Motopower 3.1A dual usb, question about wiring

[bwosw]

Hello everybody,

I recently bought one of those motopower 3.1 amp dual USB kits and installed it in my car. I piggybacked it from an existing fuse, and protected it with a new 5 amp fuse.
Now I got 2 questions:
* Is 5 amp fuse to protect my battery too high? As the motopower can output (5v, 3.1A -> so roughly 15W). With the 5Amp fuse the input of the block can be a maximum of 5A*12V -> 60W. Isn't this too much? Would i not be better of protecting my battery with a lower rated fuse? In case something happens with the motopower, that it doesn't starts drawing 60W from my battery?

* What happens if I connect a really powerhungry dashcam that draws for example 2A to one of the USBs (I know 2 amp is not realistic, but this is a hypothetical example) and then I connect an iPad also capable of drawing 2A on the other USB, so for a total of 4A to the motopower block. What would happen then? The Motopower can only deliver 3.1A? Will my car burn down ?

Thanks for the replies!

 

[Nigel]

1. The fuse is to protect the wiring, so as long as all the wiring can carry 5 amps +, then the 5 amp fuse is fine, and it looks like the device has decent wires on it, so it is probably fine. The fact that it can only supply 15W to the camera is irrelevant, it is the 12 volt wiring that matters.

2. The voltage regulator in the hardwire kit device will have a current limiter built in, if you exceed the 3.1A capacity then the voltage will drop to limit the current or more likely power drawn. At that point the dashcam will probably shut down due to low voltage and your iPad will be able to take all the 15W of power, although it probably will choose to limit itself to 10W, which should leave enough for most dashcams, so there probably wont be a problem.

 

[bwosw]

Hello, thanks for your reply.
For question 2, I'm glad my car will not suddenly burn down

For your reply on question 1. I still don't quite understand it I think. Does it make sense to allow the circuit to provide upto 5 amps to this motopower? Because 5 amps, 12 volts is roughly 60 watts, and the motopower can only deliver +- 15 watts over the usb. Would it then not be smarter to only allow +- 20 watts to the motopower (so choosing a lower amp fuse), to give it some extra, as I assume there is some loss when converting from the 12v to 5v + the extra components inside this block you mentioned.
I am just thinking here about some catastrophic failure of the motopower, where it starts drawing all the current it can from my fuse tapped (5Amp) circuit.

 

[Nigel]

You could use a smaller fuse, but there is no need, all the fuse has to do is to blow before the wiring can melt, so if you have 7.5 amp wires, which I guess is what the kit is fitted with, then a 5A fuse is perfectly safe, as would a 1A and a 7.5A. If you fitted a 20A fuse, and the the cable rubbed against some sharp metal, which is often easy to find under car dashboards, and the insulation got worn through, then the wire would blow before the fuse, and typically the insulation would melt and turn into flaming molten plastic first.

It is important to get it right since car interiors are not fire retardant, if a fire starts then it will often spread to the entire car.

 

[jokiin]

they ship it with a 7.5A fuse so your 5A fuse is a safer choice anyway

 

[bwosw]

Hello Nigel thanks for the reply.
You said 'as would a 1A and a 7.5A '. A 1A fuse, wouldn't that be too low (1*12 -> 12W)? And the motopower can deliver 15W (+ it will need some extra juice to power the block).

I also eliminated the 7.5 amp in the motopower, there is not really any reason to double fuse it...

 

[Nigel]

Hello Nigel thanks for the reply.
You said 'as would a 1A and a 7.5A '. A 1A fuse, wouldn't that be too low (1*12 -> 12W)? And the motopower can deliver 15W (+ it will need some extra juice to power the block).

I also eliminated the 7.5 amp in the motopower, there is not really any reason to double fuse it...

I said that a 1A fuse would be perfectly safe, I didn't say that it wouldn't do its job and blow when you take more than 1A!

1A at 12V is more than enough for all dashcams I've encountered, if you add on an iPad then it would be close to the limit, probably fine as long as the engine is running and there is actually 13.8V, might blow with the engine off, depends how much power an iPad actually takes from a standard USB2 cable.

 

[bwosw]

Pretty clear, thanks!