Batteries

Estimated reading time: 4 min

For the propulsion and electronics of our droids, we need a suitable, rechargeable energy source that must meet the following main requirements:

  • It must be small and light, i.e. have the highest possible energy density (measured in energy per mass = megajoule / kilogram [MJ/kg]).
  • It must provide the appropriate operating voltage (measured in volts [V]) for the drive motor
  • It must have a sufficiently large capacity (measured in ampere hours [Ah] or milliampere hours [mAh = 1/1000 Ah]) to ensure sufficiently long driving times
  • It must be able to deliver enough current (measured in ampere [A] or milliampere [mA = 1/1000 A]) without “burning out” at maximum current drain.
Battery typeAbbreviationCell voltageEnergy density
Lead-gel batteriesPb2,0 V / Cell0,11 MJ/kg
Lithium polymer batteriesLiPo3,3 V / Cell0,54 MJ/kg
Lithium Ion batteriesLilon3,6 V / Cell0,50 MJ/kg

The capacity of a battery indicates “how much electricity is stored in it”.
In other words: how long can we draw a certain amount of electricity before the battery is completely empty?

The capacity is given in “milliampere x hour [mAh]”. A 2400 mAh battery therefore provides 2400 milliamperes = 2.4 amperes for one hour (or 4.8 amperes for 30 minutes, 9.6 A for 15 minutes, 28.8 A for 5 minutes, etc.)

Which batteries are best suited for us and meet all these criteria?

There are different battery types and not every battery is always suitable for the same model.

Important factors/differences between the various battery types:

  • Voltage
  • Available amperage
  • Height and weight
  • Required safety when charging
  • Price

The 3 most common battery types we use are lead-gel batteries, LiPo batteries & lithium-ion batteries (in rare cases LiFePo batteries, but whoever uses them should know about the subject anyway).

Lead-gel batteries:

LiPo batteries:

A single Lipo cell has a nominal voltage of 3.7 volts.
Fully charged it is 4.2 volts. It should not be discharged below 3.2 volts.

In the past, however, these cells could not put out as much power. In the meantime, many lipo cells can deliver well over 20A of power and are therefore ideal for our purposes.

Lipo packs are usually sold with two, three, four, five or six cells and have a nominal voltage of 7.4 volts, 11.1 volts, 14.8 volts, 18.5 volts and 22.2 volts respectively. When fully charged, they initially have about 13% more voltage (with an 11.1V battery it is about 12.6 volts).

Lipo packs always have an indication of the maximum power output (i.e. 10C), the so-called C rating.

To get the maximum power output in amps, multiply the capacity in mAh x the value in front of the C and divide by 1000.

With a 10C 2200mAh Lipo battery, this results in a maximum power output of 22 amps.

Deep discharging a single cell can happen more easily than you think!
A single lipo cell should not be discharged below 2.9V, better 3.2V. So do not suffer a temporary voltage drop below this voltage (due to sudden load at full throttle, for example).

Lipo regulators therefore have a cut-off voltage (voltage value at which the current supply is reduced or interrupted). In any case, you should choose 2.9V or higher (best setting is 3.2V). This costs a few minutes of operational time, but the lipo battery thanks you with a significantly longer service life.

It should be understood here that most regulators do not monitor the voltage of the individual cells, but only take an average value. So with 3 cells they would cut off at 9.6V or higher. For this reason, it is important to ensure that all cells have the same voltage as possible.

Since Lipo batteries absorb a lot of energy and are very sensitive at the same time, they should be handled with care.

Overheated and/or damaged Lipo batteries can quickly become extremely hot and/or explode.
At the same time, an enormous amount of smoke develops.
Therefore always handle and charge Lipo batteries carefully.

Charging the LiPo battery

It is important that you use a special charger for Lipo batteries.

With a so-called balancer, which is connected to the lipo battery and the charger, you can charge your lipo batteries more gently.
The balancer ensures that all cells in a lipo pack are charged equally. This means that no cell can be overcharged (danger of explosion) or deeply discharged (irrevocable damage or destruction of the cell).

If the cells are not evenly charged, discharged too deeply or overcharged, they can be damaged. For optimal performance, it is important to note that batteries will self-discharge even when not in use. Therefore it is recommended to store LiPo batteries with a state of charge of 60% and a room temperature of about 20°C.

When charging the battery, the charging rate is a decisive variable that can be set on the chargers. One last new term must be introduced at this point: 1C. This is the nominal capacity of the battery (given in A).

A battery with a capacity of 2000mAh has a nominal capacity of 2A. To charge a battery gently, it should be charged at a rate of 1C. A battery with a nominal capacity of 2A should therefore be charged with a current of 2A.

Of course, with more C, the battery would be charged faster with a higher charge rate. However, this puts more strain on the battery, which shortens the life of the battery.

The maximum rate at which you can charge a battery varies from battery to battery. You will usually find the information printed directly on the batteries. There are special batteries that have a charge rate of 5C or higher. A battery with a capacity of 2000mAh that has a charge rate of 5C can be charged with 5 times the nominal capacity (2A), i.e. with a charging current of 10A (5 multiplied by 2A equals 10A). The battery is charged within 12 minutes (2A / 10A results in 0.2h, which corresponds to 12 minutes).

Please always charge, store and inspect regularly outside of the droid.

If a LiPo is deeply discharged (<2.85V per cell), it must not be charged under any circumstances. It can balloon and burst/explode.

A note on the C-Rating: This is not a controlled industry standard assigned by an independent body. On the contrary, every manufacturer can print this as he wants and interprets it on his Lipo battery. Some might mean what the lipo battery can withstand without exploding, others might mean what the lipo battery can withstand 2-3 times and then be broken, and another manufacturer might mean what the lipo battery can withstand if it is approx. Can be used 50-100 times.

Lithium-ion batteries:

Lithium-ion batteries have also been used in model construction for a number of years.

These batteries are a good 40% heavier than LiPo batteries, but can be charged 2-4 times faster (ie in about 15 minutes) and are capable of delivering 30C to 60C.

At the same time, they should be considerably safer and not be able to burn or explode.

In addition, lithium-ion batteries can easily last up to 1000 charging cycles at an average load of approx. 15C.
Their voltage level is slightly below that of the Lipo batteries (3.6VV). Furthermore, their voltage drops significantly more under load than with current LiPo batteries.

A lithium-ion battery has a discharge end voltage between 2.5 volts and 3.3 volts – it varies with the respective battery type.

A special charger is required for charging, just like with LiPo batteries.

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