How to calculate fuse on the 12V saide of a USB power charger ?

Submariner Gold

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Vicovation MF3
Bought this 12V USB dual Charger to hardwired into the car
It says it Outputs 5V 3.1 Amps per port , specifically highlights, that both are rated at 6.2A total.

When it comes to calculating the fuse on my Piggy back fuse holder.

Does it work like
6.2A x 5 V = 31 Watts output.

Therefore 31 watts / 12 volts = 2.588 A so a 3A would be fine.
As this rear cigar fuse only comes on when the engine is running is technically the calculation
31 watts / 14.3 V = 2.167A

I happen to have a 4A Fuse and a 3A so thought that either would be Fine. But would prefer to use the lower i.e. 3A
Is that the way you work it out?
I assume the conversion from 12V to 5V would consume some power

Note the other part of the piggyback has been filled with the original specified 15A fuse.

In real terms it might power one battery with a max input of 2.1 A
And one dash cam 0.6A = total Amps 2.7A
so 5V x 2.7A = 13.5 watts.
So on the 12V side , 13.5 / 12 = 1.1A.

I am trying to reduce the Amps as much as possible because the piggyback will only go in the wrong way around, thanks to a physicaly high relay block. Thus the piggy back is in parralell, where I would have prefered it to be in series.
However as its an S Class Coupe it actually doesnt actually have a rear cigar lighter, so there should be no load on this circuit, other than the wiring loom. ( note fuse diagram says for S Class and S Class Coupe.)

I did think of lowering the std. fuse to 10A and putting a 3A in here, as thats lower than the 15A in total as per theFuse diagram.
But I just wonder if it does secretly power anything else? Nothing on the fusebox diagram uses No. 117.
(The front Cigar lighter, and the boot power supply lighter socket both have individual fuses)

Other reason I didn't down grade to original fuse, is the fuse holder next to it says its for the fuel pump, and that should be 30A but its actually 15A. So I wondered if undocumented for the Coupe they Loaded some of that load on this empty circuit.
 
I think you've got all that correct, but remember that when you start your engine the 12V will drop to 11V or maybe even 10V on a cold winter day, plus the conversion efficiency from 12V to 5V is probably 90 - 95% - ie you might loose 10% of the power, probably a bit less.

Don't know why your fuel pump would have a half value fuse, maybe a difference between diesel and petrol?

Remember that the fuse is there to protect the wires in the event of a short, it should be sized for the wires that are in use.
 
I think you've got all that correct, but remember that when you start your engine the 12V will drop to 11V or maybe even 10V on a cold winter day, plus the conversion efficiency from 12V to 5V is probably 90 - 95% - ie you might loose 10% of the power, probably a bit less.

Don't know why your fuel pump would have a half value fuse, maybe a difference between diesel and petrol?

Remember that the fuse is there to protect the wires in the event of a short, it should be sized for the wires that are in use.

Thanks
I will downrate the cigarlighter fuse to 10A (from 15A) and use a 3A in this new circuit tomorrow ; so at least the combined value is less in total, than the stock fuse value.

I am also going to labouriously check all the fuses in that particular fuse box, just to make sure someone didnt mix them up i.e. if there is a 15A f
Use in this ‘fuel pump’ slot, ooooopps could the correct one (a 30A) be in a 15A slot. (Thats my worry).

I think its only worth double checking all t
He fuses in this fuse box located “behind the rear seat’s central parcel shelf” , as the the others are not nearby i.e. one in each end of the dash board, and two up front in the engine bay. Hopefully safe to assume if they mixed it up it would only be within the same fuse box.

They didnt make a diesel S Class Coupe, so cant be that.
I was wondering if the tracker engineer tapped into this circuit and pulled the fuse for safety while he installed it, and then couldnt find it and just stuffed anytging back in there. (as there has been no evidence of issues with the fuel pump).

Or maybe an ECO downgraded the fuel pump fuse.
 
I think you've got all that correct, but remember that when you start your engine the 12V will drop to 11V or maybe even 10V on a cold winter day, plus the conversion efficiency from 12V to 5V is probably 90 - 95% - ie you might loose 10% of the power, probably a bit less.

Don't know why your fuel pump would have a half value fuse, maybe a difference between diesel and petrol?

Remember that the fuse is there to protect the wires in the event of a short, it should be sized for the wires that are in use.

I think you are right
Its pretty close to it.
I just realised my Fluke multimeter has a Clamp DC Amps setting.

At the USB end
4.91V to 5.2V unconnected.
The Dashcam read 0.37A to 0.45A
The totally flat iPad Air sucked 1.98A to 2.02A

And at the fusebox 12V side
It varied between 1.1A and 1.14A.
So having a 3A fuse I used that in both the top and bottom pigtail fuse sockets. Originally it had A 15A fuse for the rear cigar lighter ... which the S Class Coupe doesnt have one !
So pretty safe to assume no one can overload that standard circuit as there is no outlet, other than my new USB charger.
As this charger has two 3.1A outputs where one can deliver 6.2amps i.e. x 5V = 31 watts
So 31 watts divided by 14.3V engine running = 2.167A x 1.2 = 2.6A.
I upped it 20% as a magin of safety to account for the conversion inefficient from 12V to 5V ... guess its probably more like 10% loss so well covered.

In reality the most it cou,d use is say charging 2 iPads, so 5V 4 amps = 20 watts / 14.3V = 1.4A on the 12V side.


But she shares the same wiring loom as the S Class Saloon ... just that element are not connected
 
So 31 watts divided by 14.3V engine running = 2.167A x 1.2 = 2.6A.
It would be better to do the calculation with 10V, just in case you are sitting with the engine off but your headlights on and the windscreen wipers on and the electric window heater on. Your 31 watts is still the same amount of power but with less voltage the amps will be higher, closer to the fuse limit, and a dark cold wet night is not the time you want to find that you didn't leave quite enough margin!

I think you have left plenty of margin anyway.
 
It would be better to do the calculation with 10V, just in case you are sitting with the engine off but your headlights on and the windscreen wipers on and the electric window heater on. Your 31 watts is still the same amount of power but with less voltage the amps will be higher, closer to the fuse limit, and a dark cold wet night is not the time you want to find that you didn't leave quite enough margin!

I think you have left plenty of margin anyway.
Thanks Nigel,
A good point but, The rear cigar lighter circuit I tapped off, and the power socket in the boot, oddly only gets power when the engine is on.
Unlike the front one that works on ign. Pos 1. And even has a delayed shut down (linked to opening the door.)

So for me its either 0 Volts, or 14.3V with the engine on.
 
Thanks Nigel,
A good point but, The rear cigar lighter circuit I tapped off, and the power socket in the boot, oddly only gets power when the engine is on.
Unlike the front one that works on ign. Pos 1. And even has a delayed shut down (linked to opening the door.)

So for me its either 0 Volts, or 14.3V with the engine on.
I doubt that you always get 14.3V with the engine on.

Measure it at the end of a decent drive home with the headlights and windscreen heaters on.

If the battery is full then it would normally charge at 13.8V and with an empty battery possibly 13.0V, you normally only see 14.3 from a few seconds after starting the engine up to around 5 minutes after. For the first few seconds after starting the engine, the alternator is normally off, and if you have a stop-start system it may also turn off for a while when it decides the battery is full.
 
I doubt that you always get 14.3V with the engine on.

Measure it at the end of a decent drive home with the headlights and windscreen heaters on.

If the battery is full then it would normally charge at 13.8V and with an empty battery possibly 13.0V, you normally only see 14.3 from a few seconds after starting the engine up to around 5 minutes after. For the first few seconds after starting the engine, the alternator is normally off, and if you have a stop-start system it may also turn off for a while when it decides the battery is full.

Now thats an interesting point.
I had fully charged the battery (6 months old) with a CTEK ‘Start Stop’ (max 14.8V and 3.56A) maintaining charger, till it got to “Care Maintenance Mode” and then left it on overnight. In that mode it charges at 13.8V and runs down to say 120mA. That morning the Battery was at 13.2V ... which dropped to 13V quickly as it lost its residual charge spike.
So my tests were done with a completely full Battery .
I am therefore surprised it wasnt charging, as you say at 13.8V.
Unless it was recovering from having the doors open and door courtesy lights on. .?

I know the electrical system is working perfectly, because Mobilo spent 4 hours completely checking everything. ( starter, Charging system, Battery, Batteryncontrol module etc.]
Long story ... I got a new MB battery and it failed to start the next day.
Long story short ... I got a new but intermittantly defective battery :-0
Once replaced, it was and remained all perfect again.
 
The car will only charge at 14.3 for a short while as a rapid charge to replace what was lost starting the engine, it allows to switch off every time you stop at traffic lights, a normal long charge should be at 13.8 since it is better for the battery life.

The CTEK will charge to 14.8 since that will remove some of the "dirt" off the plates and bring the battery back to new condition, for a long battery life it is good to do this every 6 months.

A new and fully charged battery will sit at 13.0V if you disconnect it.
 
The start stop CTEK is good for AGM batteries, as it doesnt have their standard precharge “desulphation” cycle at 15.3V! Thst their others currently have.
They finally recognised the problem of loosening sulphation, that cant move away from the plates in a glass matt environment. This has caused barrier problems and premature aging according to Varta , who make my Mercedes Battery.
This one starts at 12.4V or whatever your battery is at , and ratchets up the volts to 14.8, as it gradually reduces the amps from its max of 3.56A .
As soon as it reaches 14.8V this starts to decrease, until it hits 13.8v and stays there, continually reducing the amps until its down to aprox 60mA.
So its a slowish gentle charge.

How much better this ... only time will tell. :)
 
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