LiPos (Lithium-Polymer-Accumulator) become more and more useful in the RC hobby. The advantage of LiPo are founded in the possibility to load it 100 times without losing capacity.
It consists of one or more cells. Depending on the assembly of the LiPo it is composed of the voltage (V) and the power (I). Additional to the "+" and "-" connection-cable it has a balancer.
Each cell of a LiPo has a nominal voltage rating of 3,7 Volt and a maximum charge voltage of 4,22 Volt (4,2V is standard).
That means the LiPo is used in a "voltage-window" ranging between 3Volt and 4,2Volt.
In contrary to a normal accumulator the voltage of the LiPo cells will not sink below 3 Volt !.
If a cell is discharged below 3 Volt, this will cause irreparable damages. Afterwards a complete charging is not possible and its shelf life is very short.
During the flight the cells will be differently discharged. So of a LiPo with 3 cells e.g. one cell could have 3,4V the next 3,6V the third 3,2V. This is a total voltage of 10,2V.
The alarm of the FlightCtrl sends a charge alert at a voltage of 3,3V (adjustable in the KopterTool, but please not below 3,3V!). For our example-LiPo with 3 cells it adds up to 9,9V (3x 3,3V=9,9V).
That means if the voltage sinks to 9,9V the charge alert bleeps and signalises the LiPo has to be charged.
You should start landing immediately at the time when the charge alert sends a continual bleep. If you carry on flying the cell voltage sinks below 3V. The LiPo will be exhaustively discharged and damaged.
The effect is a incomplete charging of the LiPo. This causes a shorter flight time. The capacity of such a damaged LiPo can also reduce to less than 50% of the original capacity during the flight.
So it is possible the Kopter would crash down if it won’t get enough power of the damaged LiPo.
Schema of a LiPo-Akkus (example of 4S/1P)
The voltage is gripped left an right and put on the LiPo-Connector (above).
Below is the balancer-connector. Each cell is gripped separately. 3,7V is measured between each cell. The added voltage will be measured from the negative pole to each grip.
This balancer-connector helps the battery-charger with the correct loading of each cell!
Examples of LiPo-Akkumulators
On LiPo-Akkus the data is imprinted in short form.
Example of an imprint:
11,1V * 3S/1P * 2200mAh * 20C
14,8V * 4S/1P * 5000mAh * 20C * 4C charge
What ist the meaning of this data?
- Shows the number of cells connected in series (3S means, 3 cells are connected in series).
- Shows the number of cells connected in parallel. (1P means, no other cells are connected in parallel.)
Shows the capacity of the LiPo in mAh.
A normal battery charger or power supply is not qualified to charge LiPos!
LiPos must be charged with a suitable charger with additional balancer-connector!
Be shure that the charger have a maximum charge voltage of 4,2 Volt. If this is lower the lipo will not fully charged.
In case of no recommendation by the manufacturer, LiPo-Akkus are to be charged up to max. 1C!
Calculate the C-data
Essential ist the capacity imprint on the LiPo.
Example: A LiPo with 2000mAh capacity and no more data should be charged with 1C.
2000mAh x 1 = 2000mA = 2A.
The charge current (1C) is therefore in this indication 2A.
The charge time of 1C takes approx. 1 hour.
Is the charge power imprinted on the LiPo higher than 1C the LiPo can be charged with more power and the charge-time is shorter.
The LiPo will be stressed and the shelf life of the LiPo falls.
The maximum charge voltage
As above mentioned a LiPo has a nominal voltage rating of 3,7 Volt and a maximum charge voltage of 4,22 Volt (standard 4,2V).
This maximum charge voltage is necessary for the complete of a LiPo and is to be inscribed in the charger!
(For some chargers the maximum charge voltage is already specified and not adjustable.)
If you choose a lower voltage the LiPo won’t be charged completely!
The total voltage of the LiPo is defined by the number of cells. A 3S/1P has 3 cells connected in series and no one in parallel.
That means 3x nominal voltage of 3,7V = 11,1V
Regarding to our example we have:
LiPo capacity 2200mAh
Charge power 2A
Maximum charge voltage 4,2V
and total voltage 11,1V
If you have a battery charger with a manual choice of the number of cells (1S/2S/3S/...), charge power (e.g. 2A) and maximum charge voltage (4,2V) it is essential to look out for the correct adjustment of all data.
Incorrect data can overcharge the LiPo up to destruction.
To charge the LiPo after adjusting the data connect the LiPo (e.g. Deans) with the charger. Afterwards the Balancer-connector has to be connected with the charger (Watch out the right polarity!).
This Balancer-connector measures the voltage of each cell. If a cell voltage during the charging e.g. exceeds the maximum charge voltage of 4,2V it discharges the cell to the correct data to reach the right voltage balance of the LiPo-cells (Balancing).
(If the balacer-connector is not connected to the charger the cells can become overcharged and destroyed!)
Tip: For charging put the LiPo on a fireproof basis. A flowerpot could be helpful: Put the LiPo in the flowerpot and the cables form outside through the little hole at the bottom of the pot to the LiPo inside.
If contrary to expectations a LiPo catches fire NEVER take water to extinguish it. DANGER OF EXPLOSION! Prepare a bucket of sand to extinguish possible flames.
what happens if you overcharge the Lipos
Here some videos that show what happens if the Lipo is not charged correctly:
This is an overcharged Lipo
The Lipo-Tester shows a cell-voltage of 4.40V - allowed maximum is 4.22V
If ayou don’t use your LiPo for a longer time (>2-3 weeks) you should discharge it to approx. 50% of it’s capacity to store it (cell voltage approx. 3,85 Volt).
In this state of charge the chemical decomposition is minimal.
Never store empty cells. They will be exhaustively discharged!
A Lipo can be discharge in unused condition. So it should be checked from time to time. During long storage, this would cause a deep discharge.
- Take care of a reverse polarity protection and fixed connections of the charger.
It is better to charge the LiPo up to max. 1C (or less) even if another data is specified.
Use the LiPo with a surrounding temperature between +18°C and + 40°C.
A LiPo can expand during a false charging or a false handling. It can even start to burn or explode.
This happens mostly while false handling e.g.:
- mechanical damage
(crash; a sharp object damages the LiPo)
- Overcharging / using a too high charge rate
Charging a hot LiPo
A expanded LiPo is waste (don’t use it).
Each shop in Germany selling cells has to take them back for free.
Even if it sounds logical:
NEVER cut into a expanded LiPo!
LiPos like other rechargeable batteries should never come in contact with.
Never put LiPos / rechargeable batteries into the household waste!
If you want to keep the connector of the LiPo NEVER cutt he cables simultaneously.
First things first. Insulate the cutted ends. (A short-circuit can cause an explosion!!!)
Disconnect the LiPo after the flight. Even a light power consumption can cause a deep discharge that damages the LiPo.