How to Look After your Lithium Polymer Batteries, Vol. 2
By Chris Dorling
Continued from previous post.
From here onwards, I’m going to approach the subject based on my own personal experiences and expertise. I’ve used this technology for more than 10 years and 3 years ago I started a business specialising in selling Lithium Polymer technology which I left last year to pursue life in New Zealand. I want everyone to avoid some of the mistakes I made in the past and help make your packs last longer, perform better and avoid all the dangers associated with Lithium Polymer cells.
So, imagine you’ve just purchased a new setup. Along with your new equipment, you’ve also purchased a number of shiny new battery packs and discovered they cost a considerable amount of money, especially modern DJI batteries!
You’ll of course want to get as much life out them as possible.
Battery University holds a wealth of information on all types of batteries. Much of the information I’ve used has come from this source and if you’re serious about this hobby, I think it’s essential you build a good understanding all the types of batteries you intend to use.
New Lithium Polymer Batteries
If you’ve ever purchased a new battery and checked the voltage, you’ll discover that each cell will be at approximately 3.8/3.85V per cell. This equates to approximately 40% of capacity and over the years, manufacturers have to come to the conclusion this is the best condition for cell storage over long periods. All new batteries arrive in this condition. I’m sure most of you will have bought a new mobile phone only to discover that the battery partially charged, you now understand why!
It’s easy to check your new or used batteries with a simple battery checker such as the ones below:
When charging new batteries, it is advisable not to fast charge for at least the first few cycles. This means charging no faster than ‘1C’ (I’ll explain charging rates in more detail later). The same applies to the first few discharge cycles. Some say that this isn’t essential. However, it certainly won’t do any harm. It’s thought that a ‘Breaking In’ procedure is a good idea. The idea is to fly gently and only discharge you batteries to 50% for the first few flights. This is thought to help settle the internal makeup within the battery and help set the pack up for a prolonged life. After the first few cycles, it’s not unusual to see a slight increase in performance and usually a sign that the pack is ready for normal use.
Almost all Lithium Polymer batteries you buy today will come with a ‘C’ rating. The ‘C’ in C rating stands for Capacity. The C rating is the maximum, safe, continuous discharge rate of a pack as specified by the manufacturer, so when you see 20C printed on your label, it means it can be discharged at 20 times the pack’s capacity, ‘continuously’. Capacity is usually measured in mAh and a popular cell size is 2200mAh. Therefore, so by way of example:
2200mAh 20C Cell = 2.2A x 20 = 44A continuous discharge
Along with the continuous discharge rate, some batteries display ‘burst’ discharge rate. This is typically twice the continuous discharge rate. It usually means the battery is capable of allowing twice the current, but only for a few seconds at a time.
You will damage your batteries if you use them beyond these specifications. The idea is to stay well away from these limits. In theory, the higher the C rating, the better the battery. In high current draw applications such as 3D RC helicopters, high C rated batteries are essential. The trouble with 3D helicopters is that the cells are subjected to high current draws in hard manoeuvres. The current draw spikes are sometimes beyond the limits of the battery. If the battery is pushed beyond its limits, the battery will get hot, the performance will drop and if pushed hard enough, possibly inflate and cause irreversible damage.
How Good is your Lithium Polymer Battery?
Please, please, please beware of ‘claimed’ C ratings! Most batteries now come from China and at present there seems to be no control on what manufacturers can print on their labels and no requirements to substantiate their claims. I’ve seen countless batteries clearly over specified which cannot deliver anywhere near their claimed current draw! On the flip side, I’ve also used and sold 20C packs which clearly outperform other manufacturers claiming 45C! It can be confusing to determine the genuine specifications as vaunted. Please do your research, ask around and spend wisely – cheap and good as well as expensive and poor can still be found in the market today! I’ve tested hundreds of packs and to this day, I’ve never seen a pack perform beyond 35C. If you see anyone claim beyond 35C, you should be sceptical!
Here is a 20C pack:
Purporting to be a 60C pack:
Charging Lithium Polymer Batteries
It is extremely important you only use a charger specifically designed for charging Lithium Polymer cells. All new chargers now come with a balancing feature which is essential for both safety and battery life reasons.
The stated operating voltage of a typical Lithium Polymer cell is from 3V to 4.2V when fully charged. Discharging cells below 3 volts will invariably cause irreversible damage and charging above 4.2V is dangerous and easier to achieve than you might think! If you have a selected the ‘LiPo’ function, your charger will be set to peak detect cells reaching 4.2V at which point the charger will cut off. Set the wrong type of battery; begin charging beyond 4.2V, the pack will swell and if left, will eventually catch fire! Yes, it’s that dangerous, so please be vigilant when setting your charger!
For maximum safety, please use LiPo Bag such as this one:
More practical use and importance to be covered in future posts!
Balancing Lithium Polymer Batteries
Balancing is very important. If a pack is used and kept within its safe working limits, cells tend not to drift too far out of balance. Imbalanced cells can occur when the pack has been discharged too much. Sometimes a pack may just develop a bad cell. Balance charging will help detect an imbalance between cells and reduce the likelihood of overcharging other cells within the pack. For this reason, please ONLY balance charge your batteries.
Some great choices for balancing chargers:
When charging your batteries, temperature is important. Never charge a battery that has been stored below 0ºC as you risk an explosion at these temperatures. Cold storage for LiPo cells is actually a good thing, but before charging them again always make sure your packs have had the chance to fully reach room temperature before commencing! If your family will not tolerate you keeping the batteries in the fridge, you might want to consider a mini cooler for your man cave or garage:
When charging, check the temperature of the battery by hand. If charging at a 1C charge rate, the battery shouldn’t get warm. However, I frequently charge 3C and packs do get slightly warm. Excessive heat means there is something wrong, STOP CHARGING!
After a flight, you may find your batteries are warm to touch. It may be a good idea to give the packs a chance to cool a little before recharging.
Never try charging a pack that has been crash damaged, you run the risk of fire! Cells that are obviously swollen or have physical damage should never be used and careful disposal is required, especially if the cells are swollen.
If you have a pack sitting on the shelf, fully charged, never try topping it up.
NEVER LEAVE A CHARGER UNATTENDED!
The most important point above all, knowing how potentially dangerous a LiPo battery can be if something goes wrong, NEVER leave your batteries on charge unattended! No matter how safe you THINK your charger is, things can and really do go wrong from time to time. You are at the mercy of technology and some of the worlds most experienced users have had serious accidents with LiPo batteries, including the RC helicopter legend, Curtis Youngblood who seriously burnt his hand through a faulty charging process. So please be careful! Also never charge LiPo batteries inside your house if you can really help it. There are countless videos and images spread all over the internet from LiPo fires, please have a look and beware some of the images are shocking!
The general consensus has always been to charge your LiPos at a rate of 1C, e.g, a 2200mAh battery would be charged at no more than 2.2A. Charging above 1C was once considered bad for battery health. There are still some diehards out there who are adamant that this is still the case. Over the last two years, I’ve been surrounded by packs being charged at 3C and seen absolutely no difference in lifespan! I’ve even carried out tests with packs charged at 1C and 3C, the resulting lifespan was the same. I prefer 20 to 30 minute charge times myself and feel that almost all modern LiPo packs are more than capable of being charged at 3C without causing any damage. Some manufacturers now quote 5C or even 8C, but through my own tests, packs do sometimes tend to come off the charger a little too warm for my liking and essentially, it’s excessive heat which causes a battery to eventually fail.
To be covered in the next volumes:
- Discharge Rates & Level of Discharge
- Working Temperatures
- ‘Puffing’ – Why does this occur?
- When to Retire a LiPo
- And more useful insight.
Thank you so much for putting together these articles. There’s so much confusing information about LiPo’s out there so it’s great, especially for those newers to the hobby, to have everything you need to know in a single place. Looking forward to Volume 3!
Great articles Chris but you made one mistake in the “C-Rating” paragraph.
“C” Rating DOES NOT mean “Capacity”. You give the proper definition in the third sentence of that paragraph by calling it the “Maximum, safe, continuous discharge rate”. I like to call it “Maximum Current Flow”. This has nothing to do with “capacity. If you remove the second sentence; “The ‘C’ in C rating stands for Capacity.” the paragraph will make more sense. As it stand right now you give “C” rating two different definitions which could confuse many beginners.
You could also add to that paragraph by mentioning that many battery manufactures include two C ratings on batteries. One for discharging and one for charging. Most of my packs are capable of being charged at 5C. Knowing the difference is very important…
Hopefully this makes it a little clearer 🙂
If C = Capacity
A ’20C’ Rated pack = 20 x Capacity
So as an example:
A 2200mAh(2.2Ah) battery rated at 20C (20 x Capacity) = 20 x 2.2Ah = 44A Maximum, safe, continuous discharge (Maximum current draw).
Hope this clarifies your question.
Thanks for your feedback,