LiPo batteries are what make most electric RC airplanes practical, and they are not something you need to be afraid of. They do need to be treated correctly. Use the right charger settings, stay nearby while they charge, do not run them too low, and put them at storage voltage when they are going to sit. Do those things and LiPos are pretty easy to live with.
We spent a good chunk of Episode 180 talking about this because battery terminology can sound worse than it really is when you are new. So let us turn the labels on the side of a battery into something useful.
What Does 3S, 4S, or 6S Mean?
The S number is the number of cells connected in series. A normal LiPo cell has a nominal voltage of about 3.7 volts and reaches about 4.2 volts when fully charged. That makes a 3S pack 11.1 volts nominal, a 4S pack 14.8 volts nominal, and so on.
More cells means more voltage. It does not mean you can simply install a higher-S battery because it fits in the battery compartment. The motor, ESC, propeller, and airplane all have to be intended for that voltage.
What Does mAh Mean?
Capacity is normally listed in milliamp-hours, or mAh. Think of it as how much energy the battery can carry. A 5000 mAh pack has more capacity than a 2200 mAh pack, but it is also usually larger and heavier.
That weight matters in an airplane. Bigger is not automatically better. Use the capacity range recommended for the airplane or power system unless you have a good reason to change it.
What Does the C Rating Mean?
The discharge C rating is intended to describe how much current the battery can deliver relative to its capacity. A 2200 mAh pack is 2.2 Ah, so a theoretical 30C rating works out to 66 amps. In the real world, battery labels are not all equally useful, so I would not choose a power system by C rating alone.
The important part is that the battery needs to safely supply the current your motor and propeller combination actually pulls. That is where a wattmeter is much more useful than guessing from a label. Our Electric Power System Center goes deeper into matching the whole system.
Charge With a LiPo-Compatible Charger
LiPos need a charger designed for LiPo batteries. For a normal pack with a balance connector, use the balance-charge function so the charger can monitor the individual cells instead of treating the pack as one big battery.
Before pressing Start, verify the battery chemistry, cell count, and charge current. If the charger says something that does not agree with the label on the battery, stop and figure out why. This is not a good place for the “close enough” method.
What Charge Rate Should I Use?
A 1C charge rate is a common conservative starting point when the battery manufacturer allows it. At 1C, a 2200 mAh battery charges at 2.2 amps, a 5000 mAh battery at 5 amps, and so on. Some packs are rated for higher charge rates, but follow the battery manufacturer instead of assuming every LiPo can be fast-charged.
In Episode 180 we described 1C as roughly an hour-long charge, give or take, because the charger slows down as it finishes balancing the pack.
Do Not Leave a LiPo Charging Unattended
This is the battery rule I care about most. Stay where you can see what is happening. Charge on a suitable nonflammable surface and follow the battery and charger manufacturers instructions for fire-resistant charging and storage.
During that Episode 180 conversation, I mentioned that I had never had a LiPo fire, knock on wood, because I try to take care of my batteries. One of the other guys had experienced a pack going bad outside and was standing right there when it happened. That is the point. LiPos are not little bombs waiting to ruin your day, but if something does go wrong, you want to be there.
Let a Hot Battery Cool Before Charging
If a pack comes out of the airplane warm or hot, do not immediately throw it on the charger because you want another flight. Let it cool first. A battery that is getting unusually hot during normal use also deserves some investigation. You may be asking too much of the pack or power system.
Do Not Over-Discharge the Battery
A LiPo is not meant to be flown until it has nothing left. Your ESC may have a low-voltage cutoff, but I would rather use the radio timer and learn how much capacity the airplane normally uses than depend on the cutoff every flight.
Check the battery after landing. Once you know how a particular airplane uses its battery, set the timer conservatively and adjust from there. Leaving a little battery unused is cheaper than abusing packs.
Storage Charge Matters
This is one of the easiest battery-life improvements and one people skip because the battery still seems fine. Do not leave LiPos fully charged for weeks or months. Do not leave them deeply discharged either.
Most modern chargers have a storage program that brings the pack to roughly 3.8 volts per cell, although you should follow the battery manufacturers recommendation. In Episode 180 we talked about this specifically: if you are flying again tomorrow, nobody needs to panic over the battery sitting overnight. If it is going to sit for a week, a month, or the winter, put it at storage charge.
What About Puffing or Physical Damage?
A swollen, punctured, crushed, or otherwise damaged LiPo is not a battery I want flying an airplane. Do not try to flatten a puffed pack or keep using a damaged one because it still takes a charge. Follow the battery manufacturers disposal guidance and your local requirements for disposing of lithium batteries.
Also inspect the wires, connectors, and balance lead. A perfectly good pack with damaged wiring can still create a very bad afternoon.
Battery Connectors Need to Match the Power System
I learned not to trust a connector just because the two halves look like they belong together. I once had two connectors that looked identical but were wired with opposite polarity. I plugged them together and got smoke immediately. Since then, I check the actual positive and negative wiring instead of trusting the connector shape or even assuming the wire colors tell the whole story. A meter is cheaper than a charger, battery, ESC, or airplane.
EC, IC, XT, Deans-style connectors and others all show up in RC. The connector has to handle the current and make a solid connection. Avoid stacking adapters just to make whatever battery happens to be on the bench work in the airplane.
If you change connectors, make sure you know what you are doing around exposed battery leads. A LiPo can deliver a tremendous amount of current through a short circuit.
Estimate Charging Time and Combine Packs
The original RC Plane Lab LiPo guide included two useful battery tools, and those are worth keeping. The LiPo charging-time calculator estimates charge time using capacity, starting charge, requested C-rate, cell count, and available charger wattage. The series/parallel battery calculator shows what happens to voltage, capacity, and current capability when matching packs are combined.
Only combine packs when you understand the setup and the packs are appropriate matches. A calculator can show the electrical result; it cannot inspect the batteries sitting on your bench.
A Simple LiPo Routine
- Inspect the pack before charging.
- Let it cool if it just came out of the airplane.
- Verify LiPo mode, cell count, and charge rate.
- Balance charge and stay nearby.
- Before flying, make sure the pack is secured so it cannot move and change the airplanes CG.
- Use a timer and avoid over-discharging the pack.
- Inspect it again after the flight.
- Storage-charge packs that are not going to be used soon.
RC Airplane LiPo Battery FAQ
What voltage is a LiPo cell?
A conventional LiPo cell is about 3.7 volts nominal and about 4.2 volts fully charged. Multiply by the number of cells in series to get the pack voltage.
Should I balance charge every time?
For conventional RC LiPo packs with a balance lead, balance charging is the normal approach because it lets the charger monitor and balance the individual cells. Follow the instructions for your particular battery and charger, especially with smart battery systems that may handle balancing differently.
Can I leave my RC LiPo fully charged?
For short periods between flying sessions, follow the battery manufacturers guidance. For longer storage, use the chargers storage program rather than leaving the pack fully charged.
How do I choose the right LiPo for my RC airplane?
Start with the airplanes recommended cell count, capacity range, connector, and required current capability. Then make sure the resulting battery weight keeps the airplane within its intended balance and weight range.
Useful LiPo Tools
If you want to run the numbers instead of doing them on the back of a field-box lid, use the LiPo charge-time calculator and the series/parallel battery calculator. If you are shopping for a charger, see How to Choose an RC Battery Charger.
Where to Go Next
For the rest of the power system, use the Electric Power System Center. If you are new to the hobby, the Beginners Lounge LiPo lesson gives you the shorter course version.