Lipos (lithium polymer batteries aka li-poly) are the most popular power source for electric RC these days. They offer phenomenal energy storage/weight ratios and current output capabilities compared to older types of batteries. They are so good in fact that they are revolutionizing RC as we know it. But they can be a very finicky battery type and are often a very misunderstood one.
Lipo terminology and concepts
lithium polymer battery
Lithium polymer is one of many lithium based battery types available in the world. It is different than lithium ion, lithium ferrite and the one-use AA/AAA lithium cells. This type of battery differs from the other lithium types in several ways including very high energy density, very high discharge and charge rates, and very high volatility. The batteries themselves are a stacked type and come in flat cells that typically have a nominal voltage of 3.7V, and a full charge voltage of 4.2V.
mAh stands for milli-Amp-hour (1/1000th of an Ah) and denotes the capacity of a lipo. This is the standard method for stating capacity of all battery types. The Ah rating of a battery refers to how many amps a battery can output for 1 hour before it is depleted. For example a 2200mAh pack can output 2.2A for 1 hour.
Pack configuration (examples 3s1p, 2s2p)
This refers to the number of series and parallel cells that make up a pack. "s" refers to number of series cells and "p" refers to the number of parallel cells. For example a 3s1p pack is made up of 3 cells in series. A 2s2p pack is is made up of 2 sets of 2 parallel cells in series for a total of 4 cells. Raising "s" in a pack raises its voltage and raising "p" in a pack raises its capacity.
Every lipo has a "C" and it is simply the capacity of the lipo in Ah minus the "h". For example a 2200mAh lipo (cell count has no impact of "C") has a C = 2.2A. A 3300mAh pack has a C = 3.3A.
All electrical devices have resistance and batteries are no different. In the case of RC use lipos, the internal resistance (often referred to as IR) can be used to determine the quality and health of a pack. The lower the IR, the easier current flows from the pack. The easier current flows from the pack, the higher the voltage will be under load. Expect to see numbers from 2mohms for a large higher performance cell to 100+mohms for a weak older cell.
This is a battery concept, not just a lipo concept. The resting voltage of a battery is the voltage when that battery is "resting" or not in use. For example if you have a lipo laying on the table and it has not been used in any way for a few minutes, it is considered to be resting. You can use the resting voltage to know if a lipo is charged, or to get a rough idea of the level of charge, and to monitor the balance of the pack.
This is also a battery concept, not just a lipo concept. The load voltage of a battery is the voltage of that battery under load or in use. This voltage is a function of the load and will change continuously as the load changes. The greater the load, the greater the voltage drop. Monitoring this voltage can tell you many things including the health of a pack or whether a pack is properly sized for the application.
Minimum voltage for lipos
Many battery types can be fully discharged with no problem. In fact some, like NiCds, need to be fully cycled to keep the cells healthy. But lipos are different. They have a minimum voltage requirement and if it is not followed, you can easily damage the packs. This voltage varies from pack to pack but for simplicity sake lets call it 3.0V. Now this is not a resting voltage but rather the voltage seen under load. Since this is not an easy thing to monitor it is recommended that certain precautionary measures are taken and that leads into the next item, the 80% Rule.
In an attempt to keep lipos healthy a general rule has evolved, the 80% Rule. In simple terms it means you should not discharge a lipo more that 80% of its rated capacity. This has nothing to do with volts but rather the mAh used during a flight.
Many things can hurt lipos but heat is usually the culprit. In general the internal temperature of a lipo must stay below 140F. To be safe you should never read a temp higher than 130F on the surface of the lipo. Heat can come from either a hard discharge or form an external source like being left in a car during a hot summer day.
The best rule of thumb is if a lipo is too warm to hold tightly in your hand, it is too hot.
Lipo use for safety and longevity
Brand new packs
New lipos ship in a "sleep" state. They have a chemical stabilizer in the cells that help them more easily handle shelf life, and are shipped at a storage voltage of approx 3.85V per cell. This stabilizer is lost when the packs are cycled, so it is important to let the packs remain asleep until they are needed.
Once a new pack is ready to be used, it needs to go through a "break in" process. Generally this means treating the new pack gingerly for the first few cycles, including charging it slowly (1C) and gently discharging it to 50% capacity for the first 5 flights. This does 2 things. First it makes sure the pack is in good condition and secondly it "loosens" up the pack for normal duty.
Note on break-in: I am seeing more and more forum posts about people trying to break in new packs on a charger. I recommend against this. Not only is it very time consuming but it is also wasteful. Instead charge a new pack at 1C and the go fly it. Take it easy and use the time to work on some basics like hovering in less comfortable orientations or slow, precise flying. Using a pack for break-in in this fashion gives you more flight time and gets you more useful cycles of the lipos. Just remember to take it easy.
Basically there are a bunch of DOs and DON'Ts for charging and here are a few
- find a well thought out place to charge. This includes both a location as well as what you charge on and near. Many people like to charge in something flameproof like a cash box or lipo sack.
make sure the charger and lipo(s) are in good working order. choose a charge rate that is safe for your lipo.
always double check your charger settings and connections before you start charging. Then check again once the charge cycle starts to make sure the charger is displaying the correct info.
- leave the charger unattended. This does not mean you have to watch it every second but do not leave the area while it is charging.
- charge on or near anything flammable. This includes things like carpet (both in house and in car), solvents and adhesives, and trash cans full of paper or saw dust.
There are really only 2 things to worry about when discharging lipos, minimum voltage under load and heat. Everything else is a function of these 2 things including max continuous amps, max burst amps, flight time, and so on. The best practice is to follow the 80% Rule and to watch for excessive heat after a flight.
This topic is hotly debated by many but it seems most believe that you should 1) store lipos in a cool dry place and 2) at roughly 50% capacity.
- The cool dry place can be anywhere from your garage to your fridge. In any case they should be stored in a fireproof container for safety. Many use ammo cans, cash boxes or lipo sacks. Just make sure they are safe and don't get crushed or poked while in storage.
- While in storage the packs should be at roughly 50% capacity or 3.85V per cell. The exact level is not overly important, they should be somewhere between empty and full. Many people simply leave the packs in the same state they are after a flight (roughly at 20% capacity if using the 80% rule). Others use their chargers to partially charge their packs with their chargers "Storage Charge/Discharge" mode.
Disposing of lipos
It is a fact that all batteries will die someday and lipos are no different. In fact it often happens prematurely with lipos because they are damaged. Once you have a lipo that is either no longer useful or can no longer be trusted, you need to properly dispose of it. The good news is that it is not difficult to dispose of lipos because they are non-toxic. They just need fully discharged and then they can go into the trashcan. Another option is to give the lipo to a battery recycler if you have one in your area.