Li-Po batteries have become an inherent part of our hobby. Short charging times, efficient power output and a high life expectancy are just a few of the many advantages of this battery type. However, you should not underestimate their complexity. Especially beginners often make small mistakes when handling them which can lead to huge problems due to the Li-Po’s high energy density. Following some basic tips and tricks will help avoid this.
Li-Po is short for Lithium Polymer and is a chemistry for storing energy just like Ni-Mh, NiCad or Li-Ion Batteries. The defining parameters are capacity (mAh), voltage (V), number of cells (S), the discharge rate (C) and the charge rate (C). You will always find these parameters on the box or the batteries themselves.
A Li-Po consists of one or more cells. Each cell has a voltage of 3,7V and an end voltage of 4,2V. A Li-Po Cell my never be discharged lower than 3V so the effectible useable voltage lies between 4,2V and 3V. If the voltage of a cell drops below 3V it can be damaged irreversibly.
To demonstrate a typical battery cycle (charge, discharge, storage) we will use a typical Li-Po pack with the following parameters
The standard voltage depends on the number of cells. In our example the battery has three cells (3S). The calculated standard voltage is
3 x 3,7V = 11,1V
Fully charged, the voltage of the pack is
3 x 4,2V = 12,6V
After use, the voltage of the pack may never drop under
3 x 3V = 9V
The battery has a capacity of 3.200mAh and a discharge rate of 30C.
( 3200mAh : 1000 ) x 30C = 96A.
This means, the maximum discharge current is 96A. Exceeding this current can lead to permanent damage of the pack. The charging rate of the battery is 5C.
( 3200mAh : 1000 ) x 5C = 16A.
This means, the pack can be charged with up to 16A. However, we strongly recommend you charge Li-Po Batteries with a charge rate of 1C as it puts less stress on the pack and ensures a long battery life.
Ideally our pack has now been charged with 3,2A (which is 1C), has its full end voltage of 12,6V and can be used it the model. After discharging the pack down to 9V you should turn your model off and charge the battery again.
At the end of the season, when packs are not needed as frequently, they should be charged to the storage voltage by using the „storage mode“ feature that most modern chargers already have built in. Just like normal charging, the parameters of the Li-Po are entered and the charger charges (or discharges) the cells to the ideal storage voltage. If your charger does not have this feature you can simulate it by charging the packs to about 50% of their capacity. While in storage mode, you should check your batteries every now and then and recharge them if needed. Under no circumstances should you store an empty Li-Po because it will deep discharge below 3V over time. In the worst case, this can lead to a fire.
Another thing to look out for is the cell balance. Sometimes the cells start drifting apart giving them different voltages. While the other cells are still on the safe side, one cell may start to deep discharge while in use. To avoid this, check the cell voltages with a special Li-Po checker every now and then and if the difference between the cells is larger than 0,1V you should balance the Li-Po. This is also done with the charger. Please consult your chargers manual on the correct balancing procedure.
Using Li-Po batteries is safe and easy if you keep these simple rules in mind.