Friday, 1 July 2016

Battery Safety

One of the key aspects of the recent drone course I completed was battery safety, and this is for good reason.

Even though we take batteries, and in particular Lithium Polymer batteries, for granted is that we use them every day in mobile phones, cameras, toys, electric tools, my toothbrush, even cars. They "just work" and seem very safe. All of these are safely inside a device or in some proper hard case to use with some dedicated (or built in) charger which includes software and hardware to manage the battery and make sure it stays safe.

However, in the model and radio control hobbyist world, people often use much more "raw" LiPo batteries, which are simply wrapped in a foil or soft plastic. They are charged with simpler circuits and need a lot more care and attention.

In practice these devices carry a lot of energy, and that makes them dangerous. These "raw" batteries are much more so as it is very easy to charge or discharge them incorrectly and very easy to piece the wrapping or damage them. When damaged they can catch fire, and release that energy. The fire is hard to put out, but usually not long lasting. It can very easily cause secondary fires.

The battery safety aspect of the course covered several areas, and had some nice scary videos of LiPo fires. I did however find some of the points made a tad odd, and I checked the instructions that come with the DJI batteries, and they are interesting. The Phantom 3 batteries have instructions covering things we had on the course, like store with 30% to 50% charge. Though the Phantom 4 batteries have instructions saying 40-65% charge for storage.

On the course they also said these batteries self discharge, and will discharge to an unusable state where the battery cannot be charged. The instructions (Phantom 4) say they discharge to 65% automatically over 3 days and then will hibernate, and can be charged after they have hibernated to get them back.

Comparing the two battery instructions shows that they have improved the batteries a lot from Phantom 3 to Phantom 4, and that may be why the details on the course are somewhat confusing and perhaps simply out of date. Ultimately the course should say READ AND FOLLOW THE INSTRUCTIONS. And especially do not assume the instructions are always the same.

That said, the Phantom 4 instructions do have a troubling couple of points. They say, sensibly, "Make sure the batteries are fully charged before each flight", but also "Never transport a damaged battery or a battery with power level higher than 50%". Err, and if you are going to a field with no power, how do you manage - you have to transport fully charged batteries.

Another odd one for such an intelligent battery system is "DO NOT charge battery immediately after ight, because the battery temperature may be too high". Sensible, as I bet people do that all the time, except why the hell does the intelligent battery system not know it is too hot and simply not start charging until temperature is OK, that way you can "put on charge" right away.

There are good tips on ensuring battery contacts are taped, and better still, batteries are carried in fire-proof LiPo bags or ammo containers.

One thing to always remember with any battery, even in your phone, is that you must not charge or use if the battery is damaged. That is how fires start, and is very dangerous. A damaged battery is unsafe. When you are flying a small aircraft like the Phantom 4, you have to be especially careful if you do have a crash as that can easily cause damage to the battery - making it unsafe, or even starting a fire immediately.

So, take battery safety seriously!

10 comments:

  1. "Transport" in this case means things like air travel, bulk transshipment, etc: the sort of thing where the battery might experience changes in air pressure, bumping around and the like. If you're just going out in the field with a drone this applies no more than it does if you're taking your phone on holiday with you.

    The battery system will know the battery is too hot *if* it has a temperature sensor close enough to the battery to monitor it. This is obviously true of fitted batteries in things like tablets but I'm not sure if the battery circuit's sensors and the battery are necessarily close together on the Phantom (I know nothing about it, since I will learn to fly drones at around the same time as I learn to fly by flapping my arms).

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    1. It has separate section for air travel and says to fully discharge.

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  2. There are all sorts of circuitry the battery may or may not have in it. Some have a thermistor to detect the battery temperture. Some have a full gas gauge chip with a 1 wire serial protocol, the chip knows how much charge is in the battery and it's voltage and current flowing and temperture. But either of these require a third contact for the data from the battery pack.

    Some batteries have safety trip circuitry in the pack (CE marking mandates this, but it isn't always present) which turns off the pack if too much current is drawn or the battery gets very very hot. The charger needs to know how to untrip these with a very slow initial charge at a lower voltage.

    Different batteries can quote differnent guidance because LiPo tells you nothing about the chemistry. Many are Lithium Cobalt, but there are quite a few other options eg. Lithium FerroSulphate. This site is a good place to see all the options, though the site layou leaves something to be desired:

    http://batteryuniversity.com

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    1. The Phantom 4 has all the bells and whistles. I am pretty sure it reports each cell charge levels. I'll check next time I spin up the drone.

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  3. My P4 batteries refuse to charge if plugged in after flight, as they cool they can then charge

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    1. Which is what I would have expected. Thanks

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  4. With regard to the battery protection circuits a lot of cells (mobile batteries and some 18650's for example) have these. They is a widely used chip for this called the DC01. It has the following protection methods :-
    1 - Turns off current flowing to the battery if it is over voltage.
    2 - Turns off current flowing out of the battery if it is too discharged.
    3 - Turns off the output if too much current is taken. Normally around 2-3A
    4 - Turns off charging if too high a charge current is used.

    The discharge current limitation is based on the internal resistance of the switching MOSFET with 0.025R being a typical value and therefore it is difficult to use with equipment which is designed to have a high current draw so these often have more custom protection circuits.

    This chip does not do any temperature monitoring and this is always handled by completely separate circuitry.

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    1. Well, testing, as someone else says, seems to suggest no charging happens until it cools down anyway. The battery pack has separate per cell voltage checking, and has temperature reporting as well as in built logging of charges and discharges and flight time, so pretty comprehensive.

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