Difference between revisions of "Batteries and chargers"

Revision as of 16:00, 21 March 2014

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.

Lead Batteries

(To Do)

NiH Batteries

(To Do)

Lithium Polymer (LiPo) Batteries

(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 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.

LiPoly 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)

How to select a battery

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.