Autononmous docking mechanism and others.

@dronebot-workshop 

Hi Bill,

  I hope this is never put to the test, and this is only an amateur impression from some reading a while back, so this is only plea to

Please check up with people who actually know what they are talking about. I take ABSOLUTELY NO responsibility for anything written here.

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My impression is that fire extinguishers can (sometimes?) be near useless when facing lithium-ion battery fires. And the toxicity of the gases emitted means that spending too long near such a fire (assuming it is in a 'confined space') might mean injury or worse, you don't make it out.

(I am assuming that unlike firemen tackling such a fire, you do not normally wear full breathing apparatus and protective clothing, so advice intended for firemen may not be applicable.)

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The general principles apply to any Li-ion cell, but obviously the bigger the battery, the bigger the potential hazard. One website I came across ( https://www.airseacontainers.com/blog/how-to-put-out-lithium-battery-fire/) suggested 100Whr as a boundary.

To attempt to put this in context:

A laptop battery might be a third to a half this value, but I don't know about your bots and other higher power projects. And bear in mind the authors of the article were thinking about a cargo ship environment, not a basement, so you might 'adjust' this boundary to your circumstances and risk adversion viewpoint.

Hopefully a single 18650 powering an ESP 32 is a much smaller risk, but basic precautions are still advisable.

A 'typical' 18650 might be specified as 3.6V 3Ahr, which is nominally about 11Whr.  So a battery with 9 of these would be about 100 Whr. By contrast, 5 x 3.6V = 18V, which might be a convenient starting point.

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I am assuming Li-ion and Li-Po batteries are essentially the same from this general viewpoint. Others may like to refine this approximation.

Also, Li-ion comes in a variety of 'flavours', some maybe 'safer' than others .. e.g. lithium iron phosphate (LiFePO4) may be less prone to thermal ruanaway than its more common cobalt based relatives. (e.g. abstract at https://iopscience.iop.org/article/10.1149/2.0311410jes)

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  The possible problems include:

1. Many common lithium-ion batteries have a thermal runaway characteristic .. that is once their temperature exceeds a critical value, they continue to get hotter regardless of the availability of atmospheric oxygen. The only way to stop the process, once started, is to make it colder.
2. When a cell exceeds a certain temperature it is liable to produce gases which are highly flammable and may also be very toxic. (e.g. carbon monoxide, hydrogen fluoride https://www.nature.com/articles/s41598-017-09784-z) A fire extinguisher may be effective in extinguishing fire resulting from the flammable gases but, unless it cools the cell, typically by drenching it in a cool liquid, like water, it will not affect the thermal runaway process. And of course the fire extinguisher will not (generally) reduce the toxicity of the gases.
3. A battery implies a number of cells, usually in close proximity. If one goes into thermal runaway, there maybe little to prevent the others following the same path soon afterwards.
4. A cell may overheat due to charging or discharging at too high a rate. Obviously external circuitry can protect external current flows, but an internal short can produce the same effect.

So, does any of this affect your thinking? A few suggestions to think about...some will be more appropriate with bigger batteries.

1. If your battery is made from a number of cells, like 18650s, consider spacing them apart so that failure of one does not immediately heat up the rest. Use heat resistant materials that will not melt at low temperatures. (This may only delay a worst-case process, but time to escape is also important.)
2. Include a simple fuse in wiring to protect against short circuit ... include electronic protection but do not rely on it as the only type.
3. Include cell load matching.
4. Monitor the temperature of each cell separately and switch off if any exceed a safety value.
5. Revise fire extinguishing equipment and policy. Look for a way of immersing/surrounding an overheating (but not yet dangerously on fire) battery in cooling water? Would a hose pipe be a useful precaution?
6. Be prepared to make a rapid retreat and ensure your exit route does not take you closer to the battery.
7. The 'thermal blanket' bags maybe helpful in some circumstances, but usually cooling the cell helps whilst surrounding with thermal insulation will increase the rate of temperature rise.
8. When buying cells (especially for a 'large' battery), can you buy 'safer'? e.g. lithium iron phosphate hold a bit less energy per cell (of same size), and are probably more difficult to find, but might be safer??

Sorry this looks a bit scary, and may contain errors on my part - so please treat as a list of things to check up on ... not a definitive message.

Best wishes and stay safe, Dave

@byron 

@dronebot-workshop @will

As I think @robotbuilder has mentioned in the past regarding the vacuum cleaner bot base he uses, it has a mechanism for getting itself recharged.

I use the base not the software or sensors. It was just a cheap way to get the basic requirements. Motorized wheels with encoders. Battery and charging circuit. The brain is a Mega with a L298 module to control the motors. And I can communicate between the Mega and a laptop via the usb port.

The creative part for me is the software. The expensive part is the hardware and I need people like Bill to keep me up to speed on the electronics and any associated software.

As my interest is vision I will probably use that to locate and dock with a charger.

@robotbuilder

Posted by: @robotbuilder

@frogandtoad

Had the same thought myself.

You can get wireless chargers for some mobile phones.
https://www.nutsvolts.com/magazine/article/august2013_Bates
https://www.jjplus.com/robotics-charging/

You could also use a light source to transfer energy to the batteries using solar cells.
Outside you can just "plug into the sun" to recharge your batteries.

Indeed... modern mobile phones do have induction charging... I like the idea and think it would be a good project to design one for yourself.  There are many different docking solutions... just look at how some jet fighters refuel mid air, or small craft dock with the space station, etc...

Thanks for the links.

Cheers.