Note: It's been about 2 months since I've posted the part 3 of this article. Sorry for being late with this part.
I hope you have read my earlier articles on Li-Po batteries (Li-Po Batteries Explained Part 1, Part 2 and Part 3). Now let's look at the most important part - charging and maintenance.
First of all, why is this so important? Well, just watch the following video on what happens when you mistreat a Li-Po battery.
So....., now that you've seen the dangers of Li-Po batteries, it should be clear that they should be handled with care. Li-Po's are very different from conventional rechargeable batteries, so you should only use a charger specifically designed for Li-Po's. Doing so will increase the life span of the Li-Po battery pack and yours as well.
CC/CV Charging
A single cell Li-Po (1S), rated 3.7V, is fully charged when it reaches 4.2V. Charging it any higher will damage the cell and even make it catch fire (as seen on the above video). So, you'll need a charger that stops charging at 4.2V.
Most good Li-Po chargers use the CC/CV method, which stands for Constant Current/Constant Voltage. This method charges the cells in two stages. In the first stage, the cell is charged at a constant current until the cell reaches 4.2V. At this point the charger switches to the constant voltage mode, in which the charger gradually reduces the charge current while holding the voltage at a constant 4.2V. Once the charge current has fallen to a small percentage of the original current (typically 2%-3%) the cell is said to be fully charged.
The CC/CV method is shown in the following chart,
Maximum Charge Current
The general guideline for the charge current is never to charge a Li-Po with a current larger than its capacity. This means that if you are charging a 1000mAh Li-Po, the charge current should be 1000mA or lower. Just getting little above this limit doesn't hurt much, but anything significantly higher would cause the cell to heat up, swell and/or catch fire.
However, with the recent improvements of Li-Po technology, this limitation has been slightly improved. Now, if you have a Li-Po pack with 20C discharge rate or more, you can safely charge it at 2, 3 or even 5 times the current than its capacity.
Li-Po Balancing
What does Li-Po balancing or balanced charging mean? Simply put, it means that the charger monitors the voltage of each cell in a Li-Po pack and varies the charging so that all the cells are charged to the same voltage.
Why is it important? Well, let's take an example and see,
Think of a 2S Li-Po pack connected to a charger. It has 3 cells, each of which have a fully charged voltage of 4.2V, making the fully charged voltage of the pack 12.6V. So, the pack will be fully charged when it reaches 12.6V right? Well, it would be if all 3 cells are identical. But unfortunately, no two cells can be considered the same. So what if one of the cells in the pack is charging faster than the other two... that cell will be overcharged by the time the pack hits 12.6V, and you know what happens when these things overcharge...
How does a charger know to balance a Li-Po pack? Every multi-cell Li-Po pack has a balancer plug, which allows access to individual cells in the pack.
The charger or an standalone balancer can use this plug to monitor the voltages of each cell and discharge if a cell is overcharging.
Balancing ensures that all the cells in a pack are within 0.01-0.03 volts of each other. While you don't necessarily need to balance a Li-Po pack each time you charge, it's better to do so once you think of the risks of not doing so.
So, that concludes the charging and maintaining of Li-Po packs. Just a few things to note:
This concludes my 4 part article on Li-Po batteries. I would really appreciate your comments about this.
References
http://en.wikipedia.org/wiki/Lithium-ion_polymer_battery#Charging
I hope you have read my earlier articles on Li-Po batteries (Li-Po Batteries Explained Part 1, Part 2 and Part 3). Now let's look at the most important part - charging and maintenance.
First of all, why is this so important? Well, just watch the following video on what happens when you mistreat a Li-Po battery.
So....., now that you've seen the dangers of Li-Po batteries, it should be clear that they should be handled with care. Li-Po's are very different from conventional rechargeable batteries, so you should only use a charger specifically designed for Li-Po's. Doing so will increase the life span of the Li-Po battery pack and yours as well.
CC/CV Charging
A single cell Li-Po (1S), rated 3.7V, is fully charged when it reaches 4.2V. Charging it any higher will damage the cell and even make it catch fire (as seen on the above video). So, you'll need a charger that stops charging at 4.2V.
Most good Li-Po chargers use the CC/CV method, which stands for Constant Current/Constant Voltage. This method charges the cells in two stages. In the first stage, the cell is charged at a constant current until the cell reaches 4.2V. At this point the charger switches to the constant voltage mode, in which the charger gradually reduces the charge current while holding the voltage at a constant 4.2V. Once the charge current has fallen to a small percentage of the original current (typically 2%-3%) the cell is said to be fully charged.
The CC/CV method is shown in the following chart,
 |
| Li-Po charge curve |
Maximum Charge Current
The general guideline for the charge current is never to charge a Li-Po with a current larger than its capacity. This means that if you are charging a 1000mAh Li-Po, the charge current should be 1000mA or lower. Just getting little above this limit doesn't hurt much, but anything significantly higher would cause the cell to heat up, swell and/or catch fire.
However, with the recent improvements of Li-Po technology, this limitation has been slightly improved. Now, if you have a Li-Po pack with 20C discharge rate or more, you can safely charge it at 2, 3 or even 5 times the current than its capacity.
Li-Po Balancing
What does Li-Po balancing or balanced charging mean? Simply put, it means that the charger monitors the voltage of each cell in a Li-Po pack and varies the charging so that all the cells are charged to the same voltage.
Why is it important? Well, let's take an example and see,
Think of a 2S Li-Po pack connected to a charger. It has 3 cells, each of which have a fully charged voltage of 4.2V, making the fully charged voltage of the pack 12.6V. So, the pack will be fully charged when it reaches 12.6V right? Well, it would be if all 3 cells are identical. But unfortunately, no two cells can be considered the same. So what if one of the cells in the pack is charging faster than the other two... that cell will be overcharged by the time the pack hits 12.6V, and you know what happens when these things overcharge...
How does a charger know to balance a Li-Po pack? Every multi-cell Li-Po pack has a balancer plug, which allows access to individual cells in the pack.
 |
| A Li-Po pack with a balancer plug |
 |
| A Li-Po charger with a built in balancer |
So, that concludes the charging and maintaining of Li-Po packs. Just a few things to note:
- Never use trickle charging on a Li-Po cell, as lithium cells cannot handle it.
- Li-Po's can be stored for one or two months without losing much charge.
- However, If you want to store for a long time, charge the cell for about 40-60% of full charge.
- Some say that refrigerating (without freezing the cells) is also a good method for storing.
- And you should never let a cell get punctured.
This concludes my 4 part article on Li-Po batteries. I would really appreciate your comments about this.
References
http://en.wikipedia.org/wiki/Lithium-ion_polymer_battery#Charging
Thankyou ... I've read all four articles and I now have a much better understanding and a great deal of respect for Lipo technology ! Thanks again from Romania !
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