Amps and Charging

rogerg (Roger) · #1 21-08-2010, 10:17 PM

Hi all,

Just want to check my understanding of amps with respect to charging electronic devices.

I have devices I need to charge, which have the following ratings on thier AC power supplies:

1) 5v 0.7A
2) 5v 3A
3) 5v 1A
4) 5v 0.7A

My understanding is that I can power all of these using a 5v 1A power pack, it just means the 0.7A ones will charge slightly faster and the 3A one will take 3 times as long to charge.

Is this correct or am I going to blow something up?

Do I need to find a charger that outputs at the lowest common denominator of 0.7A ?

Thanks,
Roger.

Sylvain (Jon) · #2 21-08-2010, 10:37 PM

You are correct, but you'll need more than 5v if you want to fully charge your devices. This depends on the true voltage of the devices when fully charged. Although 5v is indicated, most of the time, when fully charge the voltage will be something like 6v or so. So you might need a 7V charger (for example) to fully charge your batteries.

Imagine that your battery (what you wanna charge) is an empty bottle of water which measures say 10cm in height..
Next to this bottle of water you have a water tank that operates by gravity (no pump if you like) with the water dropping from the top. A pipe connects the top of the tank to the top of the bottle.
If the thank is 1m high, you'll fill your bottle to the top with no problems. The speed at which you fill the bottle depends on the flow of water (so it depends on the diameter of the pipe and the aperture of the opening of the bottle).
Now, if the tank has the same height as the bottle, there will be no "slope" and the water won't be able to fill the bottle.

The heights are voltages and flow are currents (amps). The aperture of the pipe is the amp of the source, the aperture of the bottle the amp of your battery.
You have to respect voltages, but amps only define charging speeds.
In practice it is slightly different as the voltage of the battery changes with its level of charge. An empty battery might have a voltage of 4v while a full will display 6V. So in practice, you'd be able to charge a bit the battery with a charger that has the nominal voltage (5v) of the battery.
Now, if you use a 5v charger that you bought in a shop, it is most likely that it will account for the need of charging a bit over 5v, so you don't need to worry about it. If you are doing it yourself, well then you need to know the max voltage your batteries are charged up to and then set the charger's voltage slightly higher.
As far as current go, 1amp will charge the 0.7A faster and 3A slower.

Hope it makes sense.

Zaps · #3 22-08-2010, 06:44 AM

Quote:

Originally Posted by rogerg

My understanding is that I can power all of these using a 5v 1A power pack, it just means the 0.7A ones will charge slightly faster and the 3A one will take 3 times as long to charge.

Is this correct or am I going to blow something up?

The devices will draw or consume whatever they need. You could supply them from a 100A charger with no problems (so long as it's supplying 5V), because they will still only draw the current they need from it. But the 3A device won't respond well at all if you only supply it with 1A, because it requires all three.

mswhin63 (Malcolm) · #4 22-08-2010, 09:03 AM

I am still trying to get my head around all this. When relating to charging the rating is in AH (Amp/Hour). Amps only is for supplying power to non battery equipment. So lets look at both. (They the similar)

If you are hoping to supply power to all of them at the same time then add them all up 6.4A and buy a 7-10A power supply. If they are all batteries to charge which is a weird voltage do the same except by a specific charger. PSU's and Chargers operate slightly differently.

I am not selling PSU's yet but can get a hold of some.

rogerg (Roger) · #5 22-08-2010, 11:27 AM

Quote:

Originally Posted by Phiber

... So you might need a 7V charger (for example) to fully charge your batteries....

This doesn't quite make sense to me considering I'm reading the 5v figure off the current power plugs which charge/power these devices. Surely if they are supplying the devices with 5v, supplying them with 6 or 7v would be dangerous?

The rest makes sense

Thanks,
Roger.

bojan · #6 22-08-2010, 11:41 AM

Guys,
lets not mix up the issues here.. Charging and supplying power are two very different things.
Roger, what exactly (what kind of devices - are they computers, rechargeable batteries.. what ?) you want to have connected to power supply and what are the ratings?

rogerg (Roger) · #7 22-08-2010, 12:30 PM

Quote:

Originally Posted by bojan

Guys,
lets not mix up the issues here.. Charging and supplying power are two very different things.
Roger, what exactly (what kind of devices - are they computers, rechargeable batteries.. what ?) you want to have connected to power supply and what are the ratings?

You could say Charging. But really in this case the ratings provided in my original post are those of the power packs that chage, and run while charging, the devices.

The devices are media players, mobile phones and HDD's that would ordinarily be charged and run while being powered by USB or their AC adaptors with ratings as specified. That is asside from one media player which can only be charged (and run from mains) by it's 3A power plug but is still 5v.

Edit: powered by USB or AC except one media player.

Roger.

Sylvain (Jon) · #8 22-08-2010, 12:41 PM

When your batteries are fully charged, it is likely that the true voltage - as you could measure it directly - is going to be slightly more than 5V even though it says 5V on the power plug. Therefore in order to charge the battery up to this value, you'll need a source witha voltage slightly higher (you need to have some "slope" between the top of the water tank and the top of the bottle of you want the water to fill the bottle).

You do not need to worry about that if you are using an "already made" (one that you bought in a shop) charger (source).

However if you are using your own source (with variable voltage and current values) then you'll need to think about this.

I guess you just bought a 5V-1A charger, so if this is the case, then it will work just fine, no need to worry about the rest

Sorry if it is a bit confusing!

Also as Zap said, the batteries will consume only what they need, so if you haven't bought a charger yet, you'll be better off getting a 5V/3A charger. The small batteries will consume only 0.7 and the 3A one will consume all the 3A.
The amps rating on a source only shows its maximum ability to deliver current. But it does not mean that it will always deliver this max value. This is not the case with voltage though. A 12V charger will always display this voltage no matter what battery you try to charge.

I have assumed that you want to charge one battery at a time! If you want to charge all batteries at the same time, then you'll need a source with more amps. (just think that you would add more water bottles next to the first one, each connected to the same pipe in parallel: some water will go in the first bottle, some in the second etc. There won't be much water left for the last bottle. You therefore need to increase the size of the pipe)

Hope it makes more sense.

Anyway, if you haven't got a charger yet, grab yourself a 5V/3A (or more As) otherwise if you have a 5V/1A it will work ok although not optimal.

Steffen · #9 22-08-2010, 12:52 PM

Quote:

Originally Posted by rogerg

The devices are media players, mobile phones and HDD's that would ordinarily be charged and run while being powered by USB or their AC adaptors with ratings as specified. That is asside from one media player which can only be charged (and run from mains) by it's 3A power plug but is still 5v.

In this case the plug packs only serve as power supplies. The charging electronics is built in to these devices.

You need to supply the correct voltage (too low and the device's voltage regulators may not function, too much and you might fry them). You also need to supply the minimum current these devices need. If the power supply is rated at a lower max output current than the device wants to draw you could fry the power supply, or the attached device may nor work/charge properly.

For one power supply to rule them all get a 5V 3A one.

Cheers
Steffen.

rogerg (Roger) · #10 22-08-2010, 12:57 PM

OK, thanks all, sorry for confusing things by not describing the devices - I thought I had simplified it

I can't find a 5v 3A adaptor that has adaptable plugs so might stick with one 5v 1A to power all bar the 5v 3A media player which I'll continue using it's own power supply for. Jaycar has a convenient 5v 1A with USB output which make sense for all devices except the one 3A device which I was hoping to just bundle in the same bucket so to speak.

Roger.

Steffen · #11 22-08-2010, 01:01 PM

Actually, if a device it designed to be run off USB power it shouldn't draw more than 500mA, which is the maximum a standard USB2 port delivers.

Cheers
Steffen.

rogerg (Roger) · #12 22-08-2010, 01:30 PM

Quote:

Originally Posted by Steffen

Actually, if a device it designed to be run off USB power it shouldn't draw more than 500mA, which is the maximum a standard USB2 port delivers.

Cheers
Steffen.

Yeap, bound to be the case for all devices except the 3A one which doesn't charge from USB, only charges from it's plug pack.

bojan · #13 22-08-2010, 01:45 PM

Chargers are designed to charge specific battery...
LiIon battery is totally different from Lead.. or NiCd. So are the chargers.
So, don't mix them!
The only common denominator is the source, and this is 240V AC line voltage.

On the other hand, if your charger is also power adaptor (and I am guessing this it the case, because you were talking about 5V/3A or whatever rating.. ), then you can mix them - because the charger is built in the device. However, the voltage must be as specified (5V..) and current rating must be adequate, more than specified.
However, it is best practice not to experiment too much here especially if you are not sure what you are doing.
That means, use only power adapter supplied with your device.