What’s a LiPo ?
Lithium Polymer batteries are known as LiPo batteries or shortly Li-Po’s. these type rechargeable batteries are used when there is a requirement for high discharge current, for example in RC drones, Cars, boats and model airplanes. compared to widely used Li-ion batteries, LiPo’s are lightweight and have a high discharge capability, also they are more sensitive to cell damage and hence for an explosion or bursting into flames. when they are handled safely and operated only within recommended parameters, they are very safe to use.
LiPo battery basics
When selecting a Lipo battery there are main terminology that is used with, they are,
- Nominal Voltage
- No. of Cells
- Capacity
- C -Rating
- Main lead connector
- Balance plug
- Internal resistance
Lipo battery voltage
Standard lipo batteries consist with cells that has a nominal voltage of 3.7 V per cell. According to available number of cells. Total voltage is indicated in the battery cover. For example 14.8V or 4 cells ( 3.7 x4 ) is indicated in the 4 cell or “4S” battery pack.
3.7V battery = 1 cell x 3.7V= 1S battery
7.4V battery = 2 cells x 3.7V= 2S battery
11.1V battery = 3 cells x 3.7V= 3S battery
14.8V battery = 4 cells x 3.7V= 4S battery
18.5V battery = 5 cells x 3.7V= 5S battery
22.2 V battery = 6 cells x 3.7V= 6S battery
29.6 V battery = 8 cells x 3.7V= 8S battery
37.0V battery = 10 cells x 3.7V= 10S battery
44.4V battery = 12 cells x 3.7V= 12S battery
All lipo’s are designed to operate safely within specific voltage range. 4.2 v being the max and 3.0 v is being the lowest. Charging a battery more than 4.2v will lead to a explosion and fire. Also discharging below 3.0v will damage the battery irreversibly. As a general practice its best to limit to 3.5v as the minimum discharge level. This will increase the battery lifespan.
Nowadays there is a increasing popularity for LIHV battery, these are referred to as high voltage lipo batteries. These have a nominal voltage of 3.8v per cell. And can be charge up to 4.35v per cell.
when comes to industrial drone batteries there are many types which can have the standard or different nominal voltages
Figure 2: LiHV battery
Li-ion battery has a nominal voltage of 3.7V
Lipo Cell configuration
Usually lipos have only series connected cells, in this case if the battery is 4s, only 4 cells are available. But in some cases there batteries in parallel connection as well ( to achieve a high capacity and/or high currents )
Figure 3: 4s2P Lipo
If the battery indicates as 4S2P, then there are 4 x2 = 8 cells in total.
If you have a identical 4s battery from same brand and same specifications, it can be used in as parallel or series as per your requirement.
When the need is to increase the capacity, batteries can be paralleled using connectors, or if to increase the voltage can connect in series.
Capacity
Battery capacity is rated in milliampere-hour (mAh). Which denotes as how much current you can discharge continuously for a hour , until its fully discharged. For larger batteries this is denoted as Amp hours ( Ah ) . this means 1000mAh = 1 Ah.
For example, 1500mah can be used to draw 1.5 amps continuously for an one hour.
Or 3 amps for a half hour
Or 6 amps for a quarter hour, etc.
Lets say fpv drone require 30amp continuous current draw. Lets calculate the flight time with 1500mah battery ( estimated ).
capacity (mAH)= constant discharge current (mA)× discharge time (H)
1500mAh=30A x H
1500mAh=(30x1000)mA x H
H=0.05 h
H=0.05 x 60 mins= 3 mins
Total estimated flight time will be 3 mins.
C rate or Discharge rating
C rating indicates the maximum current that can be drawn from the battery without harming it. If you fail to choose the proper c rate for your battery, it might puff up, explode or catch fire. So C rate is much important.
Maximum current that can be safely drawn is directly related to the C rate and the battery capacity.
Maximum Current (A)=C rate x Capacity( Ah)
If you draw more than this maximum amount it can cause to over heat, damage battery ( reduce life) , puffing or catching on fire.
For example lets consider 1500mah battery with 120c rating
Max Current =120 C x 1500/1000 Ah=180 A
This is purely theoretical value. Battery manufactures often over inflate there numbers for marketing purposes. As a good practice we recommend to use 70-80% range of the indicated C rate when selecting for your applications. Also this is very different from brand to brand and their manufacturing process.
even though your specs are well within parameters. still it can be get hot. This is due to internal battery factors and external factors (temperature etc).
For high performance applications its always best to select the High C rated battery. But its not the other way around.
for smaller power requirements its best to select low c rates. Cause it will save the cost, extra weight as well.
Battery main lead and Balance connector
Lipo battery usually have 2 sets of wires . main large wire lead for discharge . and small wire set for balance charging and cell monitoring. Except for in 1S battery. It only have 1 set of wires.
Balance lead comes with number wires ( number of cells +1 ) amount, with a JST-XH connector.
For example 4s battery balance lead have 5 wires.
Figure 4: JST XH connectors
Figure 5: JST Xh 3pin ( 2S ) connector
For small batteries this is almost same. But with large UAV batteries it can change.
There area many types of main leads . with different max ampere ratings and connector types.
these will have different sizes of gauge wires and different connecting mechanisms too.
Most of the common connector type is XT series
Figure 6: XT connectors
The number indicates the maximum continuous current rating. For XT30 its 30amp, xt60 its 60amp and for xt90 its 90 amps.
There are many types of old and very new innovative connectors available.
These are still in used.
For micro drones and whoop. Jst PH2.0 and Bt2.0 and BT3.0 types plugs are used. Cause they are smaller and lighter.
For larger uav’s and drones, these QS8/QS10/QS12 type connectors are used.since they require high amp rating.
When connecting 6S or higher battery packs. There is a significant sparking effect. If a spark arrestor system is not installed. This could lead to heavy carbon depositing in the connectors and it could stop working. To minimize this effect some connectors has a feature call “ anti Spark” system.
Which is a small resistor connected inside the connector and it will reduce the sparking effect.
This is available in XT90-s , AS150 and QS series connectors
Figure 7: XT90-s anti Spark
Figure 8: AS150U anti spark
These advance connectors have extra pins for data communications as well. It can be used to connect to CAN bus or I2c communication line to get the battery or other sensory data information easily.
Internal Resistance
As other electronic components, batteries also have internal resistance (IR). Which measure with mΩ.
(milli Ohms). This is what limits the maximum current output of a battery. High C rated batteries have a very low resistance compared to low C rated ones.
This can be used to measure the battery health.
With lower IR battery can deliver the full power effectively. If the IR is high there will be a significant voltage drop. Which is noun as voltage “Sag” or sagging.
With time and usage this IR will increase slowly. It’s a irreversible process. That’s the reason batteries have a excepted life cycle.
but over discharging , over charging , overheating and exceeding the max C rate limits will accelerate the battery aging, and reduced the cycles.
