An interesting problem possibly a PICAXE soloution

[David HK]

In July 2023 my family purchased a Mercedes Benz EQA 350 4matic as the family car. We live in Hong Kong.

We then purchased from Tao Bao China, a charging cable which plugs into a domestic 220 Volts 13 Amp wall socket andf thence the car socket.

It all worked well until a week ago when my daughter and I noticed that charging had stopped. Investigations revealed that the 13 Amp fuse inside the 13 Amp plug had runaway with a heat load, melted the area around the fuse, and finally the fuse burnt itself out. In other words the fuse had done its job correctly to protect the circuit.

I excavated the burnt section of the plug fuse holder and it proved impossible to scrape and polish the fuse clips back to nice shiny metal capable of holding the fuse tightly.

The 13 Amp plug was therefore cut off the cable and a new 13 Amp plug has been installed. The advantage with this new plug is that the plug cover can be removed and the fuse clips can be examined for tightness and polished - whichever is necessary. Every two weeks I inspect the metal ends of the fuse itself to ensure that these are nice, clean, shiny pieces of metal - not tarnished with age and humidity effects.

The old and the new plugs pulled 11.18 Amps when charging and do become comfortably warm when charging, but not excessively so. The ambient temperature in the daytime is around 33 to 36 degrees Celsius, and averages around 33 degrees Celsius overnight. Quite warm!

It had crossed my mind that it may be possible to put a heat sensor on the outside of the plug to detect temperature above a certain level, and perhaps a PICAXE may be suitable for this purpose. The alarm should be audible, and there is no need for fancy stuff such as temperature recording, time, and so on.

Any ideas or comments from readers would be most welcome. (Perhaps a PICAXE is overkill).

David

 

[bpowell]

A PICAXE, a DS18B20, and the READTEMP command should get you exactly where you want to be!

 

[PhilHornby]

Some random thoughts/observations...

A "13A" 3 Pin socket is actually rated for a 10A continuous load. You can probably "get away" with up to 13A for long periods, so long as you follow your procedure and keep a close eye on the plug components. I would also ensure that there is nothing close to the mains socket that could catch fire! Most electrician types will tell you not to do any of this. (My personal experience is of a Pottery Kiln belonging to my 'other-half' - which represents a very similar load. )

The better Granny Chargers (as they are colloquially known) apparently have a temperature sensor, of the type you describe, as part of their pre-fitted plugs. It causes problems, when the unwary cut off the plug, with a view to extending the cable!

The other things to watch out for, are fake fuses! I recently bought a hot air gun from Amazon and thought I'd check what fuse had been fitted. I found a 13A fuse protecting a cable capable of 3A! Then I noticed the blurry writing on the fuse and decided to crack it open. They're apparently supposed to be filled with sand to contain any 'explosion', but this one wasn't :-( Just how much money did they save on the fuse ?!?

 

[PhilHornby]

A PICAXE, a DS18B20, and the READTEMP command should get you exactly where you want to be!

I'd be wary of a DS18B20 in this application - for some reason, they seem to be quite poor at surface-temperature sensing. (See my shenanigans with a DS18B20 and central heating radiator!)

 

[David HK]

Phil and bPowell thank you for the notes.

Yes, I know the 13 Amp plug is on the limit. I have studied the situation and followed the general public guidance notes issued by the Director of Electrical and Mechanical Services who is the titular head of the Hong Kong Government for anything electricl.

Its all electrcal theory accepted by engineers world wide. My situation is temporary pending the installation of a new power supply arrangement into my property by China Light & Power.

Other points and ideas mentioned above are noted. In my estate there are 5,024 residential homes and charging battery electrical vehicles with the method desribed above is quite common. Residents withTesla's and other makes with upgraded domestic power supplies charge at a higher amperage.

So far so good.


David

 

[AllyCat]

Hi,

A PICaxe (08M2) is probably not "Overkill" in itself, but the need for a DS18B20 and a low voltage power supply, etc. might be. Deriving the supply voltage from the 220 Vac mains needs some care (and a high voltage-rated resistor or capacitor), but instead, the PICaxe could sleep most of the time and use a small battery. Perhaps a 3 volt coin cell (e.g. CR2032) or a small 3.7 V lithium rechargeable cell could be salvaged from a "Disposable Vape" (but these are banned in some countries).

Many commercial temperature sensors still use a Thermistor (NTC resistor) which is sensitive and low-cost, but non-linear so needs a Lookup Table which can be tedious to develop. Alternatively, in most cases the self-heating of the PICaxe is negligible so it's possible to use the chip itself (perhaps glued directly to the thermal surface) but (IMHO) the READINTERNALTEMP command is completely useless. However, I did spend considerable time developing an alternative subroutine CHIPTEMP (the "universal" version in post #3 or #7 should be good enough) which can read the temperature within about two degees (C), but it MUST be calibrated (at one spot point) before it can be used.

Cheers, Alan.

 

[The bear]

Hi,
Good luck with your replacement 13 A plug.
I have always found MK (TradeMark) did a well made plug.
Bear.............

 

[Solar Mike]

As an electronics tech, I would not be messing around attempting to measure the plug for over heating, just get a new mains cable run to the main switch board and terminated in a proper rated 20 amp switched socket outlet, better get a 40amp circuit installed, then you have redundancy, and fit a mating plug to your lead. Overloading standard wall outlets is dangerous and can burn your house down, its just not worth the risk.

 

[David HK]

Bear and Solar Mike - thank you.

Solar Mike I know exactly what you are concerned about. All of your comments crossed my mind hence my own research. The car charging supply goes directly to the meter and the fuses adjacent to it. There is no way I would plug this into and existing in-house 13 Amp socket because I know nothing about the in-house wiring.

I have followed the Government recommendations for this type of circuit , wiring diameter, and intermediate fuse trips, on/off switch and its rating, and finally the 13 Amp socket (female). All wiring is in conduit and when China Light & Power and myself upgrade the incoming power supply later this year there will be some changes for example the wiring diameter.

In my residential street there are electrical engineers and technicians and we all talk to each other regarding aspects of battery electric vehicles - especially charging set ups. Typically the electric vehicles in the street are - Tesla, Mercedes Benz, Build Your Dreams, Ora, BMW and others I cannot revcall as I type.

Mercedes Benz also manufactures a specific wall mounted charger unit and my family are intending to purchase one in due course.

The thoughts expressed in this thread are an interim arrangement.

I hope this topic does not conflict with forum rules.

David

 

[PhilHornby]

... just get a new mains cable run to the main switch board and terminated in a proper rated 20 amp switched socket outlet, better get a 40amp circuit installed, then you have redundancy, and fit a mating plug to your lead. Overloading standard wall outlets is dangerous and can burn your house down, its just not worth the risk.

I'm not sure that actually addresses the problem though...

The 'Taobao Charging Cable' is an EVSE, and as such, should be advertising that the maximum current available is 10A (assuming that is what it is set to). The actual charger is part of the car and it should draw no more than the EVSE says is available.

So why then, is it routinely drawing 11.18A and occasionally drawing enough to blow a 13A fuse to bits? It's not so much that the wiring isn't "man enough for the job", it's more that it's being overloaded for some reason.

If you take the 'UK-style' 3 pin socket/plug and fuse out of the equation, then what is going to protect the 'flex' to the 'TAOBAO' and car in the event of another massive overload. The 20A MCB at the distribution board may not trip before the 'flex' is damaged...

I might be tempted to try and find a better 'Granny Charger' than the Tao Bao. Something like this maybe ? (I'm surprised the car didn't come with one)

 

[David HK]

Hello Phil,

I know the domestic voltage in Hong Kong is 220 volts. To workout Amps i use the Benz App on my smartphone which shows 2,600 Watts. The arithmetic comes to 11.82 Amps if I have fingered the keypad correctly.

The ambient temperature in Hong Kong is currently around 33 Celsius overnight and rises from this to anything around 36 Celsius during a sunny day. We also have the June rains which approach Noah's Ark requirements and are frequently in the 90% humidity range.

The Tao Bao cable we are using was/is a quick fix until later this year.

I do perform a palm of the hand test on the replacement plug top when charging and note a comfortably warm sensation, but nothing alarming.

It merits note that when the 13 Amp plug and fuse failed that the ambient temperature was around 36 Celsius, the weather cover door part of the 13 Amp housing was closed, and the whole thing, was perhaps a glorified green house. After the event I used a small electrical screw driver to excavate deep into the fuse part of the Tao Bao moulded plug and extracated the fuse. This was not a five minute job. After removing the fuse I dug deeper to expose the the usual clamps that grip each end of the fuse.

I was not impressed with these fuse clamps. The entire plug unit seems to be of moulded rubber and I wonder if the ambient heat tends to cause a softening of the moulding which in turn decreases the clamping pressure on the fuse itself. I take the point about MK plugs. The plug I have since fitted is easier to open and inspect and I also no longer close the 13 Amp weather cover door when charging - this does allow light winds to swirl around the assembly.

The Benz company does offer an in-house desgined wall mounted charger assembly which we did not purchase at HK$12,000 (thereabouts) at the time of purchase because connecting it to the box containing the China Light & Power meter and primary fuses actually sits in a box located in the neighbours area. Later in the year we will have a new China Light & Power incoming supply in our own propery area so this will dovetail into upgrading the charger and higher amperage charging. The existing Tao Bao charger cable will become an emergency spare.

Battery electric vehicles are a new learning curve especially on the charging side of things.

Thank you for taking an interest in this subject.

David

 

[The bear]

Hi David,
Non technical: The previous four days haven't managed to get above 14C . Today it's peaked at 17C, it was 13C this morning.
Hopefully we have had our monsoon season. I'm not complaining, we are used to it.
I don't fancy your temperatures. I guess you have the added bonus of the odd snake too.
Bear........