Current Draw

[aaron]

Hey guys, just confused (Pun not intended.....) with choosing a fuse.

I have a 12v lead acid sealed battery, it states it's 4.2AH, I must be missing something. If I wanted to pick up a fuse to protect the battery, do I just pick up a.. 12v fuse? 5Amp max? Do I have to short it with a multimeter and measure the current draw, what's a safe amperage fuse for it? Or is it best to measure how many amps are being drawn from it from the device using it, and then get one that's of similar amperage?

Just having trouble finding information about this exact problem.

[poodyp]

4.2AH is the storage capacity of the battery. Don't worry about that. What's the maximum current draw of your circuit?

[dr_p]

The battery CAPACITY is 4.2 Amphere-hour (Ah). It more or less means that if you draw 4.2Amperes it will last 1hour, and if you draw 2.1A it will last 2h However, on the battery you should see what the maximum current draw is (e.g. "max load 2A").

Generally, the fuse has to be rated just above of your PROTECTED DEVICE's maximum current (sinked or sourced).

Meaning that if you want to protect a circuit that draws (sinks) more or less 1A, you use a fuse "slightly" bigger than 1A, for instance 1.6A.

If you wish to protect the battery, than you have to figure out what is the maximum current that the battery can safely deliver (source). It's either written on the battery, or you can guess what it is. For instance let's take 2A as your battery's maximum (instantaneous) current rating. You'll need a "slightly-more-than" 2A fuse, for instance a 2.5A fuse. You will tend to choose a higher value here, because there might be high current surges on startup. However, a regular 2.5A fuse doesn't blow instantly at 2.6A. See .

All fuses have to be rated to the maximum working voltage, in your case 12V and that doesn't present a problem, as 5x20mm glass fuses are generally 250V compliant. Voltage rating is mostly about arcing and that tends to be a problem mostly at higher voltages, in the hundreds of volts area.

[aaron]

Thanks, that's exactly the information I was after

I've left the multimeter at another house so I won't know what the current draw is, but I can work it out and it should all be good

Thanks guys

[ve7xen]

Do I have to short it with a multimeter and measure the current draw, what's a safe amperage fuse for it?

Your actual question was answered, I just wanted to say DON'T DO THIS! Lead acid batteries are capable of 100s of amperes of current when shorted. You'll at best blow the fuse in your meter, at worst explode it in your hands.

[Christe4nM]

The battery CAPACITY is 4.2 Amphere-hour (Ah). It more or less means that if you draw 4.2Amperes it will last 1hour, and if you draw 2.1A it will last 2h However, on the battery you should see what the maximum current draw is (e.g. "max load 2A").

Generally, the fuse has to be rated just above of your PROTECTED DEVICE's maximum current (sinked or sourced).

Meaning that if you want to protect a circuit that draws (sinks) more or less 1A, you use a fuse "slightly" bigger than 1A, for instance 1.6A.

If you wish to protect the battery, than you have to figure out what is the maximum current that the battery can safely deliver (source). It's either written on the battery, or you can guess what it is. For instance let's take 2A as your battery's maximum (instantaneous) current rating. You'll need a "slightly-more-than" 2A fuse, for instance a 2.5A fuse. You will tend to choose a higher value here, because there might be high current surges on startup. However, a regular 2.5A fuse doesn't blow instantly at 2.6A. See .

All fuses have to be rated to the maximum working voltage, in your case 12V and that doesn't present a problem, as 5x20mm glass fuses are generally 250V compliant. Voltage rating is mostly about arcing and that tends to be a problem mostly at higher voltages, in the hundreds of volts area.

A fuse's current rating is specified at the maximum current at which it will NOT blow. A 1A fuse will surely conduct 1A fine. It will almost certainly not blow at 1.1A though. Or it might...: In a fuse datasheet there should be a diagram showing the current vs. duration at which the fuse will blow. A 1 amp fuse might even withstand a 10A surge, if only it has a very very short duration. Only the manufacturer's datasheet can answer that for certain.

So concluding, as I understand you generally don't have to choose a fuse above the expected maximum current. Be aware though of inrush current surges for example, since their magnitude in amps over time can or cannot result in a blown a fuse.

As far as protection goes; if you want to be certain your 'to be protected component/circuitry' is indeed protected, do not (only) rely on a fuse. Use other current limiting and protection devices (e.g. current limiting resistor, diodes conducting to voltage rails, etc)

[IanB]

Adding to what is said above, fuses are generally to protect the wiring between the source and the device being powered. So if your device happened to develop a short circuit a lot of current would flow through the wiring and this might cause smoke, fire or other damage. The fuse is intended to blow before this happens, but the other side of the coin is that the wiring must be sized to handle the rated fuse current without getting hot.

[Smokey]

To make it more confusing, there are a few different time constant fuses, two of which are fast acting and slow blow.  The fast acting ones are for cases where you want the circuit to open quickly when the current exceeds the rating.  These usually protect sensitive stuff.  Slow blow are for cases where there might be transient currents above the current rating, but you mainly care about the continuous current.  Slow blow will survive the short transients and only open if the continuous current is above the ratings.  They do degrade a little with each transient, so there are realistically only a certain number it can withstand depending on the amplitude and duration.  Just like everything else, when you get down into it even fuses have data sheets.

[aaron]

Thanks for the information, I'll try get a fact acting one I think.

And don't worry ve7xen, I won't do that unless I was absolutely sure of it first.