View Full Version : Two phones use the same exact charger plug but different amperage

07-17-2007, 09:56 AM
Essentially this is the deal. If any of you got the new motorola phones you know they now are using the Mini-USB port as the charging port. I have a Motorola V325 and my work bought me a Motorola 8703e Blackberry. I also have the Bluetooth H500 earpeice for the blackberry. All three devices use the exact same mini-USB port. However, the blackberry's charger says on it 5V 0.75A while the V325 and the bluetooth earpiece charger both say 5V 550mA. Essentially I'm kind of scared that one day I might accidently plug the wrong charger into the wrong phone. I would think a company like motorola would make their phone chargers so that they can't be mismatched with each other.

I don't know much about how the phone charging devices work so essentially my question is if I plug my 5V 0.75A charger into my bluetooth will it damage it or will it just use enough Amperage (like a car battery ) to charge it. I found that if I plug the lower amperage charger into the blackberry for some reason it still charges.

Any comments.

07-17-2007, 12:06 PM
The current rating is simply the amount of current it is rated to source, but that doesn't mean any time you make a connection that it will be exactly that current coming through the wires. The phone will draw whatever current it needs to charge the batteries, so using the higher power charger shouldn't be a problem since it simply won't draw all of that power. I don't know for sure, but using the lower current charger to charge a phone which might need more than it can supply could potentially be a bad idea. In order to get the power it needs to charge the battery, it may increase the voltage due to insufficient current. I don't know if that will actually happen but it can with different types of circuits.

07-17-2007, 01:52 PM
For USB Battery Charging Specification, check out the following site:

In the document, it states that: "A dedicated charger is required to specify on its packaging or casing, the current at which the output voltage of the charger drops to 3.6V."

So if the Motorola charger shows 5V --- 550mA, then I think it means that the charger output voltage is typically 5V, but when the USB device starts drawing 550mA from the charger, it drops the charger voltage to 3.6V.

Additional details from the specification state that a dedicated USB battery charger can have a maximum current output of 1.5A. So if your BlackBerry still draws higher than 550mA, it can still do it without damaging the charger. But at that point, the USB charger's voltage is automatically lowered to about 3.6V.

07-17-2007, 04:49 PM
I really doubt there would be any problem using the wrong one, DEFFINITELY not with using the larger one for the small phone and I'd say its highly unlikely using the undersized one would be bad for the larger phone.

07-17-2007, 07:17 PM
Use only the charger with the highest current rating, throw the others in a drawer somewhere. Never worry again.

The charger needs to be rated for at least the amount of current that the device it's charging needs. Being rated higher is fine, being rated a bit lower is often OK but not desirable.

Mark R
07-18-2007, 03:05 AM
The charger is in the phone - the plug just provides power.

Worst that will happen is you plug the phone which needs more power into the weaker charger. Either the plug will get quite warm, or the charger in the phone won't be able to get the power it needs and will abort charging.

07-18-2007, 05:50 AM
...which could lead to Bad Things, but it's rather unlikely. At any rate, only using the biggest charger would definitely work:)

07-18-2007, 08:19 AM
From reading the other posts, I think many of you are confusing a traditional adapter use to, say, power a cordless phones. Those square like adapters weigh a little bit because by design they contain a step down transformer. On the casing of these traditional adapters, you will see the voltage and current ratings. However, for a USB charger, it follows a different standard.

See this image:
Background (http://bayimg.com/paeOoaabA)

The USB Battery Charging specification clearly defines the difference between a USB host or hub and a USB charger. Read thru the Charging Specification document in the link previously provided for more info. In that document, you will also find a schematic on how a portable USB device can determine whether it is connected to a dedicated charger or a host/hub.

See this image:
USB Dedicated Charger (http://bayimg.com/AaepCaaBA)

You'll also read about the current charging limit for this setup:
Current Charging Limit (http://bayimg.com/AAEPDAABa)

It specifically states that a portable device can draw more current than the dedicated charger output current (min 0.5A to max 1.5A). Doing so, will put the charger in current limiting mode, which will cause its voltage to drop down. The overall power dissipation is thus kept within acceptable levels despite.