Novice paranoia: USB device drain on 12v batteries?

[GTom]

But those 2.1 amps are at 5v so it will be closer to 1 amp per hour allowing for a little loss converting between 12v & 5v.

Still, I did manage to drain my car battery twice charging my tablet. True, that's an old ~40Ah FLA.

 

[Mistroma]

Still, I did manage to drain my car battery twice charging my tablet. True, that's an old ~40Ah FLA.

I suppose it depends on how you charged the tablet, size and age of the tablet and age/condition of car battery.

Something like an iPad Air would be expected to take about 5.6Ah from a 12V battery if charged using an inverter (12V DC > 230V AC > 5V DC).
It would be more efficient to use a 12V DC > 5V DC charger and might only use 4.4Ah.

Even using the inverter you'd only be taking about 14% out of a fully charged 40Ah car battery and an older or larger tablet might take around 18%.

Of course your car battery won't like deep discharges and wouldn't have anything like 40Ah if it's old and capacity would also be further reduced in cold weather. On the other hand, capacity increases with higher temperature and also with very low current draw, roundabouts and swings.

I wouldn't be too surprised to find that your tablet would continue to use a modest amount of additional power if left charging overnight. It does look likely that your car battery is getting on a bit if charging a tablet completely flattened the battery (I assume you meant it wouldn't start the car).

 

[Seastoke]

Golly, s'been awhile since I've checked in.

No worries about thread Hijacking, I'm very happy just to read and try and ingest as much knowledge as I can. Thanks VERY MUCH for the info about the cold's effect on the batteries, I tangentially knew that temperature had an effect on battery life but didn't even consider how dramatically it will affect ours. Luckily we're staying in a marina overwinter, so shore power and battery lights will keep most of the wear off of our main batteries.

In the meantime, I think I'll simply have to hunt down a multimeter and try and start applying some of this stuff, at least out of academic interest. Thus far we've been pretty good at staying frugal and keeping battery usage to a minimum, especially considering we've no demands regarding cooling/heating (yet at least, may consider that diesel heater in the dead cold of winter...), and even a few choicely placed AA powered lights have made our lighting needs pretty slim.

Cheers and thanks for humoring!
-D

Have you moved to Conwy by any chance. Eleg

 

[Darwining]

Have you moved to Conwy by any chance. Eleg

Why yes, we in fact just did as of a few days ago!

 

[Baggywrinkle]

Amps drawn from the battery are amps drawn from the battery whether you draw them at 5v or 12v. However, as has been said, that will be a minor blip on the total draw from living aboard.

I think you are getting confused with a 2A fuse blowing at 2 amps at any voltage less than the rated voltage.

The voltage rating, as marked on a fuse, indicates that the fuse can be relied upon to safely interrupt its rated short circuit current in a circuit where the voltage is equal to, or less than, its rated voltage. So a 32V 2A fuse will reliably blow at the rated Amps at all voltages up to 32V.

Power however is a function of voltage x current - and a different voltages, the same current delivers different amounts of power.

The USB charger steps down the 12V supply to 5v for the USB devices.

25W for example is 2,08A@12V but 5A@5V

As an example, a typical USB charger might have two USB outlets rated at 2,1A@5V .... (2 x 2.1A x 5V = 21W) ... the charger itself usually comes with a 3A fuse on the 12v leads.

Why a 3A fuse when the USB ports deliver 2,1A x 2?

... because 21W at 12V is only 1,75A (21W/12V=1.75A) .... safely below the 3A fuse rating.

That means when the 2 USB ports are on fast charge and delivering 2 x 2,1A @ 5V ... the device draws less than 2A from its 12V supply.

Note: These calculations ignore losses in the USB charger for simplicity.

 

[Graham376]

Why yes, we in fact just did as of a few days ago!

Whereabouts - marina, mooring or pontoon? Marinas are O.K. but be careful on pontoons and moorings, lots of things grow legs there. A few of the locals have sticky fingers and others travel down the A55 from the east.

 

[Seastoke]

Why yes, we in fact just did as of a few days ago!

I saw you arrive , I was smiling as one of you had your life jacket on all day , I am off Sea Star just below you give us a hello if you see me about , if you need any help give me a pm.