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Li-Po Batteries Explained - Part 3

If you've read my previous posts about Li-Po batteries (Li-Po Batteries Explained Part 1 and Part 2), you'll now understand the basics of Li-Po's. Now let's look at some standard ratings of Li-Po's that you'll need to consider when you buy them.



Voltage

A single Li-Po cell is rated at 3.7V. This however doesn't mean that the fully charged voltage of a Li-Po cell is 3.7V. Actually, Li-Po cells have a minimum voltage of 2.7V (fully discharged) and a maximum of 4.23V (fully charged). The 3.7V is just the average operating voltage of a single cell.

Many Li-Po battery packs have two or more cells connected in series to provide higher voltages. The cell count of the battery pack is denoted by the 'S' number. A 2S battery pack means that there are 2 cells connected in series, thus providing 7.4V. Here are some examples of standard size battery packs:

1S = 1 cell x 3.7V = 3.7V
2S = 2 cells x 3.7V = 7.4V
3S = 3 cells x 3.7V = 11.1V
and so on....

As you might have noticed, the 'S' means the cells are connected is 'Series'. There are packs where the cells are connected in parallel to increase the capacity. This would be indicated by a 'P' number. For an example, a 3S2P pack means it has 2 sets of serially connected 3 cell packs connected in parallel.




Capacity

The capacity is the amount of power the battery can hold. It is measured in milliampere hours (mAh). It indicates that how much load the battery can hold for it to be fully discharged in 1 hour.

Here's an example to understand this,
If we take a battery of 1000mAh, and put it on a load of 1000mA (a 1000mA current would flow through the load), the battery will be fully drained in 1 hour. If the battery was put in on a 500mA load, it would take 2 hours to drain.

As you can see, the larger the mAh value the higher the capacity, so longer the time you can power a device. Unlike voltage, you can put different capacity batteries to a device without harming it. If you need it to work longer, just put a higher capacity battery.

Discharge Rate

The discharge rate indicates how fast a battery can be discharged safely. It is indicated by the 'C' rating. The letter 'C' comes from the word Capacity, so as you would have guessed, the discharge rate is directly related to the capacity. A battery marked as 10C can be discharged at a maximum of rate of 10 times the capacity of the battery.

For an example, if we take a battery of 1000mAh and 10C,  it can be discharged at a rate of 1000 x 10 = 10000mA without harming the battery. This means that a load drawing 10000mA can be connected to the battery safely. Anything higher could damage it. If you do a quick calculation, you can see that such a load can drain the battery in about 6 minutes.

Most newer Li-Po batteries will also indicate a 'Burst rating' as well. The burst rate is the discharge rate for short bursts of higher power. The burst current of a battery is usually higher that the continuous current. So, an example C rating would be '10C Continuous/20C Bursts'.

Putting a battery with higher discharge rate is the best option for any device. But, higher C rate batteries tend to be expensive. So, keep a balance between discharge rate and the price when you're buying Li-Po's.

Internal Resistance

Compared to the above ratings of a battery, Internal Resistance can be less important. It is however a good indicator of the health of a battery. Higher discharge rate batteries usually have a lower internal resistance, about 2 to 6 milliohms per cell when they are new. Lower discharge rate batteries can have up to 200 milliohms.

As batteries age, their internal resistance increases. As the resistance increases, the hotter the cells get when they operate. The operating temperature of Li-Po batteries should be kept below 60°C, so higher internal resistance can cause harm to a battery. So, measuring the internal resistance can be a great way to monitor the condition of your batteries.

So, I hope this cleared up few things you'll need to know when buying Li-Po batteries. Let's talk about charging and maintaining Li-Po's in the next post...

References
http://batteryuniversity.com/learn/article/the_li_polymer_battery_substance_or_hype
http://en.wikipedia.org/wiki/Lithium-ion_polymer_battery

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