Square Root of Negative One

LiPo Batteries

Posted in EnS by cheng on December 20, 2009


good sourse:BatteryUniversity

10C from 3S4P? Naming conventions explained.
How fast a battery can discharge is it’s maximum current capacity. Current is generally rated in C’s for the battery. C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour. All RC batteries are rated in milli Amp hours. If a battery is rated at 2000 mAh and you discharge it at 2000mA (or 2 amps, 1 amp = 1000mA) it will be completely discharged in one hour. The C rating of the battery is thus based on its capacity. A 2000mAh cell discharged a 2 amps is being discharged at 1C (2000mA x 1), a 2000mAh cell discharged at 6 amps is being discharged at 3C( 2000mA x 3).
All batteries have limitations on how fast they can discharge. Because of this many LiPoly batteries are put in parallel to increase the current capacity of the battery pack. When 2 batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have 2 2000mAh cells and you wire them in parallel then the result is the same as 1 4000mAh cell. This 4000mAh cell has the same C rating as the original 2000mAh cells did. Thus if the 2000mAh cells could discharge at a maximum of 5C, or 10 amps then the new 4000mAh cell can also discharge at 5C or (4000mA x 5) 20 amps. This method of battery pack building allows us to use LiPoly batteries at higher currents than single cells could produce.
The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the XSXP method. The number in front of the S represents the number of series cells in the pack so 3S means it’s a 3 cell pack. The number in front of P means the number of cells in parallel. So a 3S4P pack of 2100mAh cells has a total of 12 cells inside. It will have the voltage of any other 3S pack since the number of cells in series determines the voltage. It will have the current handling of 4 times the maximum C rating of the 12 individual cells. So say our 3S4P pack had a maximum discharge of 6C. That means that it has a nominal voltage of 10.8 volts (3×3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).
[http://www.rcgroups.com/forums/showthread.php?t=209187]

11.1 volt – 2000mAh -10C
2000 milliamps = 2 amps
2 Amps x 10 = 20 amps continuous discharge

[http://www.commonsenserc.com/page.php?page=c_ratings_explained.html]

Lifetime
Expect to get at least 300 cycles during the lifespan of the batteries, if you take care when charging/discharging.

Do not store lipos at 100 percent charge, and you should not store them at near minimum voltage. Either of these situations will result in the battery internally “rusting away”, and will decrease it’s capacity over time. The best way to store a lipo is at about a 50% charge, in a cool, dry location.

No charger will undo the permanent chemical reactions which take place within an over discharged cell.

Lithium batteries like heat, but not too much. In the winter time, try to keep your batteries from the cold as much as possible. Leave them in the car while your flying, or keep them in your cargo pants… etc. At the same time don’t let them heat up too much. Try to keep your batteries from reaching 160F after use. This will prolong the life of the cells. A good way to measure temperature is a handheld IR meter, they can be found for around $50.00 at most hobby shops.

[http://www.rctoys.com/pr/category/rc-information/lithium-polymer-battery-info/]

Safety

  1. Store lithium polymer batteries in a flame proof LipoSack while charging.
  2. Read the manual
  3. Don’t charge batteries unsupervised.
  4. Use the right battery charger
  5. Keep a fire extinguisher, or bucket of sand near the charging area
  6. Don’t charge lithium polymer batteries near flammable substances
  7. Check lithium polymer batteries for swelling prior to charging and each use – A puffed battery is unstable, and can be in danger of exploding. If you see a puffed battery, immediately disconnect it from the charger or aircraft and put it in a bucket of water. Dissolve a few tablespoons of salt in the water to aid conductivity, and leave the battery in the bucked for about 4 days. The salt water depletes any power remaining in the battery by creating a short, and it can’t catch fire while underwater. After the four days are up, take the battery out and cut off the connectors (which may come in handy for other projects). You can then dispose of the battery in the trash. The battery no longer contains toxic metals, won’t harm the environment, and by using the salt water you’ve guaranteed that it won’t catch fire. This should be done as soon as you see a puffed battery. You can’t salvage a puffed battery, the best you can do is to dispose of it safely.
  8. Never charge a lithium polymer battery in a model
  9. Make sure the charging leads are connected properly –
  10. Don’t overcharge batteries – By their very chemistry, lithium polymer batteries cannot be discharged to a potential of less than 3 volts without damage. For the same reason, don’t charge them to over 4.2 volts. This means that you have to land your rc aircraft before the motors stop turning. Some aircraft come equipped with a voltage cut-off, others do not. If you don’t have a voltage cut-off, then land as soon as you sense the propeller or rotors slowing down.
  11. Balance lipo batteries – Lithium polymer batteries have balance connectors, designed to make sure that each cell in the pack has the same charge. If this isn’t the case, some cells can become overcharged and explode.
  12. Never let the battery leads touch
  13. Don’t ever store / charge lithium polymer batteries in your ca
  14. In the event of a crash, remove the battery and supervise it for at least 4 hours

[http://www.rctoys.com/pr/category/rc-information/lithium-polymer-battery-info/]

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: