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I have heard about batteries going into flames when left charging too long. And no doubt that can happen with a power supply that supplies just enough amps.

Is it possible for a battery to be badly designed such that it uses more amps than it needs? So, a power supply that can provide 2A to a device that needs 300mA, and the device mistakenly 'requests' all the amps it can get, and it gets all 2A. Or worse, the power supply can give 10A and it gets all that. And could it then ever go into flames from that? Is that possible?

So, if somebody has a cheap device like a cheap toy with a cheap battery in it, and they charge it from the mains..and the power supply can provide more amps than the thing needs. Are they more at risk if the toy or battery within the toy fails?

And if a supply can provide lots of amps like 10A, and a device only needs 1A, and there's a fault with the supply, is there more risk from a faulty supply rated at 10A?

Added- Assuming a good power supply, i.e. not faulty. And supplying a cheap faulty device. If the cheap device 'requests' more amps.. Could that happen? How likely is it? And in the event that it does, then would a good 1A supply be safer than a good 10A supply? Since, at least with the good 1A supply, the fuse would prevent 10A being supplied.

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People use fuses for limiting current under fault conditions. This makes things safe usually. –  Andy aka Apr 20 at 17:19
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The rating of the supply is what it was designed to supply. Good ones will turn off or blow a fuse. Cheap 1 amp supplies may happily try to supply 10 amps while its wires melt and it catches on fire. Fuses and other current limiting devices are the proper solution here. –  Grant Apr 20 at 17:28
    
can it try to supply more amps due to a fault in the device? as well as a fault in the supply? so, can either be a cause of it trying to supply more amps? a good supply that can do lots of amps is a danger then if device it is supplying is faulty? whereas a good supply that can do just 1A is safer than a good supply that can do 10A.. In the event the device is faulty? –  barlop Apr 20 at 18:07
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2 Answers 2

There are lots of variables at play, but in general, it's never a good idea to operate a small load from a huge supply unless the small load has safety features designed to protect against what a huge supply could do under abnormal conditions - appropriate fusing certainly comes to mind.

Imagine your load fails and presents a short-circuit to the source.

A 1A current-limited power supply will deliver some percentage above its 1A rating - most manufacturers target overload thresholds between 110 and 125 percent of nominal to allow some margin. So, your load can potentially suck 1.25A unless something interrupts the current.

Imagine now you have a 10A supply. That 1.25A is now potentially 12.5A, and now if your downstream device doesn't have some integral protection, you're in trouble.

Imagine you now have a power supply that doesn't current limit, but instead limits the power going to the load. The amps will increase but the volts will decrease. A 5V, 1A power supply could become a 1V, 5A supply, or a 0.5V, 10A supply. Now imagine a 5V 10A supply which behaves this way.

Also, fuses tend to work better when there's sufficient current available to clear them, and may not work at all if the source isn't stiff enough. A 10A supply should easily clear a 1A fuse under abnormals - a 1A supply would likely deliver current indefinitely into a short through a 1A fuse.

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Generally it depends on power (in watts) that power supply can deliver to device and kind of failure in device.

Small power adapter typically shutdown if is overloaded, or voltage drops on its output. That may significantly reduce fire risk in faulty device.

Large power adapter may deliver more power. More power means more temperature. 10A may melt insulation on thin wires etc.

If you have only big power adapter - use small fuse between charged device and power supply.

And one more thing - if you are charging "unsure", low quality devices - do it at some "fire safe area" - far from flammable clotches, books, animals and womens watering flowers :)

That applies to normal batteries without thermal protection too, charging in "fire safe area" is recommended by some battery and charger manufacturers, like Turnigy (RC equipment manufacturer).

How likely it happends? Not likely, but it just may happen.

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I hope he isn't hooking a LiPo to a bench PSU. That might lead to nice surprises. –  Vladimir Cravero Apr 20 at 20:16
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