Batteries and chargers

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Contents

General issues

There are different types of batteries, mainly distinguished by the technology used to store energy. They are also different in capacity, how to charge/discharge them, weight, etc. The characteristics of each type of battery are reported below. A quick comparison among them is at the end of the page.

IMPORTANT! Each type of battery has its own type of charger. Using the wrong charger can cause fires.

ALSO IMPORTANT! Do not directly connect the two terminals of any battery (e.g. by pushing the battery against a metal object). This can cause fires, release of dangerous chemicals, or both.

AND FINALLY! Never touch any chemical coming from a battery with your naked hands. Some of those chemicals are highly corrosive and can eat away your fingers. If you see a leaking battery, don't touch it and report immediately to your supervisor.

Lead-acid Batteries

Lead-acid technology is the same used for car batteries. It is stable from decades, dependable, cheap, and effective. The main disadvantage of Lead-acid batteries is their weight. For robots, the most frequently used type of Lead-acid battery generally used is the sealed one, where the battery is a closed box. This is safer for handling, because Lead-acid batteries are filled with very corrosive sulfuric acid: with sealed batteries, this cannot escape the battery. The other common type of Lead-acid battery is the one with the refill caps for distilled water on top. We do not use this in AIRLab.

Lead-acid batteries should be kept charged. They are on a table below the window in front of of the entrance of the lab, together with the chargers. Once you take charged batteries, put the exhausted ones on charge. Take into account the power supplied by the charger (usually 24 V, i.e. two batteries in series with the appropriate cables). At the moment (October 2014), for lead-acid batteries, the charging table is equipped with 4 chargers for 24 V batteries (for instance, battery packs composed of two 12V batteries) and 3 chargers with a 12 V output (for instance, single 12 V batteries). One of the 12 V chargers is also capable of charging 6 V batteries, if needed.

NiMH Batteries

Nickel–metal hydride batteries are commonly used for cheap low-power electronics. Besides being inexpensive, they are very easy to buy: you can usually find them in supermarkets. Unfortunately, these are their only advantages. They suffer from low energy density (i.e. ratio between stored energy and weight) and are subject to memory effect and other capacity-reducing effects. In AIRLab are located in the cabinet between the two doors of mechanical and electronics workshops. Chergers are on the table close to the glass door on the right of the entrance of the lab. Use the appropriate ones.

Lithium Polymer (LiPo) and Lithium Iron Polymer (LiFePo)Batteries

WARNING! Unlike Lead-acid batteries, LiPo batteries are FLAMMABLE. If they are not used and (especially) recharged properly, they can cause FIRES. Please be careful when using these batteries. A good safety primer in Italian can be found here. LiFePo have similar properties, but they are not flammable.

(What follows is taken from http://www.hobbyking.com/hobbyking/store/lipo_basics.asp)

It can sometimes be difficult to know which battery is best for your application. A huge variety of batteries are available and, while many may suit your application, your ultimate goal is to purchase a battery pack that will: -be within your budget -have a long cycle life -have the correct size and weight -give you the longest flight or operating times -be able to deliver the correct voltage/amp (Power)

You may have noticed by now that batteries have different ratings, sizes, plugs, wire, charge rates and chemical makeup. Lets decipher;

Capacity (mAh). This is usually the biggest number shown on the pack and is measured in mAh (Milliamp/hour) or Ah (Amp/hour). The capacity is the first indicator of the batteries size. To keep things simple, think of capacity (mAh) as the amount of fuel in your cars gas tank. A higher capacity tank will run your car for longer. A 4,000mAh battery will run for twice as long as a 2,000mAh battery. A 2,000mah battery will (in theory) run for 1hr if drained at a constant 2,000 Milliamps.

Discharge (C) Discharge is the amount of power the battery can 'push' out and the number shown '20C' is an multiplication of the capacity. For example; A 20C battery can discharge at 20 x 2,000mAh which is 40,000mAh or 40Amps. This is an important number if you know your motor requires a certain power level. In addition to this, batteries have a 'Burst' rate, which is the amount of power the battery can discharge for a short period, usually 10-20 seconds. A typical battery label may show 20-30C, this would mean a 1,000mAh battery can discharge 20,000mAh constantly or give a sudden and short 10-20 second 30,000mAh (30A) burst of power. Tip: A higher 'C' rated battery will last longer if run at a lower 'C' rate. Example: a 30C battery run at 20C maximum will have a longer cycle life than a 20C run at 20C each flight.

Voltage (S) All lithium Polymer cells in any industry have a nominal voltage of 3.7v per cell. When fully charged a LiPoly cell should be 4.2v and when discharged it should never be below 3v. You will notice that LiPoly RC packs are made up of layers of multiple cells. If the battery's rating is 3S this means it is 3 x 3.7v which is 11.1v. It has 3 layers of 3.7v each. In other words, its a '3 cell pack'.

Weight/Size For a battery to be right for your system it must fit within the system battery compartment and also balance the sstem correctly. It's temping to choose the biggest and most powerful battery your system can handle, but this may sacrifice performance and, if your packs voltage is too high, destroy the ESC or Motor. Check with your ESC and Motor specification to ensure you have the right voltage pack then check the system CG (Center of Gravity) to decide on the right battery weight.

LiPo Charging Always use a lithium Polymer battery charger and never charge the battery above 4.2v per cell. (example: 2S, never above 8.4v) Never leave a charging battery unattended. Never allow the battery's voltage to fall below 3.2v per cell. (example: 3S, never below 9.6v)

In AIRLab, if they are not on the robots, they are in a box on the table under the window in front of the entrance of the lab, together with the appropriate chargers. When charging, LiPo batteries require that 'both' the power cable (thick, 2 conductors) and a sensing cable (thin, multiple conductors) are connected. The sensing cable is necessary to monitor the status of the battery cells to prevent damaging the battery during the charge process, or even fires. Unfortunately, there is not a universal standard for plugs and sockets on LiPo batteries, especially for sensing cables. So you will need to find the "right" charger (among the ones available on the table) and, sometimes, to use an adapter cable (with the right number of pins!) and/or adapter PCB to connect the sensing cable of the charger to the one of the battery. Please carefully put all this back on the table when you finish charging!

How to choose the right type of battery for your robot

Some general issues are mentioned here

Except for low-performance applications (where cheap NiMH batteries are still popular), when designing a battery-powered robot the choice is between Lead-acid and LiPo batteries.

Actually, the choice is easy, as the only advantage of LiPo batteries over Lead-acid batteries is weight. More precisely, LiPo battery have a much higher energy density, i.e. they store much more energy than a Lead-acid battery of the same weight.

This advantage is offset by several disadvantages of LiPo with respect to Lead-acid are:

  • Higher price. LiPo batteries are much more costly than Lead-acid ones with the same energy capacity.
  • Lower robustness. LiPo batteries are delicate and can be easily damaged, especially if not recharged correctly.
  • Not suitable for low temperature application. Exposure to cold environments (outdoor usage during winter) damages LiPo batteries.
  • Safety issues. Unlike Lead-acid batteries, LiPo batteries burn, as some of their components are flammable. Moreover, metal "dendrites" tend to form inside incorrectly used and/or recharged LiPo batteries, which in turn can cause internal short circuits and even fires.

So: if your robot requires that batteries are as light as possible (i.e., a flying robot with low payload), LiPo is the best choice for you. Otherwise, Lead-acid is probably the best choice.