Let's get started!
Let me start off by saying that I am a fan of Lithium Polymer(LiPO) battery technology. The LiPO battery has opened up many avenues in Radio control that would just not have been possible without it. This guide is related to how the technology is used in Radio Control.
I will try and keep this guide simple and I won't go into extreme details like chemical makeup and so on, but please take the time to read through charging section in this article, as this is arguably the most important section in the guide.
The best way to understand LiPO batteries are by listing known advantages and disadvantages.
High Power Density - The batteries contain more power per weight over older technologies like Ni-CD, NiMH and PB batteries.
No Memory Effect - Older battery technologies suffered from charging and discharging patterns which meant that power delivery from the batteries became unreliable.
Low Self-Discharge Rate - The battery does not lose its charge over time when not in use.
High Energy Discharge Rates - The battery can supply high demands for power.
Smart Charging - Battery chargers can tell us exactly what percentage of battery power remains in the pack, how healthy the battery is and the voltage of each cell indicating how balanced the battery is.
Low Profile - The LiPO battery comes is a vast variety of size configurations.
Over Heating - When the discharge rating of the battery is exceeded, the battery is easily damaged.
Safety - Care must be given in handling the batteries. Use the batteries within the given specifications only.
Transportation - Air freight is often not possible due the the high watt hours contained in LiPO batteries. LiPO batteries are often therefore shipped via land and sea freight.
Fire Hazard - More modern Lithium Polymer packs are a lot less susceptible to fire, however the risk remains and so fire resistance bags should be used for charging, storage and transportation of the batteries.
Higher Maintenance - LiPO batteries need to be sorted at a specific voltage to ensure longevity. Under voltage batteries could be over discharged below 2.9v per cell making them expensive paper weights or if the battery is stored fully charged could swell the battery making it unusable.
With proper care, Lithium Polymer Batteries are amazing and allow us to play with all sorts of Radio Controlled models with more powerful motors and longer run times.
What are all the numbers on my battery?
The capacity on LiPO batteries are typically specified in mah which stands for milliamps per hour and can be converted to amp hours simply by dividing the number by 1000 because 1000mah = 1Ah. Amp hours you say? Yes! In the example above, the capacity of the battery is 3300mah. If I were to put a load on the battery equal to its capacity, the battery would last 1 hour. 3300mah / 3300mah = 1Hr. If I reduced the load to 0.9Ah the battery would last 3.6Hrs (3300/900=3.6hrs) and if I increased the load to 50Ah the battery would only last around 3.9 minutes (3300/50000=0.066hrs).
Voltage and Cell Count
The voltage and cell counts mean the same thing, but are indicated slightly differently. A single LiPO battery cell(1s) has a nominal voltage of 3.7v. You can see in the picture above the battery is specified to be 3S. This mean that the battery has been created using 3 separate lipo cells setup in series giving it a total voltage of 11.1v. 3 x 3.7 = 11.1v. The cell count is important so that your charger can be correctly configured to balance the cell voltages evenly when charging the battery. I'll cover more of that a bit later.
You will notice on this battery label there are two C ratings. The first one the left says 5C charging and the one on the right says 45C. Not all batteries including two C ratings printed on the label, so if you only see one number followed by C you can safely assume this is the discharge rating. In fact the larger number of the two will always be the discharge rating.
That's all well and good... but what is a C Rating? Thankfully all battery manufacturers use the same principle and so the same math can be applied to all batteries.
C = Battery Capacity, in this case 3300mah. So the safe Discharge Capacity of this battery is 45 x 3300 / 1000 = 148.5A.
I always recommend charging a battery at 1C to ensure the longevity of the battery. In this case 1 x 3300 / 1000 = 3.3A. The manufacturer have specified on this label that charging is safe up to a maximum of 5C. In this case 5 x 3300 / 1000 = 16.5A. I imagine a charge rating this high may reduce the efficiency of the pack or reduce overall battery life. So if you are not in a hurry, stick to 1C charging.
Lithium Polymer batteries must only ever be charged using a compatible charger that is designed specifically with the ability to understand how to charge a LiPO battery pack. Charging a LiPO battery with multiple cells requires the use of cell balancing functionality. This balancing ensures that the cells are kept at the same voltage during the charge cycle ensuring that no cells exceed the maximum charge voltage of 4.2v per cell. As we established earlier, failure to follow the voltage specifications for the battery could result in a damaged battery or worse, even a fire.
An easy to understand example of how this could occur looks like this:
We have a 3300mah battery consisting of 3 cells. During the discharge process because of imperfections in manfacture, the batteries were discharged at slightly different rates. This doesn't matter too much, but over time the cell voltage differences would compound and therefore increase over time.
In this example, the cells discharged in the following way.
Cell 1 = 3.84v
Cell 2 = 3.44v
Cell 3 = 3.12v
For Cell one to be fully charged would require that the voltage be increased by 0.36v. 3.84v + 0.36v = 4.2v (Fully Charged). The problem comes in that without the balancer keeping the cell voltages the same, the charge in the second cell would have only been 3.44v + 0.36v = 3.8v (Under Charged) and in the third cell 3.12v + 0.36 = 3.48v (also under charged).
The net effect here is that we have an under charged battery. A fully charged 3s battery will have a total voltage of 12.6v. In the above example we have cell1 4.2v + cell 2 3.8v + cell3 3.48v = 11.48v. As you can see, the total battery voltage is under the voltage of a fully charged pack. A Lithium Polymer charger without balancing would not stop the charge at this point because the battery has not as yet reached 12.6v. The charger would continue to add current until the battery in this example reached 12.6v. The voltage difference is 12.6v - 11.48v = 1.12v. Lets divide that into the three cells and see what the results are. 1.12v / 3 = 0.37v to be added to each cell.
Cell 1 = 4.2v + 0.37v = 4.57v (Over Charged, likely going to get damaged)
Cell 2 = 3.8v + 0.37v = 4.17v (Just slightly undercharged which means wasted capacity)
Cell 3 = 3.48v + 0.37v = 3.85v (Completely under charged and wasting approximately 40% of its capacity)
The battery is showing fully charged 4.57v+4.17v+3.85v = 12.59v however you and I know that these cells are out of balance and the battery is not going to last and could be a potential fire hazard.
All modern Chargers and LiPO batteries therefore have balancing plugs that are connected so that the Charger can ensure each cell has the same voltage. What we are looking to see is:
Cell1 = 4.2v (Fully Charged)
Cell2 = 4.2v (Fully Charged)
Cell3 = 4.2v (Fully Charged)
Although your charger will do everything it can to fully balance your batteries cells, you may find very slight voltage discrepancies, however these will be far less than in the example above. 4.19v, 4.2v and 4.18v for example is perfectly acceptable and can be considered balanced. You will note, none of these cell voltages exceed 4.2v
Maintenance and Care
Handle your batteries with care. The more effort you take in caring for your batteries, the longer they will last. I have batteries that are over 10 years old and are still in use today.
Make sure they are securely mounted when in use and that they are properly stored when you are done.
The proper storage voltage for your batteries are 3.8v per cell. In the example above we have a 3s battery so your storage voltage is calculated as 3 x 3.8v = 11.4v. Your battery may requiring charging or discharging depending on its state after usage or sometimes you charge the battery with the intent of using it and just don't get around to it.
Modern chargers have a storage mode. Once the correct battery has been set on the charger, the charger will increase or decrease the voltage of the battery so that the battery voltage meets the recommended voltage of 3.8c per cell. The charger balances this battery during the storage process. Batteries that have been placed in storage should require little to no maintenance until the next time you want to use them.
DO NOT discharge the LiPO cells below 2.9v per cell.
DO NOT overcharge the LiPO cells above 4.2v per cell.
DO NOT charge the batteries until they are room temperature.
DO NOT charge the batteries unsupervised.
DO NOT charge the battery without a LiPO Bag.
DO NOT use visibly damaged or swollen batteries.
DO NOT exceed the battery C Rating.
DO NOT leave a battery fully charged for more than 3 days.
DO NOT use a battery if the connector is loose in any way.
DO NOT use a battery if the main leads or balancing leads are not fully insulated.
DO enjoy your hobby because no amounts of "DO NOTs" supersedes the amazing value these batteries bring to the hobby!
How do I prevent over discharging my batteries?
I have goods new and bad news. The good news is that most electronic speed controllers(ESC) have battery protection built into them. In land vehicles they will stop providing power to the motor, but allow the radio to continue to work. In many aircraft speed controllers, the controller will pulse the motor so you know it's time to land. The ESC's often also allow you to set a minimum voltage so that the warnings start earlier.
Let me be clear in saying that the battery protection in your ESC should always only be your backup plan. As the operator of a model, especially in the case of an airborne model, you must know what the runtime is for your particular model. I typically land with around 30% battery remaining. This provides room for go arounds and second chances, but that's for another guide around flying best practice.
I always have a rough idea how much power my model will consume by looking up the maximum amp consumption. This is my starting point. So for example If my model has a 30A speed controller, I calculate runtime as follows using our 3300mah battery example. 3300mah /1000 / 30 = 0.11hrs * 60 minutes = 6.6 minutes. This is the absolutely worse case scenario. When I fly, I very seldom fly with the throttle fully open. I set my timer to 6 minutes and fly as I normally would. I take my battery once cool and fully charge the battery. I take note of the mah that the charger put back into my battery and then calculate my average flying time from there.
Let's say that the charger put 2000mah back into the battery to get the battery to full charge. That means with the model I flew, I have an average consumption of 2000mah / 6min = 333.33mah consumed for every 1 minute of flying. To get my full flight time 3300mah / 333.33mah = 9.9mins total flying duration. Of course I need a few minutes extra in case I don't land on the first go, so I will likely set my transmitters timer to 8 minutes giving me an additional 2 minutes to land when the battery becomes low.
Although this is an excellent way to protect your investments, it is never full proof which is why the ESC battery protection is your backup plan.
No, I didn't forget about the bad news. The bad news is that some manufactures, including LiPO batteries and their ESC's, do not include battery protection. So by the time you notice a huge drop in power, it's often too late and the battery has already over discharged.
Thankfully there is a little gadget that you can plug onto the balance leads of your battery called an Alarm Buzzer. This small gadget will make a loud buzzing noise warning you that your battery levels are getting low. In this instance follow the same rules, plan 1, calculate your runtime and plan 2 have the alarm buzzer as your backup plan.
How to dispose of LiPO Batteries
Discharge the battery to 3v per cell. In our continued example the total battery voltage would be 9v.
Dissolve a fair amount of salt into water and submerge the battery completely in the salt water solution.
Check the battery voltage daily until the voltage measure 0.0v
Once you are sure the battery is inert, its always better to recycle, so go ahead and find the closest battery recycling bins near you.
Finally if you don't want to go through the process yourself, feel free to contact us. We offer a LiPO disposal service.
Thank you for taking the time to read this article. Should you feel that this article requires an update or should you wish to ask me a question, please do so by emailing email@example.com