This guide aims to give you a basic understanding of Li-Po batteries.
Li-Po (Lithium-ion Polymer) batteries are rechargeable batteries that are capable of discharging their stored energy at a high rate (current). With the battery being able to discharge itself at a high current there is enough power for a model motor to turn a propeller and generate thrust for flight.
These batteries are made from lightweight materials which helps to improve the power-to-weight ratio essential for flight.
Li-Po batteries are sold in one or more cells and are named by their configuration, e.g. 11.1V 3S1P 30C 2200mAh. If these numbers don't mean much at the moment don't worry, we'll go through each of these in turn through the guide.
To understand batteries better we need to have some electrical theory under our belt. Don't worry, there won't be a test later and we'll keep
it simple.
A good analogy is to think of an electrical circuit as a river. The width of the river is defined by a measurement of volts.
The higher the number of volts the wider the river. The speed at which the water flows down the river is defined by the current value noted as a measurement called amps. Again, the higher the current value (amps) the faster the water flows. In our case we replace the water with electrons.
V is shorthand for Volts named after Alessandro Volta who invented one of the first chemical batteries.
This term, mAh, is the measurement of the electrical current capcity of the battery in milliamphere-hour so over one hour the battery can discharge this amount of current.
There are one thousand milliampheres in an amp so in our example 2200mAh is equal to 2.2-Amp.
"C" is the value that relates to the maximum rate the battery pack can either be charged or discharged in relation to its capacity. A battery pack will have separate values for charging and discharging. The discharge rate is generally quoted in the product name and is normally higher than the charge rate, although as battery chemistry improves faster charge rates are expected.
Discharge
To calculate how much current a battery pack can discharge at
once we perform the following sum. As we know 1000mAh = 1-Amp, so we can use
this to give us a total amp figure:
2200 (mAh) divided by 1000 = 2.2 (Amp), multiplied by 30(C) = 66-Amps
Charging
To calculate what the maximum current a battery pack can be
charged at we perform the following sum:
2200 (mAh) divided by 1000 = 2.2 (Amp), multiplied by 5(C) = 11-Amps
Charging Time
Using the charge rate we calculate the approximate time it will take to charge the battery
with the following sum:
60 (minutes) divided by 5(C) = 12 (minutes)
The actual charging time will vary depending on your charger.
Li-Po batteries are made up of one or more cells. Each cell has the nominal (operating) voltage of 3.7
Volts.
The "S" represents the number of cells configured in Series and the "P" represents the number of cells configured in Parallel.
In our example the 3S1P has 3 cells configured in series. This means that the pack has the nominal voltage of 11.1
Volts.
If you increase the number of cells in Series you increase the total voltage of the pack. If you increase the number of cells in Parallel you increase the total capacity of pack.
One of the critical areas with Li-Po batteries is how to safely charge and
maintain your battery pack as the material the cells are made from is sensitive
and has volatile nature if handled incorrectly. 
A fully charged Li-Po cell should have a maximum voltage of 4.2 Volts. Any heigher and the cell can become volatile. When a Li-Po cell is discharged its voltage should never fall below 3.2
Volts. Any lower and the battery chemistry can be degraded so shortening the life of the cell and the overall pack.
When a pack of cells is used (being discharged) it is likely that one or more cells will discharge current more than another. When this occurs the the voltages of all the cells in the pack will differ from each other so you end up with a pack that is termed
as unbalanced.
To maintain a healthy Li-Po battery pack a separate connector on the pack, called a balance tap, is used when charging a pack. The battery tap allows a Li-Po Balancer unit to monitor each cell's voltage and control which cells get charged thoughout the charging cycle. By controlling which cells receive charge, the Balancer can maintain equal voltages across the cells in a pack so
that they are always balanced.
Balancers come as either single units (see left) or integrated into the charger (see right). Integrated chargers offer greater convenience and simpler connection. Separate balancers are cost effective when using an existing Li-Po charger that doesn't feature a balancer.
Over time Li-Po cells lose a small amount of power. When you come to store your Li-Po pack it's best to store the pack charged at around 80% of the total pack capacity. Most chargers include a storage feature that will perform the task of charging the pack to around this percentage.
If you are leaving your batteries for a long time it may be necessary to check on your batteries to make sure they have not discharged themselves below the safe 3.2V cell voltage after a month or two. Re-running the storage cycle on the battery will keep it in good condition.
Due to the potentially volatile nature of Li-Po batteries there are some important safety points which should always be followed:
- Always follow the instructions that come with the battery pack.
- Always follow the instructions that come with the charger and balancer.
- Always charge and transport Li-Po batteries in a sealed container such a Li-Po safety bag.
- Never leave a battery unattended when charging or discharging a battery connected to a charger.
- Never puncture a battery pack.
- Never charge a battery over the charge rate (charge C) noted in the
manufactures instructions.
- Never charge a battery over the capacity (mAh) noted in the manufactures instructions.
- Never discharge a battery below a cell voltage of 3.2 Volts.
- If you incorrectly connect a battery pack or cause a short circuit immediately move the battery pack to an open outside space and observe the pack from a distance for 20 minutes. If the pack "puffs" up, do not use the pack and dispose of the pack according to the manufactures instructions and local authority disposal guidelines.
Have a browse of our Li-Po batteries by clicking here. If you need any help, just call us on 0845 539 2103 (local rate) or email us at Shop@TopProps.co.uk where we'll be glad to advise you.
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