RC LiPo Battery Labels Explained
The soft and hard pack (not cylindrical) LiPo batteries we use in the RC hobby are prismatic die-cut Lithium-Ion Polymer cells, arranged in series and parallel configurations to achieve the different voltages and capacities available.
Each individual cell’s nominal voltage is 3.7 volts DC. LiHV is another option with higher voltages per cell but the calculations are all the same.
This paper will explain the common identifiers found on LiPo batteries. Here’s an example label of a LiPo battery:
Step 1 – what really is “C”?
Each battery has a “C” factor. It’s used in all our calculations. “C” is determined easily:
Capacity / 1000 = C
In our example above we would simply divide the capacity (5200mAh) by 1000 = 5.2 . A battery smaller than 1000mAh still works the same way. For example, an 850mAh battery would be 850 / 1000 = .85
What is “S”
“S” refers to the number of cells in Series wired in the pack. For example:
1S is 3.7 x 1 = 3.7 volts
2S is 3.7 x 2 = 7.4 volts
3S is 3.7 x 3 = 11.1 volts
4S is 3.7 x 4 = 14.8 volts
Standard LiPo cells have the following voltage range:
Fully charged = 4.2V DC (per individual cell)
Lowest safe discharged = 2.7V DC (per individual cell)
Nominal Rated Voltage = 3.7V DC (per individual cell)
NEVER run your cells too low while in use. The voltage drop is so fast at lower charge states you'll end up over-discharging the cells and the pack will be ruined. Most ESCs have a safe cutoff at or over 3.2V per cell. Applications dictate different safe cutoffs to be sure the cell voltage never drops below 2.7V, i.e. Airplanes, Drones, Boats, Trucks, etc.
Now that we understand each pack can have different voltages based on the number of cells in series, we need to understand capacity. This is the amount of time the battery will last when used (runtime). It will also determine a few other factors that we’ll explain later.
Each battery we sell has a capacity, measured in milli-amp hours, or mAh. For example, a common battery pack capacity for surface and marine vehicles is 5000mAh. While this is important for runtimes, it also has an impact on performance. While the discharge rating (in our example above) seems like an absolute, it isn’t. The capacity of the battery determines the amperage rating of the battery. Ultimately, all we care about is how many amps the battery can deliver and for how long.
The discharge rate is indicative of the internal resistance of the cells when manufactured. So, if a battery has higher internal resistance, the rating is lower. Lower internal resistance (how easily the battery gives up the juice) results in a higher rating on the battery. We use LiPo batteries for this very reason because they have the highest discharge rate of all lithium batteries.
Note: state of charge and temperature have a big effect on internal resistance so never assume you're getting the actual discharge rating of the battery written on the sticker! Always allow 20% overhead for your powerplant calculations. Determining your power requirements is done by using the formula: motor wattage / supply voltage = amperage. Amps are variable, volts and watts are constant. Higher volts = lower amperage.
In our example, we can determine the continuous discharge amperage of the battery by the following calculation:
(5200mAh / 1000) x 35 = 182 Amps
HOWEVER, if you have a lower capacity battery, the continuous amps are lower. "C" is calculated from capacity, so our C Factor drops. If our sample battery was only 3300mAh, it would be (3300mAh / 1000) x 35 = 115.5 Amps . The lesson here is larger capacity batteries can deliver higher amperage with a lower C rating. This is where it can get confusing. We'll compare the amperage delivery from a huge 7500mAh battery rated at only 25C versus a premium priced 3300mAh 50C rated battery:
(7500mAh / 1000) x 25C = 187.5 Amps
(3300mAh / 1000) x 50C = 165 Amps
You're paying for both capacity AND discharge rating when buying LiPos. Be sure to determine what your powerplant really needs and run the numbers against the capacity before you start spending big bucks for a discharge number you may not need!
The standard charge rate for all LiPo cells is 1C. Looking back at our C Factor calculations, for the pictured example I would set my charger to charge at 5.2 Amps, 7.4 volts (2S). A higher discharge rated battery usually has a higher charge rate as well. 50C, 75C, and 100C+ batteries will usually state a higher charge rate than a 25C battery. For the longest cycle life, charging at 1C is always best. Also, with LiPos, charging at a rate slower than 1C has ZERO benefit. There is no "trickle" charging in the LiPo world since it uses a charge termination methodology (CC/CV) totally different than Nickel batteries (Delta Peak). If your LiPo states a maximum charge rate, great. If not, best to stick with 1C.
Lets calculate the pictured example!
1C charging would be: C Factor, or 5.2 Amps
2C charging (maximum stated) would be: C Factor x 2, or 10.4 Amps
We now know that from all our calculations, we have a battery that can deliver 182 Amps of current at full charge, can be charged at an amperage range of 5.2A - 10.4A and has a maximum safe (fully charged) voltage of 8.4V DC (2S x 4.2V) and a minimum safe voltage of 5.4V DC (2S x 2.7V) no load.
Hopefully this gives you a good starting point for helping understand the labels found on LiPo batteries.
Since this blog is about LiPos, here's the standard reminders..
- Never store LiPos full for more than a couple days. This is the most common reason for puffing. Discharge (or charge) to 3.85V per cell
- Never let your LiPos get hotter than 145 degrees Fahrenheit - FIRE BAD!
- Never discharge LiPos lower than 2.7V per cell
- Never use anything but a LiPo Balance Charger for charging LiPos - FIRE BAD!
- Never poke or burn LiPo batteries - dispose of them in a bucket of salt water for 2 days then drop them off at a recycling center
- Puffed LiPos are DANGEROUS LiPos - don't use them!