LiPo battery charging safety is very important. As my picture shows, this is what can happen if you charge them incorrectly. It is important to understand the basics of these batteries as well as the more complex aspects of them.
There are several ratings you hear when discussing LiPo batteries and proper LiPo Battery Charging. First, I will cover each of the ratings so you get a better understanding of each number, then I will discuss what this means when you go to use them. They are Voltage, Capacity, and discharge rate. After that, I will discuss how low you should discharge them and how LiPo battery charging is done safely afterwards.
Unlike many of the battery types of the past that were 1.2v per cell, LiPo batteries are 3.7 volts per cell. Depending on how many cells the battery has in series (or S), will give you the overall voltage. In order to produce higher voltages, the cells are put in series. To note, it is VERY important to follow these exact ratings when it comes to your rc plane. Using the wrong voltage on your plane can cause damage to many components and/or cause failure while flying which could end in a violent and sudden stop against the ground (aka crash).
Capacity rating is the measure of how much power the battery can store. It is generally rated in mAh which stands for milliamp hours. This specifically refers to how much drain can be put onto the battery to drain it completely in one hour. For example – If I had a 1000 mAh battery and placed a 1000 milliamp (1 amp) load onto it, after 1 hour it would be completely drained. If I were to take that same battery and place a 10,000 milliamp (10 amps) load on it, it would only last 6 minutes. Unlike voltage, this number is flexible. The bigger the capacity, the longer the flight times. You are only restricted in weight and size of the battery of what will fit into your plane. Obviously, the higher the capacity, the bigger the battery thus the heavier it is. This rating also comes into play when figuring the LiPo battery charging amps. I will discuss this later.
The discharge rate of a battery simply is how quickly you can discharge the battery safely. This is known as the “C” Rating. The math behind the number is quite simple once you understand it. If you are a beginner pilot that will not be flying aggressively, this number is not as important than if you were planning to fly highly aggressive, 3D maneuvers and maxing out your draw on your battery.
If you have a battery rated at 10C, that means you can safely sustain a draw of 10x the total capacity of the battery. Let’s use our 1000 mAh battery example again. A 10C battery that is rated 1000 mAh can safely sustain a 1000 (mAh) x 10 (C) = 10,000 milliamp draw. In other words, this battery can handle a 10 amp sustained draw. If we wanted to see how long this battery would last with this sustained draw, we would take 10,000 milliamps and divide that by 60 minutes which equals 166 mAh per minute. Now, divide that into the packs capacity (1000 mA) which equals out to 6.02 minutes.
Let’s try this with a different sized battery. Say we have a 15C 1300mAh battery. First, we take 1300(mAh) x 15 (C) = 19500 milliamps or 19.5 amps. This means that we can safely draw a sustained 19.5 amps from this battery. Now, let’s find out how long it will last if we were to draw 19.5 amps from it consistently. 19500 milliamps / 60 min = 325 mAh per minute. 1300 mAh / 325 per minute equals out to 4 minutes. Now, this doesn’t mean that if you have a 1300 mAh, 15C battery that it will only last 4 minutes. It depends on what you are drawing off the battery, which depends on your components and how aggressive you fly including throttle management. Your motor is going to be drawing the most power generally. C rating is also discussed when talking about LiPo battery charging. I will discuss this later.
You will hear a lot of people tell you that you should never discharge your LiPo batteries below 80% of it’s capacity. Using our 1000 mAh battery as our example again, you should never drain more than 800 mAh. You can also determine this by how many volts your battery has after you have used it. You shouldn’t drain it below 3.75 volts per cell. In other words, a 4 cell battery should not be used below 15 volts (4 x 3.75). Now wait a second, you may say to yourself. If earlier you said batteries were rated at 3.7 volts per cell, why are you telling me now not to use it below 3.75 volts per cell? I was confused by this when I first got into this sport as well. You see, a fully charged 1 cell LiPo battery will be at 4.2 volts, 8.4 for 2 cell etc. The number 3.7 volts is in reference to it’s nominal state. Others say that you shouldn’t go below 75% of the batteries capacity, and this will in fact extend the life of your batteries, but I haven’t had a problem with 80% myself.
First off, to charge a LiPo battery, you MUST be using a charger designed for LiPo’s. Failure to do so WILL result in a minimum of ruining your battery and a possibility of fire and/or explosion of your battery which could cause serious harm and damage. I have personally witnessed someone accidentally catch a LiPo battery on fire. Proper LiPo battery charging is very important.
I recommend using a computerized charger to do all of your LiPo battery charging. That will help stop you before you charge your battery incorrectly. LiPo battery chargers use the constant current / constant voltage charging method (cc/cv). This means that a constant current is applied to the battery during the first part of the charge cycle. As the battery voltage nears the 100% charge voltage, the charger will automatically start reducing the charge current and then apply a constant voltage for the remaining phase of the charge cycle. The charger will stop charging when the 100% charge voltage of the battery pack equalizes with chargers constant voltage setting (4.2 volts per cell) at this time, the charge cycle is completed.
It is also important to choose the right charge current. It is recommended that you only charge your battery at 1C. In other words, if you have a 500mAh battery, you should charge it at .5 amps. A 1000 mAh battery, it is recommended that you charge it at 1 amp. A 2000 mAh would be charged at 2 amps. Now, I’ve seen batteries that claim you can charge them at 5C. I personally won’t go over 1-2C, but once again, I recommend following your batteries manufacturer recommendations for all LiPo battery charging.
Another important factor while charging your LiPo batteries is balancing the cells so that they are each charged to the full 4.2 volts evenly between all cells. Without balancing, your 2 cell battery could end up being charged 4.0 volts in cell #1 and 4.4 volts in cell #2 (for example). This, of course, is bad and will ruin your battery. You will hear from 10 different people 10 different methods of when you should balance and when you shouldn’t. I am going to give you my opinion on this. I don’t want to ruin my batteries. They can be expensive and I feel it’s worth the extra time to balance charge your batteries every single time. You have two options. Either you can charge directly through the balancing plug if your charger has this option, or you can charge through the ESC plug and connect the balancing plug to the balancing connector of your charger or an external balancing unit. I think the second option is the best, especially if you are charging your batteries at greater than 1C. If your charger doesn’t have the balancing feature, you will need something like the Astro Flight Blinky Lithium Battery Balancer: A123 which plugs directly into the JST-XH style balancing plug (see next section for more info on the JST-XH)
A great tool to own for testing Lipo batteries is the Great Planes CellMatch LiPo 2-6S Balancer Meter w/LCD
. It is a precision measurement tool that measures and displays individual cell voltages and remaining battery power. CellMatch also identifies voltage differences between cells and discharge-balances all cells. I have one and use it all the time.
There are 3 popular balancing plug types. The first is known as the TP or Thunder Power plug. It is commonly found on the higher end batteries such as Thunder Power, Flight Power, Outrage, Voltz, etc.
The second, and probably the most common plug is the JST-XH. This is found on many model brand batteries including Align, E-Flite, ESky, Electrifly, as well as many cheaper brands like Zippy, Turnigy, Mega Power, etc.
The third is the Hyperion (Polyquest) balancing plug. It is found on the Hyperion battery packs.
My advice on this is to buy your batteries with the same balancing plugs and make sure that your charger has this style balancing plugin. If not, the most simple solution is to by adapters for your specific types. You can solder your own and modify your plugs, but I don’t suggest this for the beginner pilot. You can quickly fry your battery, your charger, yourself, and your house if you aren’t careful.
I want to say that LiPo batteries have on occasion been known to catch on fire when charging or in use so adhere to all recommendations by your specific battery manufacturer. This post is meant to be a guide and speaks in general terms to give information and is not meant to override the manufacturers recommended uses. LiPo battery charging can be dangerous, you should never leave a LiPo battery charging unattended (or any other battery for that matter).