacts with the humidity in the atmosphere and heats up the battery. This heat excites the unstable bonds, which break, releasing energy in the form of heat. The Thermal Runaway starts, and you again get a very hot and dangerous fire.
The entire process of building up that lithium oxide usually takes around 300-400 charge/discharge cycles to reach a tipping point. That's a typical lifetime of a LiPo battery. But when we heat the batteries up during a run, or discharge them lower than 3.0 volts per cell, or physically damage them in any way, or allow water to enter the batteries (and I mean inside the foil wrapping), it reduces the life of the battery, and hastens the build up of Li2O.
In light of this, most manufacturers have taken to putting a Low Voltage Cutoff (LVC) on their speed controls. The LVC detects the voltage of the battery, and divides that voltage by the cell count of the battery. So it would see a fully charged 2S LiPo as 8.4V, or 4.2V per cell.
This is where the advantage of balancing comes in. Because the speed control does not read off the balance tap, it cannot know the exact voltages of each cell within the battery. The speed control can only assume that the cells of the battery are all equal. This is important because, as I mentioned above, discharging a LiPo cell lower than 3.0V causes a usually permanent degradation of the cell's ability to absorb and retain a charge.
A LiPo cell should NEVER be discharged below 3.0V
Use it as though you have a car. Your car tank has a 10mm tube feeding the fuel injection and a pump that can easily supply it with enough fuel. The engine will only pull down so much fuel that it uses.
Now I change the Tube to a 20mm tube and a bigger fuel pump, the injectors still only use the same amount of fuel, so while the possible speed and volume can be much more, the rate of fuel usage is the same.
The three biggest killers of Lipo's are
Heat, Age and Low Voltage
Heat generated by excessive current draw. Aka putting a low C battery into something that needs high current draws. Again not really an issue with tanks. But a huge issue with high performance RC Cars and boats where the motor generates high momentary loads on accelerations (Water is brutal!). Heat also means just soaking up heat from outside or from poor chargers/charging practices (Lipo's do not like trickle charging, nor overcharging)... Use a GD Charging container (Metal Ammunition can without seals /Firebat/Even a Lipo Bag at least).
During usage and even just sitting doing nothing, there is a chemical reaction that occurs. The process is always occurring but adding heat increases the rate of chemical buildup of Li2O. (Venting a puffed lipo does not stop this process it just hides the signs).
Low voltage on a cell actually causes degrading of the cell itself as well. Meaning more susceptible to heat.
Now I have been using LiPO's for about the last 4-5 years (I have a score of them). I finally had to retire a Lipo battery that had started to swell/puff (it had been used in my RC Cars as well as my RC Tanks).
I noticed months ago that it had puffed to the point of splitting one edge of the hard case. I left it outside for the last 2 weeks and more puffing (due to being in direct sunlight and heat for days on end...) It puffed to split 3 of 4 sides of the hard case.
So the signs are easy to spot on Lipo's. Rate of puffing is not some instantaneous effect that occurs when not in use and as such can be easily monitored. Heat is the #1 killer for Lipo's.
Disposing of a Lipo is pretty simple as well. First make sure it has ZERO Charge remaining. I mean ZERO.
Cut off the leads if you want to save them for later usage (one cable at a time please...) and submerge the battery in small bucket of salt water for a couple days. If the voltage = 0 then the risk of a fire hazard from puncturing is very minimal. Then dispose the battery as per your normal local laws (aka if allowed to throw in trash or bring to a battery disposal location). The salt water bath method is the easiest to do, especially if the to-be-tossed Lipo has a decent voltage left and is more than slightly puffed.
I also use Low Voltage alarms on my tanks and RC cars regardless if they have a built in LVC. Cause I've forgotten a connected LiPo in my Taigen KV-1 once (ONCE!) and I heard the alarm go off 3 days later with the tank on the shelf. (That was the above battery, 1 year ago).
To those that use NiMH! Good! I'll never berate anyone's choices of power supply to their toys (Solar? Nitro? Yessir! Go run em!)
To those that are looking to use or are using LiPo's. Good! Just be aware of the challenges and you should get years of safe usage from your batteries.
