Whenever you read those news headline of someone losing their house to a LiPo fire, it is always because of some sort of charging mishap. Lithium Polymer batteries are at their most volatile when they are being charged, and one little slip up in the process can result in disaster. This article focuses on the how to charge LiPo batteries safely, with an emphasis of where things can go wrong and how you can prevent it.
This is part 2 of our LiPo battery safety series. Please check out part 1 if you haven’t already.
Part 1 – How LiPo Batteries Explode
Part 2 – Charging Safety for LiPos
Part 3 – LiPo Handling Guidelines
Before you go out and buy your batteries and charger, you should understand the basics of how charging works.
LiPo batteries charge differently from pretty much every other battery on the market. The charging cycle starts with a constant voltage being applied across the battery terminals that is slightly higher than the voltage the battery is currently resting at. This causes a current to flow through the battery, beginning the charging process. This current must be regulated by the charger to not over-saturate the battery with charge. Once the battery is reaches a set voltage, the current flow is slowed and eventually stop. LiPo batteries can never be “trickle charged” and do not have a safe saturation point like car batteries where they will no longer accept charge. If you overcharge a LiPo battery, it will eventually explode – catastrophically.
This is the most important thing to keep in mind when charging lithium batteries – if you attempt to overcharge them at any time – they will accept the charge, slowly grow in size, and eventually explode in a giant fireball. It is also the big difference between LiPo batteries and other chemistries. If you plug a LiPo battery into a car battery charger or a NiMH or NiCd charger, it will very likely explode. You must get a LiPo charger to charge a LiPo battery.
If you started in the hobby with a toy micro quadcopter like a Hubsan or a Syma, you received the most basic type of LiPo batter charger as part of the package. These chargers plug into the wall and the battery just docks into them, charging automatically. Unfortunately, as you progress in the hobby to more powerful aircraft, things will not be so simple. A lot of this is because of the concept of cell balancing.
All high power Lithium batteries are made by stacking several cells in series. This is the meaning of the “S” value in “3S” and “4S”, for instance – the number of cells in series. These cells work together to raise the power level of the battery in total. They also work together to make your charging life more difficult. The problem is that when you charge a LiPo battery with many different cells, you must ensure that each cell is taken care of separately. Remember the golden rule – if any LiPo cell is charged too much, it risks damage or explosion.
This is where the concept of cell balancing comes in. Balacing a LiPo is the process of regulating the charge state of all of the cells in the battery so that they are close enough to be safe to continue charging. This is why all LiPo batteries have two sets of wires coming out of them. The main (generally larger) set of wires are the main power leads – the ones you plug into your quadcopter. The smaller wires are cell taps that allow you to monitor and adjust the charge state of the individual cells of the battery. There will be one wire for each cell in your battery, plus a wire for ground. All LiPo battery chargers will have ports that accept these wires in an effort to keep the battery healthy and safe.
You may ask – why do LiPo battery cells fall out of balance in the first place? Generally, it is caused by mismatches in the chemistry of the individual cells of the battery. One cell may have slightly more electrical capacity than the others, or one cell may be more eager to give up its electrons than the others. Either way, this is largely a battery-quality issue. Buying high quality packs from reputable vendors and using them to the manufacturer’s specification is your first good step towards never having balancing issues.
There are many charger options on the market, from cheapo AC wall units that cost $20 to high current, multi-output chargers that cost in the hundreds. In practicality, you should expect to spend between $40-$60 for a basic charger that will plug directly into your homes AC outlet. Moving up, you can expect to spend between $50-$80 for a nice charger that will last you forever and allow you to charge multiple batteries concurrently. The problem as you get into these more professional chargers is that you will also need to buy a DC power supply to plug into the wall, which is another $20-$40. On the real high end, you can get a really nice multi-battery charger that has a built in AC power converter for around $180.
Since the only really safe way to charge multicell batteries is to also balance them, all decent chargers will come with a balance port built in. In fact, many of the cheapest chargers on the market consist of ONLY a balance port. These chargers are actually no more than glorified variants of toy quadcopter chargers that simply charge each battery cell individually.
What you are generally paying for with more expensive chargers is power output. This is important for being able to charge your batteries in under an hour – or for charging multiple batteries at once. As you progress in the hobby, you will accumulate a large stack of batteries that you will fully discharge every time you fly. Charging all of them separately would be a time consuming and frustrating process. This is where the more expensive chargers on the market come in. Some of these chargers will be able to output a tremendous amount of power, allowing you to charge multiple batteries at once with a paraboard (see below for more details) – like the Turnigy Reaktor linked above. Others come with multiple chargers packaged into a single enclosure, like the Hitec X4.
Regardless of whether you get a premium or normal charger, they almost all have the same computerized interface – so once you learn how to use one, you will know how to use them all.
The key to never having charging mishaps is to form good habits in your battery charging procedures. When developing these procedures, you want to develop redundancy into them so that if you forget or misinterpret a check, then another check should help you in catching the problem. Here are some good procedures to get in the habit of doing:
- Verify charger is in “LiPo” mode.
- Verify charger is in cell count mode (e.g. 3S, 4S, etc)
- Verify charging rate is accurate for battery. To get this, divide your battery’s mAh number by 1000. A 1300mAh battery pack should be charged at 1.3A.
- Crosscheck battery cell count by physically looking at battery or balance connector.
- After starting charging, crosscheck series mode with cell count again.
You’ll be happy to know that your charger has some built in safety checks, as well. These are:
- A confirmation screen before charging begins asking you to confirm the battery cell count.
- Automatic charge cancellation if a cell is not detected (when balance charging).
- Automatic charge cancellation if a cell has voltage that is out of spec (when balance charging).
You can see from the above that you get the most built-in protection while charging in balance mode. For this reason, I would suggest charging in balance mode whenever possible.
Using “Charge” mode
Sometimes, you may want to charge your batteries using the “normal” charge mode – which is active when “CHARGE” is shown on the charger’s screen. When you charge using this mode, you need to be aware that you will be losing all of the safety measures built into the charger to prevent individual cells from being overcharged.
This can be safe if you are extra careful. You should purchase a cell monitor and verify your battery has no cell which measures a voltage of .2V greater than another cell. You should also be extra careful about selecting the correct cell count when charging.
When you want to charge many batteries as quickly as possible, the best solution is parallel charging. While we won’t instruct you on how this can be done in this article, we will discuss some safety concerns with parallel charging.
First of all, make sure your parallel charging board has a built in short circuit protection fuse. Some more expensive boards also have fuses for each battery on the board. These will prevent situations where batteries are accidentally, rapidly charged by other batteries plugged into the board with substantially different charge levels. We use this paraboard an can recommend it, but you may want to opt for more safety options.
Next, you need to make sure the batteries you plug into the paraboard are all of similar charge levels. If you stop flying a battery pack partway through your normal flight time, make sure that battery is marked so you do not plug it in with the rest of the batteries. If you do not use a battery and it is left fully charged, that battery should also be marked. Check out our article on battery XT60 covers for some interesting ideas on how to do that. When in doubt, you should get out your cell balance checker and verify.
If you use batteries with difference cell counts, you need to make absolutely, 100% sure that you never plug the different batteries into the paraboard concurrently.
We have a massive article dedicated to parallel charging here. Check this out before attempting to parallel charge your batteries!
If you practice good charging habits, you should never have a problem while charging batteries. Nevertheless, we are all human. You should have one final redundancy in case the worst happens – a safe charging area.
The first line of defense in your charging set-up is putting it somewhere where you can watch it. This is especially important if you charge your batteries indoors. Never leave your house unattended while a battery is charging. If the area where you are charging a battery is in any way flammable – you should not even leave the room. Either way, make sure the room you are charging in has a fire alarm.
How do you know if something is going wrong? LiPo batteries that are becoming overcharged or are damaged and getting ready to explode will expand in size first, called “puffing”. This process also emits a sweet smell. If you see a battery doing this, immediately unplug it and rush it outside. It should be left alone for a few hours in a safe place, then discharged and disposed by placing it in a salt water solution.
Putting your batteries inside of a fireproof container while they are charging is the next logical step to prevent or reduce damage in the case of a mishap. Whatever container you decide to use – make sure that you never seal batteries inside of an airtight container – when LiPo batteries “go-off” they release pressure and heat – perhaps enough to make a bomb if sealed up.
Metal ammo boxes, which can be purchased from army surplus shops, Amazon or eBay, are a favorite storage location for batteries. Just make sure you remove the seal from around the lid so it is not airtight, or drill some holes in it. You can construct a handy charging station with an amazon box by drilling holes to pass wires through at the bottom, then passing the charging leads through those holes.
Some manufacturers have come out with firesafe bags for charging batteries in. When these bags work, they contain any flames caused by an explosion so that your house doesn’t get lit on fire. Be aware, though, that these bags are not regulated and there are many which can be purchased from otherwise reputable vendors which will not stop a flame at all. We are planning on doing a detailed comparison test on these bags in the future to find out which ones work and which ones do not. Honestly I would have a tough time trusting any bag unless you have proof that it works.
If you’re feeling ambitious, you can stop by your local hardware supply store and buy a few bricks or cinderblocks and build a fireproof “house” for your LiPo batteries. This is my favorite method for charging at home – I know for a fact that even if several batteries erupt in flame, I’ll only be burning bricks and concrete.
While researching this topic, I found an amazing video that shows you what you can expect the worst case scenario to look like when using an Ammo Box or LiPo Bags:
Thanks to Rajucam Aerial Services for the video.
As you can see, with big batteries or multiple batteries, any containment will have large flames shooting out of any open hole. That being said, I would rather have the contained fire of an ammo box over the LiPo bags. Either way, this leads us into:
As you saw above, a LiPo charging fire likes to shoot flames in all directions. You should really try to charge your batteries in places where a flame like this will not catch your house on fire. If you have a garage or basement with concrete or brick walls, that’s a fantastic place. A bathtub without a shower curtain is another decent idea. Another excellent option is just on a driveway outside of your house.