LiPos, also known as LiPoly, are light weight and have a high capacity, this makes them perfect for multicopter applications.
To select the most suitable LiPo, the most important characteristics have to be considered.
- Pack voltage
- C rating
This article also cover some aspects related to battery charging, as well as dangers and safety instructions when handling LiPo bateries.
The pack voltage measures the output voltage of the battery. For multicopter the most common voltages are 11.1V and 14.8V. Although some extreme heavy lifting multicopters use up to 22.2V
Because LiPos are constructed with individual cells, which have a nominal voltage of 3.7V, the total nominal voltage is always a multiple of this quantity. So 11.1V are obtained with 3 cells in series, and 14.8V with 4 cells.
Batteries are most commonly denoted by their cell configuration, instead of their voltage. So 3 cell batteries are referred to as 3S and 4 cell batteries as 4S. Also, cells can be added in parallel, which increases the total capacity. A battery denoted by 4S2P has 4 pairs of cells in parallel which are connected in series, having 8 cells total (4x2).
Individual cell voltage can greatly vary from the nominal value. When fully loaded the voltage of each cell can be up to 4.32V, but is usually limited by battery chargers to about 4.2V.
Also, when fully discharged it can become as low as 2.7V. But compared to other battery types like NiCd which require fully cycling them to extend their lifetime, a complete discharge will irreversibly kill a LiPo cell. It is recommended to discharge it to a maximum of 3.3V, which is about 90%. Thus, most low-battery indicators are set to a threshold value of 3.4V. This indicator can be separately bought and attached to any application that uses LiPos.
Another reason to avoid fully discharging a LiPo cell is that at low voltages (below 3.3) starts dropping very fast. This means that discharging from 3.3 to 3.2V takes only a fraction of the time needed to discharge from 3.7 to 3.6 at the same output current. So the pilot has only little time under 3.4V cell voltage to safely land before the battery is completely discharged and a crash becomes inevitable.
Capacity is measured in mAh, mili-amperes per hour. It indicates how much current a battery can deliver in one hour.
The approximate amount of energy in mWh (mili-Watt-hour) that a battery can deliver is the product of the nominal voltage (depending on the cell count) by its capacity in mAh. It is only an estimation because the voltage changes non linearly over time and due to inner losses.
This means that a 4S 1000mAh battery stores more energy than a 3S 1000mAh battery. Or that a 4S 1000mAh battery stores approximately the same energy as a 3S 1300mAh battery.
The C rating measures how fast a battery can discharge as a multiple of its given capacity in mAh.
That is, a 1000mAh battery with a C rating of 1 can deliver maximum 1000mA, and this during 1 hour. If the same battery had a C rating of 2, it could deliver 2000mA, but only during 0.5 hour before it is fully discharged. Applying the maths, a 1000mAh 30C (C rating of 30) battery has a maximum output current of 30A and would discharge in 2 minutes, but can also deliver 1A during 1 hour as in the first example.
So, to know the maximum output current of a battery it is as simple as multiplying its capacity by its C rating. It is important to remark that a battery discharge rate is proportional to the amount of current it is delivering.
Manufacturers usually give two C ratings. The lower is for continuous operation, and it is safe to demand current from the battery up to that limit. The second if for few seconds bursts (usually 10 seconds).
Weight is a key factor for multicopters. Weight increases as the capacity of the battery does (bigger cells or more cells in parallel) and its C rating (thicker conductors).
As reference, here are some weights for LiPo batteries:
- 3S C20 2200mAh - 190g
- 3S C30 2200mAh - 200g
- 3S C30 3300mAh - 300g
- 4S C30 3300mAh - 390g
LiPo if not treated carefully, and sometimes even then, can explode. The most illustrative way to see it is to search "LiPo fire" on the Internet.
This explosion can be caused by overcharging them past 4.2V per cell or by overheating during operation. In case a the battery starts swelling in mid-air the multirotor has to be immediately landed to avoid mayor damage. Sometimes LiPos recover from swelling, but it could indicate that it is approaching the end of its lifespan.
In order to safely charge a LiPo a computerized charger is recommended. This kind of chargers allow for monitored charge up to a maximum voltage, avoiding damage to the battery.
Controlled charging current
Also, they allow for a controlled charge current. This way, a 3000mAh battery can be charged in 1 hour at 3A, or in half an hour at 6A. The maximum current depends on the power rating of the charger, yet it is highly recommended to charge the battery bellow 1C in order to extend its lifespan.
Because a LiPo gets permanently damaged if its charge gets to low, it is not advised to store them close to their minimum voltage because they will discharge over time. Also, storing them at maximum load is also a bad idea because they could start swelling doe to chemic reactions. This means that storing LiPos should be done at their nominal voltage.
Most computerized chargers have a storage mode which charges, or discharges, LiPo to that extend.
Because not all cells inside a LiPo are identical, they have slightly different capacities, inner resistance, voltage, etc. This causes during discharge that some cells discharge faster that the other, although in the range of less than mV. When charged again this difference gets augmented due to the same cause. Over time the voltage difference between cells increases, causing the battery to age faster.
To avoid this problems, most LiPo batteries have an additional connector which allow computerized charges to monitor individual cell voltage and balance the load between them.
Most computerized charges allow connection for an external temperature probe which can be attached to the battery. In case it starts heating up and swelling the charger automatically disconnects.
- Never leave a charging LiPO unattended.
- Charge LiPos on a fire-proof surface or use a fire resistant container.
- Never charge a damaged Lipo.
- Never charge a LiPo right after use. After use a LiPo is usually warm, wait until it cools down.
- Never discharge a LiPo past 3V. Recommended is 3.3V. Use a low-voltage indicator on multicopters.
- Never charge a LiPo past 4.3V, recommended is 3.2V. Use a computerized charger.
- It is recommended to monitor the LiPo's temperature during use and when charging.
- Inform yourself before deposing a Lipo. Some considerations have to be taken.