How to charge a lipo battery safely

One of the most critical things you need to know when you get into FPV and drone racing is lipo battery safety.

Lipos are most vulnerable in two states: when they are being charged, and when they are being discharged. That’s why you must take care during both those times.

A quick primer on how lipos work

Lipo batteries are made of one or more cells hooked up in series. Each cell has a voltage range from 3.7 as a base voltage to 4.2 as fully charged.

Along with voltage, each battery has a milliamp-hour capacity and a C rating. The mAh is the capacity of the battery – how much current does it hold, and the C rating is an indication of how much current it can deliver.

When the cells are hooked up in series, the voltage will compound. So a 3 cell battery is 3.7 x 3 or 11.1 volts. However, even in the case of 11.1 volts, it is vital that each individual cell is at the same 3.7 volts.

 

 

 

Such a battery is considered to be balanced.

It is crucial to always make sure your batteries are balanced. You can do this by using a lipo checker or the battery meter function on your charger.

Similarly, when you charge a battery, each individual cell will be charged up from whatever voltage it is at(resting voltage is between 3.7-3.8 per cell) up to 4.2 volts per cell.

When you are using the battery or discharging it, the C rating and voltage come into play. The C rating is a measure of how many amps the battery can deliver.

To calculate the current-providing capacity of a lipo you can multiply the C rating by the mAh rating. So a battery with a C rating of 65C of capacity 1300 mAh(or 1.3 Ah) can theoretically provide 65 x 1.3 or 84.5 amps.

Batteries are usually rated with 2 C ratings – the first number(65 in the above case) is the constant rating, which is how much current the battery can regularly provide. The next number is the burst rating, which is how much current the battery can supply in short bursts.

Current will depend on voltage and what kind of motors and props you are running. When the motors spin props, they will draw amps from the batteries.

So if you know that your motors draw 15A each on full throttle on 3S(11.1 V), you must use a battery that is capable of delivering that much current – the battery in the example above would be fine, since 15 x 4 is just 60, and the battery can deliver 84.5 amps.

If you use a 4S pack, the motors will draw more current, since the props will spin faster. In this case, the motors may be drawing 20A each, so you now need 80A – so a 4S battery rated at 65C of 1.3 capacity would still be fine.

The reason I’m getting into this here is that if you try to pull more current from a battery than it is capable of delivering, the voltage will drop too much and the chemistry of the battery will get damaged.

The battery’s life will decrease and it may even puff up.

Charging safety

When you charge a lipo battery, try to keep it as far away from flammable materials as possible. Some people like to charge in their garages where the floor is concrete.

You can charge in a metal box which would be able to contain flames in case of a fire. Some people also put a bag of sand on top of the charging box so in case the battery does catch on fire, the fire will burn a hole in the bag, and the sand will fall on the flames and extinguish them.

As long as your batteries are not physically damaged and the cells are balanced, charging them is safe. But it is still a good idea to take precautions.

The most important precaution you can take is to not charge the batteries unattended. If something were to happen, you must be able to react quickly. The first thing to do is disconnect the power from the charger, then either throw the batteries outside where they can burn out or be extinguished safely.

Before you charge, check each cell of your battery to make sure it’s balanced with the rest of the pack.

You also want to inspect the battery for puffing or damage. If there is any, charge very slowly and be very watchful.

How to charge

I suggest you invest in a decent charger before trying to charge any lipo batteries. Usually computerized chargers have a bunch of modes you can use: Charge, Fast Charge, and Balance Charge.

I am not a huge fan of fast charge. The only methods I use are charge and balance charge, and I stick to balance charge most of the time.

When you select the balance charge option, you’ll have the options of choosing how many amps to charge at and the cell count of the battery.

Make sure you choose the correct cell count! Even though most chargers will warn you before you try to charge mismatched cells, if you somehow manage to start the charge cycle anyway, it will not end well.

As for amps, the Ah rating of the battery will help you decide how many amps to charge at. The amps you choose here is how fast current will flow into the battery.

Generally, you want to charge batteries on 1C, or the same as the mAh rating of the battery. So if you have a 1.3 Ah battery pack, charging it at 1.3 amps is considered charging at 1C.

Some batteries are rated to be charged faster(at 2C or even faster), but generally charging faster means you are risking shortening the life of the battery.

If you feel a battery is suspicious, charge at 0.1C – in the case of the 1.3 battery, that would be 0.1 or 0.2 amps. Better to be safe than sorry!

For most charging, you can stick to 1C and if you know what you are doing, bump it to 1.5C or 2C.

To charge a whole lot of packs very quickly, you can parallel charge.

When you’ve got the settings correct on your charger, you’ll have the main power lead plugged into the charger through the banana plugs and you also will connect the balance lead to the corresponding plug on the charger.

Then press and hold the start button and the charger will start charging.

Keep an eye on your batteries as you are charging to make sure they are not getting hot or puffy. If that happens, stop charging immediately, unplug the charger, and take the batteries out of the house someplace fireproof immediately!

Conclusion

Lipo charging is pretty safe as long as you follow the guidelines, use common sense, and don’t ask more of the batteries than they are capable of. Good lipos can last for hundreds of cycles before going bad, so there’s plenty of flying to be done!