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Batteries

Various sizes/voltages of lipo batteries

Various sizes/voltages of lipo batteries

LiPo Battery Safety

Batteries. Batteries are a necessary evil of the RC community. While they provide absurd amounts of power for us to be able to fly hard and have a good deal of fun, with great power comes great responsibility. LiPo and LiHV batteries are capable of delivering over 100Amps at a time, and as a result, carry a ton of potential energy. While most of that energy is diverted into ESCs and motors to allow us to fly our models, we have to be careful with that power so as to not cause accidents.

First off, let’s just start with some general safety information.

LiPo Safety

1) Do not charge batteries unattended.

Batteries operate by running a chemical reaction. This high-energy chemical reaction is most active when the batteries are either being charged or discharged. As a result, it’s at these two times when there is most potential for a catastrophic incident. Most often, when they are being discharged, it’s when you’re out flying the model (of course there are exceptions), so this means that you’re watching it and paying attention to it. However, when charging the battery, it’s easy to forget to pay attention to the batteries. Always be sure to stay available to keep an eye on charging batteries.

A simple example of lipo charging gone horribly wrong during a livestream. Credit to EZ Drone.

2) Keep LiPos in a fire retardant bag or fire-safe storage container

While the vast majority of LiPo incidents occur during charge or discharge, it is possible that an unknown issue or slight break in the protective layers of the battery could cause the battery to combust while being stored. As a result, never leave your batteries in a place that could potentially cause damage to property — they should always be stored in an ammo box, LiPo bag, or inside of some cinderblocks where they cannot catch something else on fire.

LiPo charging in a fire-retardant bag.

LiPo charging in a fire-retardant bag.

3) Do not over-charge or over-draw LiPos

If you over-charge a battery, the potential for catastrophic failure increases dramatically. While most LiPo chargers available today protect against over-charging, it’s possible to accidentally use the wrong settings on the charger to push a higher voltage into the batteries than is recommended. This especially will cause dangerous LiPo fires.

Conversely, if you overdraw batteries, the chemical reaction inside the battery will also become unstable, seriously harming the lifetime of the battery, and potentially causing explosive results.

A friend of the author accidentally charged a 3S lipo to 4S voltage, and he was extremely lucky to have noticed the battery before it caused a larger issue.

4) Do not draw more amps simultaneously than the battery is capable of delivering

We’ll go into more details later with this when we talk about C ratings, but drawing more amps simultaneously than a battery is supposed to deliver will also reduce battery lifetime and destabilize the battery’s chemical reaction.

We don't want lipos to cause fires.

We don’t want lipos to cause fires.

LiPo break-in

When you first receive your LiPos, one step that you should take is to ‘break-in’ the batteries. At the beginning of their lifecycle, the lifetime voltage and resistance of the battery will benefit from being slowly brought up to pulling higher and higher Amps. While some argue that it’s probably not as important for small miniquad 3 and 4s batteries, it is especially important for larger packs. That being said, all batteries can benefit from at least some break in procedure.

This article espouses the following benefits of lipo break in:

  1. Increased overall pack life.
  2. Volts per pack/cell are generally higher during the course of your flight.
  3. Packs are less likely to puff due to abusing them as they’re broken in properly before ever being flown.

Personally, we don’t break in our lipo batteries to the same extent that the article’s author suggests, but we do make sure to fly the first few cycles on the battery very gently, aiming to never draw more than 20A across the four motors. After that, open the throttle and let ‘er fly!

Storage

It’s been stated before, and it will be stated again: store LiPos in a secure environment, where they can not cause collateral damage if an incident occurs. Since battery cell voltages are supposed to range from 4.2 to 2.7 volts, when the LiPo chemical reaction is most active (on either end of the spectrum), the proper voltage to store your batteries at is about 3.85 volts per cell. This is also, consequently, the voltage at which your battery will be delivered to you from the manufacturer, and is the safest voltage for traveling with your packs.

More Details: https://www.servocity.com/blog/?p=492

Internal Resistance Measurement

Hobby King Accucel 6 80w set to check internal resistance.

Hobby King Accucel 6 80w set to check internal resistance.

A useful way to gauge the health of your batteries, and determine when you should be considering retiring a battery is to check its internal resistance (IR). IR is a measurement in milliohms to see how much resistance there is against the current flow from the storage of the battery through to that which it is trying to deliver a current. You must provide a load to the battery to be able to check IR, so you cannot simply use an ohm checker on a multimeter.

That being said, many modern LiPo chargers have an IR measurement tool built in, which you can use to see the IR of your batteries.

The following article has an awesome chart against which you can make decisions about your packs:http://www.helifreak.com/showthread.php?t=233111

  • 0-5 milliohms per cell — brand new or like new
  • 5-10 milliohms per cell — flown less than 100 times
  • 10-20 milliohms per cell — well-loved battery that has been flown many many times
  • 20-25 milliohms per cell — probably should be retired, and probably won’t fly your machine for long

To check resistance, find the option on your charger for checking Internal Resistance, plug the battery into the balance lead and main power connector, and hit start! A few seconds later, and it should tell you the internal resistance of the battery.

Internal resistance measurements of the cells of a 4s lipo.

Internal resistance measurements of the cells of a 4s lipo.

What is a C-Rating?

C-rating is a measurement that is often surrounded by a little bit of confusion as to what it means (or at least what it’s supposed to mean, since apparently manufacturers aren’t always totally truthful about it). The “C” in C-rating stands for “Continuous current draw.” This is a measurement of the maximum amount of current that you can safely draw from the battery at any given time. To calculate what that current level is, simply multiply the mAh (miilliamphours) of the pack against the C rating, and you’ll have the maximum amount of power the machine can safely draw from the battery.

Thus, a 1300 mah battery with a 45C rating will produce

45 * 1300 = 58500 = 58.8A

For comparison, your traditional miniquad with 5” props on a 4S battery will draw about 25A per motor at full throttle, which totals to around 100A draw at max throttle burn.

But keep in mind that many batteries also have a “burst” C-rating, which is another measurement of maximum current draw, which you are generally allowed to draw for up to 10 seconds at a time from the battery safely. So most batteries come with two numbers on them: continuous current-burst current. An example would be 45-90C, where 45C is the standard max draw, and 90C being the burst rating.

Thus, a 1300mah battery with a 90C burst rating will produce

90 * 1300 = 11700 = 117A

The 117A draw is well above the 100A that our motors can draw, so this 45-90C battery should suit our needs well.continuously at mid-high throttle or so, and for 10 seconds at a time at high throttle.

At the same time as there are C ratings for discharging (flying), there are also C-ratings available for charging. Many batteries technically support between 4-8C charge rates, but it’s always a good idea to charge at as low of a C-rating as you can, while charging at a pace that will actually get you out to the field. Thus, we always aim to charge at 1C (which is always the same amount of mAh as the battery itself (i.e. charging a 1300 mAh pack at 1300mAh will be a 1C charge), but we don’t hesitate to charge a little bit faster when in in a rush (usually never more than 2.5C)

C Rating on a Tattu 1800 mah 4S

C Rating on a Tattu 1800 mah 4S

Conclusion

LiPo batteries are awesome little packages of tons of potential energy to hurtle around in the air at 85mph on a quadcopter. But, despite the fun that you can have with them, pilots need to be careful that they take careful steps to make sure that they are not abusing their batteries or potentially putting their property and themselves at serious risk. Having All the information will help immensely. Stay flying and fly safe!