# DC currentWatch

#1
If I have a battery of 12V.When the positive leg is connected to the ground (-) Will that drain all the power from the battery in say 30 minutes.If I connect an electric device (say a DC fan between the poles). On the device is writen, that the current is 0.1 A.Is that a maximum current that can go through?The device has a resistance of 12/0.1 Ohm?I know Ohm's law. How to apply it here
0
1 year ago
#2
(Original post by Teunis)
If I have a battery of 12V.When the positive leg is connected to the ground (-) Will that drain all the power from the battery in say 30 minutes.If I connect an electric device (say a DC fan between the poles). On the device is writen, that the current is 0.1 A.Is that a maximum current that can go through?The device has a resistance of 12/0.1 Ohm?I know Ohm's law. How to apply it here
More information is needed here, in particular the capacity of the battery normally stated in Ampere Hours (Ah).

If the battery -ve and positive terminals are shorted, then a very high current will flow and only be limited by the wire resistance. This is a fire hazard and burn risk as well as risking rupture to the battery casing. The shorting wire will get very hot and may even vaporize like a fuse wire.

A 12V car battery may have a typical capacity of 80Ah so will provide a current of 80 Amps for 1 hour. If the leads were shorted (dangerous and not to be attempted) then with a lead resistance of <0.1 ohms, the battery could fully discharge in under 30 to 40 minutes.

A smaller capacity battery like 6 x 1.5V AA cells in series, will provide 12V and have a capacity of typically 2500mAh. If the terminals were shorted with a resistance of 0.1 ohms, this would drain the battery in perhaps a minute or so.

A 12V fan rated at 0.1 Amps will run from a typical car battery for weeks.

The same fan run from AA cells may run for perhaps a day.
0
#3
Now I would like to be more specific.
The 12 V DC fan has 0.6 A written on it.
Is that the maximum current it can handle? In other words is the resistance of the
fan always the same.
This device only takes 0.6 A current. Can I also connect a device with a smaller current? say 0.06 A? When is the point reached that I blow up the device?
A second question:
And how about a charger (12 V DC 1.5 A) Will it blow up the fan?
0
1 year ago
#4
(Original post by Teunis)
The 12 V DC fan has 0.6 A written on it.
Is that the maximum current it can handle? In other words is the resistance of the
fan always the same.
The 0.6A is the manufacturers average rating for all the fans manufactured to that design when running at full speed and venting into free air. Because of manufacturing tolerances, any given fan picked at random may be slightly under or over that current draw but will usually be within 10% or so. The current draw will increase if the fan has to push or pull air via an air filter etc.

During start up, as the fan runs up to speed, the fan presents an inductive load to the battery which means the current rises from zero rapidly to a peak and as the motor achieves full speed, the current will settle to the average rating of that fan.

This device only takes 0.6 A current. Can I also connect a device with a smaller current? say 0.06 A? When is the point reached that I blow up the device?
The device will not blow up. If they are both rated as 12V and connected in parallel with the battery, they will both work simultaneously but the current draw will now be 0.6A + 0.06A = 0.66A total and the battery will discharge around 10% faster.

A second question:
And how about a charger (12 V DC 1.5 A) Will it blow up the fan?
I don't really understand what you want to do?

Battery chargers contain electronics to regulate the charging current to the battery which changes through the optimum charge cycle.

If you try to connect a fan to the battery whilst trying to charge the battery at the same time, the charger may try to keep supplying a constant charge current.

This is bad news for the battery as overcharging will cause the electrolyte of a car lead-acid battery to decompose and generate hydrogen gas. This is an explosion risk because hydrogen is odourless and colourless and a small spark could cause ignition - especially when connecting and disconnecting cables or even the fan slip rings.
0
#5
I don't really understand what you want to do?

Battery chargers contain electronics to regulate the charging current to the battery which changes through the optimum charge cycle.

Sorry I meant a charger from the wall outlet with specs 110 AC -> 12V DC 4.5A
Is the charger actually deliver a 4.5A current to the 0.6A fan?
0
1 year ago
#6
(Original post by Teunis)
I don't really understand what you want to do?

Battery chargers contain electronics to regulate the charging current to the battery which changes through the optimum charge cycle.

Sorry I meant a charger from the wall outlet with specs 110 AC -> 12V DC 4.5A
Is the charger actually deliver a 4.5A current to the 0.6A fan?
DO NOT DO THIS.

Using an electrical device for anything other than it's intended design purpose, is a potentially dangerous and even lethal mistake.

There are several different ways a battery charger can work one of which is constant current. This means the charger may attempt to force a high current through the fan by increasing the supply voltage.

If that happens, the fan can easily overheat and cause a fire risk or even mechanical failure.

If you attempt this, you may risk an accident and invalidate your buildings and contents insurance in the event of a fire. Personal liability insurance (mandatory for UK businesses) may also be invalidated if the action results with injury to an employee or a member of the public and you could face criminal negligence charges.

As I said, do not attempt this. I cannot advise you further.
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