How long can I leave a LiPo battery fully charged?

How long can you let a fully charged LIPO battery sit around before it gets bad? Why does a LiPo battery get bad at all?

Most people in the RC hobby know that you should not let your LIPO batteries sit around fully charged. LIPO batteries are very sensitive, they don’t want to be over-discharged, charged or discharged too quickly, they don’t want to sit around either fully discharged nor fully charged.

In this article I want to have a look at why that is and look at different phenomena you might know from your LiPo batteries and why they happen.

Basic primer

A LiPo battery consists of one or multiple single cells that are usually connected in series to each other. But they can also be connected in parallel or in series and parallel. In the quadcopter hobby you will usually only find those that are connected in series to each other. Like 4S oder 6S, meaning they have 4 or in the later case 6 cells connected in series to each other.

A single cell consists of an anode, cathode (the plus and minus connections on your battery) and a separation layer between them. Those layers are very thin (less than 100 micron each). The three layers are tightly wound around a core and are then put in a foil pouch usually made from deep-drawn aluminum foil. This foil is then laminated on three sides. After lamination the packed winding is dried in a vacuum under complete exclusion of moisture. Once dried, the electrolyte is injected and the cell is sealed. The electrolyte is what allows ions to flow between anode and cathode. At this point the cell is completely discharged and must be activated by initially charging it - this can expand the cell by up to 10%.

If you want to learn more about the chemical processes, I would highly recommend you go through this paper about LiPo batteries which explains everything in great detail.

Why does my LiPo battery smell sweet?

When your battery smells sweet, what you are smelling is the electrolyte - it simply has a sweet smell to it. This means that the seal of one or more cells is damaged. You should safely dispose of this battery and under no circumstances use it any longer.

Why do LiPo’s get puffy?

There are multiple reasons why a LiPo could get puffy, lets look at them in some more detail.

Wrong handling

If a LiPo battery is handled wrongly, for example, discharged above its C rating or over-discharged, the electrolyte inside of the cell starts to vaporize. This will happen over time even if you take good care of your batteries, but it will happen quicker when you handle them badly.

This process is called electrolyte decomposition. During electrolyte decomposition the electrolyte breaks down into lithium and oxygen and the oxygen is the main reason behind your battery swelling up. The oxygen is also what makes a puffy battery dangerous since it burns easily, and is hard to extinguish once it is burning, since it will self fuel with oxygen.

Electrolyte decomposition is also the reason for the internal resistance of the battery going up. High internal resistance is the reason for sag.

Contamination

Another reason for swelling is contamination during assembly of the cell. The contamination could be water or any other substance that might cause lithium oxidation in the cell.

A swollen battery does not automatically mean that it will burst into flames when you are charging it - but the risk is definitely higher. A swollen battery is one more reason to not charge unattended.

Depending on the amount of swelling I would recommend to safely dispose of a swollen battery, or at least use it for low discharge applications - but only in cases where it is not swollen too much.

Can I fix a puffy battery?

There are some articles and videos online that claim you can fix a swollen battery, but you simply can’t.

It might sound harsh, but LiPo’s are consumables in the rc hobby, you might break them in a crash, you might reach the cycle limit or they might get puffy. No matter what, at one point they will be dead - so is it worth risking a fire and burning down your house, just to get a couple more cycles out of an already dying battery? Take a look at this list of puffy LiPo’s gone bad.

HOW LONG?

After all of this preface I still did not answer the question of how long. In fact, the answer is very simple: as short as possible.

Letting a LiPo sit around fully charged causes stress inside the battery and the battery degrades over time and will even swell up. High temperature makes things even worse. A battery sitting at room temperature fully charged will loose about 20% of its capacity in a year. A battery sitting at 60 degrees Celsius will lose 40% of its capacity in just 3 months.

And all of this adds up, the more often you let your LiPo battery sit around fully charged the worse it will get. It is not something that will show after a couple of days of letting the battery sit around, but it will eventually add up.

Best practice is to discharge the batteries to storage voltage when you are not intending on using them in the next couple of days. Usually when I return from a session with fully charged batteries I will discharge them if I do not intend on using them the next day.

Safely disposing of LiPo batteries

To safely dispose of your LiPo batteries you should first completely discharge them. There are multiple ways to do so. Two of them I want to take a closer look at.

Discharging with a load

Probably the safest way is to discharge your LiPo with a load. A 12V, 20W halogen light bulb or tail light bulb will work perfectly fine for up to four cells. It will discharge slowly and after a couple of hours your battery will be empty.

Double check with a multi meter to be sure that your battery really is empty. For more than 4 cells you can simply add another light bulb in series.

If you want to get fancy you can also get a LiPo killer - this is a simple circuit that will discharge your LiPo battery for you, the great thing is, that you can also use it to discharge your batteries to storage voltage.

Discharging in salt water

Another way is to discharge in salt water. You simply fill a bucket with water, mix salt into it, throw your battery in and let it sit for a couple of days. This method is especially good when you have a couple of batteries to discharge at once. After you put the batteries into the salt water you should see bubbles coming from the battery terminals, that’s how you know it is discharging. Also with this method, double check with a multi meter that the batteries are empty.

Recycling

Once discharged - with whichever method you prefer - bring them to a battery recycling point. You can often find them in super-markets or electronic markets. Your local hobby shop might also take them back.

Youtube

Some interesting videos on the topic of LiPo manufacturing:

Sources

Also check out

Battery Specs

Battery Specs

Parallel Charging

Parallel Charging

Serial Charging

Serial Charging

Serial charging 2S

Serial charging 2S
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