Calculate the Real Maximum Current of a LiPo – RC Models, DIY Hobby Elektronics, Arduino projects, RC Airplanes

How to calculate the maximum current capacity of a LiPo battery?

The current power of a battery should match or exceed the maximum current draw of the motor we intend to use. Otherwise, the motor will not reach its maximum performance, and the battery will overheat. In some cases, the battery cells may even get damaged.

LiPo batteries typically display their capacity in milliampere-hours (mAh) and their discharge current rate, denoted by the letter “C” (C: current). Using these two values, we can calculate the maximum current power.

Maximum Continuous Current = Capacity × Discharge C Rating

For example, let’s calculate the maximum current for a battery with the following specifications: 11.1V (3S), 2700mAh, and 30C.

Capacity: 2700mAh (2.7 Ah)
C Rating: 30

The maximum continuous current this battery can provide is 2.7Ah × 30C = 81 Amps. BUT IT IS REALLY THAT SIMPLE?

If we lived in a perfect world, it might be. However, in reality, many of us have observed that we often get much less than the theoretical current calculated, or when using a motor that requires the calculated current, the battery overheats and fails to deliver the expected power.

So, Where’s the Problem?

Real-World Factors and Losses

Internal Resistance (IR) Loss
- Every battery has internal resistance. As internal resistance increases, heat losses rise, and current output decreases.
- Even a new battery with low internal resistance may struggle to deliver its full capacity.
Voltage Sag
- When high current is drawn, the voltage of the cells drops.
- For example, a 3S (11.1V) battery under high current may momentarily drop to 10V or lower.
- As voltage drops, power (Watt) decreases, which in turn affects current output.
Chemical and Thermal Limits
- LiPo batteries cannot fully deliver their nominal C rating (the value written on the label).
- When batteries reach a certain temperature, their internal resistance increases, further limiting current output.
- For example, the 30C rating in our example is typically a maximum value under ideal laboratory conditions and cannot be sustained continuously.
Connection and Cable Losses
- Battery connectors (such as XT60, XT90), cable thickness, and soldering points can limit current capacity.
- Poor soldering or thin cables can prevent the full current from being drawn.

How Can We Make a More Practical and Realistic Calculation?

First, we need to know the internal resistance (IR) of the battery cells. Internal resistance values are critical for determining current power. You can measure internal resistance using digital chargers or specialized devices like the SM8124A. Alternatively, you can calculate it manually.

Manual Calculation: By measuring the voltage drop under different loads, you can calculate the internal resistance using the following formula:

Where:

and are voltages under two different loads.
and are the corresponding currents.

Once you know the internal resistance, you can use the following formula for a more realistic estimate:

Realistic Maximum Practical Current Calculation;

Where:

$V_{n o mina l}$ = Nominal voltage of the battery (e.g., 11.1V).
$V_{min}$ = Minimum safe voltage limit of the cells. Typically, 3.2V per cell is acceptable (e.g., 9.6V for a 3S battery).
$R_{a}$ = Total internal resistance of the battery (in milliohms, mΩ). For the formula, convert the total mΩ to ohms (Ω) by multiplying by 0.001
$I_{ma x}$ = Maximum current.

This approach provides a more accurate and practical estimation of the maximum current a LiPo battery can deliver under real-world conditions.

Note: Calculating with the voltage value under load instead of the nominal voltage produces more realistic results. However, the voltage value under load will not remain the same during operation. The voltage will decrease as time progresses. Therefore, the nominal voltage allows us to get a more accurate result for the worst case scenario.

FOR EXAMPLE (FOR 11.1V 2700mAh LiPo):

Ra: 12.2 + 11.3 + 11.3 = 34.8 m

The maximum current capacity of the battery whose internal resistance I measured is 43.10 Amperes.

   Let’s assume the total internal resistance of the socket and cables is 0.0005Ω.
   If we add this to the internal resistance of the battery, we get 0.0348 + 0.0005 = 0.0353Ω.
   11.1V-9.6V / 0.0353 = 42.49A
   In this case, the maximum current capacity will be 42.49A.

In addition to the battery’s internal resistance, external resistances also negatively affect the current power. External resistances include the resistance values created by the battery’s connector, cables, and solder joints. For a more realistic estimate, external resistance values should also be included in the formula.

However, in practice, it is often observed that only about 50-70% of the nominal C rating can be drawn as continuous current. For example, a battery labeled as 30C can typically operate efficiently at a continuous current of around 15C to 21C, depending on the quality and condition of the battery.

For instance, the practical continuous current that a 2200mAh 30C battery can sustain is approximately:

So, in the real world, this battery can provide a continuous current of approximately 40.5A to 56.7A. A higher current like 81A is usually only possible as a burst current (short-term) but cannot be sustained continuously.

Digital Charger