Sweed
02-18-2007, 01:23 PM
Overview:
Li-Poly batteries have been getting real popular lately, what with the high discharge rates and a high energy storage/weight ratio they make a perfect fit into our 1/28 scale XMODS.
Are Li-Po's for me?
Li-Poly cells are not recommened for the casual RC hobbyist. You do have to know how to properly charge, discharge, and store these cells or they can cause a fire. Follow the proper rules for the use and care of Li-Poly cells and they are perfectly safe to use. Despite popular opinion, Li-Poly batteries do not explode, they vent toxic gasses and flames. Here is a video showing a Li-Poly pack venting.
http://www.helihobby.com/videos/LithiumBattery.wmv
If you are the type that likes to push the limits and throw caution into the wind, then Li-Poly cells are not for you. You have to be very responsible when using Li-Poly cells. You must follow all the safety requirements.
Charging Lithium-Polymer Batteries:
Li-Poly cells are very different than NiMH or NiCad. Each Li-Poly cell delivers 3.6V when under load vs. 1.2V for NiMH and NiCad.Due to this voltage difference, we only need 2 cells to match the voltage used in 6 cell NiMH XMODS.
Each Li-poly cell is fully charged when the cell tests at 4.2V (no load) and is fully discharged when the cell tests at 3.0V (no load). It is very important that you do not let the cell go above 4.2V or below 3V or you will damage the cell. It is not recommended that you charge cells that have discharged below 3V. To ensure that you do not over discharge a Li-Poly cell, protection PCBs are available that monitor your cells voltage and discharge rate to protect the battery from over charging/discharging. If you are new to Li-Poly, it is highly recommended that you get one of these protection PCBs.
Li-Poly cells are very different than your NiMH or NiCad cells. You must use a charger that is specifically designed to charge Li-POly cells. Using any other type of charger is dangerous. Some of these chargers have a cell number setting to charge 1,2,3, or 4 cell packs. Make sure the charger is on the proper cell number for your pack. The majority of us use 2 cell packs unless you have a voltage regular board to limit the voltage to the XMOD PCB. A Stock XMOD cannot handle a 3 cell pack.
It is not recommended that you use a cell phone charger to charge Li-Poly packs. Although they were designed to charge these cells I have found that many places discourage this practice. Li-Poly chargers are widely available and affordable on the Internet. Be safe and use a proper charger for your Li-Poly battery pack.
What not to do....
Never charge Li-Poly batteries unattended, this is the leading cause of houses being burnt to a crisp by a Li-Poly fire. If you do charge unattended, do so outside away from any flamable objects.
DO NOT puncture the cell, ever. If a cell balloons quickly place it in a fire safe place, especially if you were charging it when it ballooned. After you have let the cell sit in a fire safe place for at least 2 hours. Discharge the cell/pack slowly. This can be done by wiring a flashlight bulb of appropriate voltage (higher voltage is ok, lower voltage is not) to the battery. Wait until the light is completely off, then throw the battery away.
Never charge the cells at more than 1C unless specifically authorized by the pack vendor.
What is C you say? Well, I am not an electronics wizard, but I found this description on the net that I thought I would steal from a RC airplane forum. It explains it alot better than I could.
The "C" Rating:
*This has been re-writin by Sweed, but may contain original phrases*
The "C" rating is how fast a battery can discharge it's maximum current capacity. Current is generally rated in "C"'s for batteries. One (1) "C" discharges the batterie in one hour. Two (2) "C" discharges the batterie in half-an-hour (30min), and so on. RC Batteries are rated in "Milli-Amp Hours" or "mah".
If said battery is rated at 2000 mAh and you discharge it at 2000mA (2 amps) 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 two batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have two 2000mAh cells and you wire them in parallel then the result is the same as one 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 Li-Poly batteries at higher currents than single cells could produce.
Name Convention of Li-Poly Batterie Packs:
The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the xSyP method.
-- The variable "x" in front of the "S" represents the number of cells in series. For examble, a 3S pack has three batteries in Series.
-- The variable "y" 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 four times the maximum "C" rating of the twelve 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 (3x3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).
------------------
Copied from XmodWorld with permission from the original author, Mango.
-This article has been edited for better flow, and to make it easier to look things up.
Li-Poly batteries have been getting real popular lately, what with the high discharge rates and a high energy storage/weight ratio they make a perfect fit into our 1/28 scale XMODS.
Are Li-Po's for me?
Li-Poly cells are not recommened for the casual RC hobbyist. You do have to know how to properly charge, discharge, and store these cells or they can cause a fire. Follow the proper rules for the use and care of Li-Poly cells and they are perfectly safe to use. Despite popular opinion, Li-Poly batteries do not explode, they vent toxic gasses and flames. Here is a video showing a Li-Poly pack venting.
http://www.helihobby.com/videos/LithiumBattery.wmv
If you are the type that likes to push the limits and throw caution into the wind, then Li-Poly cells are not for you. You have to be very responsible when using Li-Poly cells. You must follow all the safety requirements.
Charging Lithium-Polymer Batteries:
Li-Poly cells are very different than NiMH or NiCad. Each Li-Poly cell delivers 3.6V when under load vs. 1.2V for NiMH and NiCad.Due to this voltage difference, we only need 2 cells to match the voltage used in 6 cell NiMH XMODS.
Each Li-poly cell is fully charged when the cell tests at 4.2V (no load) and is fully discharged when the cell tests at 3.0V (no load). It is very important that you do not let the cell go above 4.2V or below 3V or you will damage the cell. It is not recommended that you charge cells that have discharged below 3V. To ensure that you do not over discharge a Li-Poly cell, protection PCBs are available that monitor your cells voltage and discharge rate to protect the battery from over charging/discharging. If you are new to Li-Poly, it is highly recommended that you get one of these protection PCBs.
Li-Poly cells are very different than your NiMH or NiCad cells. You must use a charger that is specifically designed to charge Li-POly cells. Using any other type of charger is dangerous. Some of these chargers have a cell number setting to charge 1,2,3, or 4 cell packs. Make sure the charger is on the proper cell number for your pack. The majority of us use 2 cell packs unless you have a voltage regular board to limit the voltage to the XMOD PCB. A Stock XMOD cannot handle a 3 cell pack.
It is not recommended that you use a cell phone charger to charge Li-Poly packs. Although they were designed to charge these cells I have found that many places discourage this practice. Li-Poly chargers are widely available and affordable on the Internet. Be safe and use a proper charger for your Li-Poly battery pack.
What not to do....
Never charge Li-Poly batteries unattended, this is the leading cause of houses being burnt to a crisp by a Li-Poly fire. If you do charge unattended, do so outside away from any flamable objects.
DO NOT puncture the cell, ever. If a cell balloons quickly place it in a fire safe place, especially if you were charging it when it ballooned. After you have let the cell sit in a fire safe place for at least 2 hours. Discharge the cell/pack slowly. This can be done by wiring a flashlight bulb of appropriate voltage (higher voltage is ok, lower voltage is not) to the battery. Wait until the light is completely off, then throw the battery away.
Never charge the cells at more than 1C unless specifically authorized by the pack vendor.
What is C you say? Well, I am not an electronics wizard, but I found this description on the net that I thought I would steal from a RC airplane forum. It explains it alot better than I could.
The "C" Rating:
*This has been re-writin by Sweed, but may contain original phrases*
The "C" rating is how fast a battery can discharge it's maximum current capacity. Current is generally rated in "C"'s for batteries. One (1) "C" discharges the batterie in one hour. Two (2) "C" discharges the batterie in half-an-hour (30min), and so on. RC Batteries are rated in "Milli-Amp Hours" or "mah".
If said battery is rated at 2000 mAh and you discharge it at 2000mA (2 amps) 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 two batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have two 2000mAh cells and you wire them in parallel then the result is the same as one 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 Li-Poly batteries at higher currents than single cells could produce.
Name Convention of Li-Poly Batterie Packs:
The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the xSyP method.
-- The variable "x" in front of the "S" represents the number of cells in series. For examble, a 3S pack has three batteries in Series.
-- The variable "y" 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 four times the maximum "C" rating of the twelve 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 (3x3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).
------------------
Copied from XmodWorld with permission from the original author, Mango.
-This article has been edited for better flow, and to make it easier to look things up.