Parameters defining the charging rate

It depends on the output current rating of the USB source, and how it is configured.

A standard PC USB-2 port has a maximum current rating of 500mA.
Red USB-2 have higher current ratings, also USB-3 sockets are usually rated at higher currents.

Different cables can have a drastic effect, as the cheap / poorly made ones can have very high resistance in comparison to a good charging cable.

USB-C ones often have a charging power rating, though that will usually relate to the maximum USB-C output voltage - eg. a "60W" cable may be rated for 3A at 20V.

With USB-A power units, there are different combinations of connections or bias voltages that can indicate to the connected device what maximum current it can draw is.

When you check the resistance between the two ends, it evidently shows a zero resistance for all the cables.
Don't you think that it's common to have no resistance in between the ends for all cables? This is a doubt i have, since when i check the resistance, most of the wires show zero or beep for short.

Electroenthusiast said:

When you check the resistance between the two ends, it evidently shows a zero resistance for all the cables.
Don't you think that it's common to have no resistance in between the ends for all cables? This is a doubt i have, since when i check the resistance, most of the wires show zero or beep for short.

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If you're getting a reading of zero, then you're probably using the wrong range - it would normally be low, but very unlikely to read zero. The 'beep' doesn't mean zero either, it just means below a certain resistance - what resistance depends on your meter.

Thanks rjenkinsgb , Nigel Goodwin

But, how does such a resistance decrease the charging rate so drastically? I have seen that costlier USB cables don't do that. My question is even with such a low resistance, compared to to input impedance of the cell phone increase the charging time to an extent that it takes hours together to even move ahead to 5%.

Electroenthusiast said:

Thanks rjenkinsgb , Nigel Goodwin

But, how does such a resistance decrease the charging rate so drastically? I have seen that costlier USB cables don't do that. My question is even with such a low resistance, compared to to input impedance of the cell phone increase the charging time to an extent that it takes hours together to even move ahead to 5%.

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Many chargers have internal resistors, utilising the unused data pins, the phone measures this and sets the charging rate accordingly - essentially the value of that resistors tells the phone what the capability of the charger is.

If there's no resistors, or the lead doesn't connect those pins (many charging leads only connect two wires), then the phone assumes it's the worst possible charger, and charges accordingly.

Have a read here:

Electroenthusiast said:

But, how does such a resistance decrease the charging rate so drastically?

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The phone (or device) charge control circuit needs to be able to give out 4.2V to fully charge a single lithium cell.

The charge control circuit, that has to control both voltage and current, will itself cause some voltage drop to be able to actually operate and regulate the charge.

Under ideal conditions, there is just 0.8V for the charge circuit to work within, both measuring and controlling the current and voltage, assuming a perfect 5.0V high current supply and zero cable resistance.


The charge control circuit would typically try stepping up the charge current in stages, aiming for whatever limit the USB power source is configured for.

If the input voltage at the phone drops enough to effect charging at a particular current, it would step the current back down to a level that can be sustained without too much voltage drop.


Testing one of the very few (all old) USB A-B cables I have that will run such as a USB scope or audio interface, so lower than normal voltage drops, the power cores measure around 0.11 Ohms each; 0.22 Ohms total.

A brand new - and supposedly "high current" - A-B cable measures around 0.16 Ohms per core, 0.32 Ohms total. Those are both around 5mm diameter cables.

I don't have any typical thin unbranded phone cables to test, I've thrown them out and only have high current rated types..

However, that heavy 0.32 Ohm cable would cause a voltage drop of 160mV at 0.5A, 0.32V at 1A or 0.64V at 2A

With only the 0.8V available for the charge regulator, you can see how much even an apparently trivial resistance cause voltage drops that could prevent it from functioning, at anything other than low current.



You also have to trust that the supplier is honest and gives genuine ratings for the cables they sell:

This is an example of a really abysmal cable - a 3m A-B that did not work on anything.

The cable is marked as each core being 24AWG:



However, 24AWG copper wire has a resistance of around 85 milliohms per metre, so this should measure around 250 milliohms.

It actually measures just under TWO Ohms on each power core... Around 1.7 - 1.8 Ohms each...

I cut the plugs off as soon as I discovered it was useless, so it could not get confused for a usable cable. It looks like copper wire, but it certainly does not measure as it should - thinner and possibly plated aluminium??

Great! I know am understanding a little. And, also this makes me be careful while i buy USB cables next time. BTW, i am curious to know how the control circuit at the cell phone end will manage to step up and step down the current/voltage based on it's (intelligence) requirement.

The link posted by Nigel, gives me enough understanding at the charger end. So does the explanation from you, jen. Glad that i'm able to understand this much which many know but don't understand. Please can anyone elaborate with the circuitry at the battery end, which would vary the charging rate(?).

The 'battery end' doesn't vary the charging rate, the charging circuit (and software) in the phone does.The charger itself, via the lead and resistors, tells the phone what the current capability of the charger is - the software then controls the phones charger circuit for the correct voltage and current, and the safe charging of the battery.

Basically you charge via constant current (set according to the afore mentioned resistors) until the battery reaches it's terminal voltage (normally 4.2V), you then switch to constant voltage whilst monitoring the current - when the current has fallen to your target value, then it's considered fully charged, and charging is stopped.

It's all pretty straightforward with a switch-mode charger circuit, and the micro-controller in the phone measuring current and voltage, and adjusting it accordingly.

Nigel Goodwin , Can you explain the two tables in the above link (Minty Boost). I couldn't understand it after continued look at it.

Electroenthusiast said:

Nigel Goodwin , Can you explain the two tables in the above link (Minty Boost). I couldn't understand it after continued look at it.

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You just need to study it - as I just have

Think x/y from R4 and R5, each can have four options, and draw a line from each option to see where they cross - so R4 as float and R5 as >3V meets at 250mA.

BTW, can we also saythat a bulkier cable would have better charging capabilities, since resistance would be low.

Electroenthusiast said:

BTW, can we also saythat a bulkier cable would have better charging capabilities, since resistance would be low.

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Depends on the difference in size, and how much you would consider 'better'?.

This was because i found one which was bulkier was charging faster, while a China made leaner one seemed to take too much time.

In other words, the resistance decreases with girth of the wire. Agree that this is not enough to say anything is 'Better'.

Electroenthusiast said:

This was because i found one which was bulkier was charging faster, while a China made leaner one seemed to take too much time.

In other words, the resistance decreases with girth of the wire. Agree that this is not enough to say anything is 'Better'.

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You would need to make a large change in the wire to make any appreciable difference - and the existing wire will be rated to cope with the output current of the supply. So making the wire thicker will make no difference - if the phone wants 1A, it will take 1A, you can still make the wire as thick as you like, it will still only pass that same 1A.

A fatter cable does not necessarily mean thicker wires. It could just mean more insulation and/or fillers.

You'd need to cut it open and measure the conductor diameter, or measure the resistance (with milliOhm resolution) to know for sure.

I was few days ago into a electric vehicle dealer. And, the sales man there said this: "Fast Charging in anything will lessens the battery lifespan. Be it mobile battery, or Electric Vehicle Battery."

How does anyone explain this? Also, we are now in an era where every gadget or device comes with a fast charging adapter.

Electroenthusiast said:

"Fast Charging in anything will lessens the battery lifespan. Be it mobile battery, or Electric Vehicle Battery."

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That depends how you define each, and how well things are designed!

There is Fast charging vs. Trickle charging - so an hour or two "fast" vs. a 14 hour or overnight charge - or Fast charging by pushing things beyond the various parts manufacturers specifications or causing things to get unduly hot.

Electroenthusiast said:

I was few days ago into a electric vehicle dealer. And, the sales man there said this: "Fast Charging in anything will lessens the battery lifespan. Be it mobile battery, or Electric Vehicle Battery."

How does anyone explain this? Also, we are now in an era where every gadget or device comes with a fast charging adapter.

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There's an optimum charging rate for pretty well all types of battery, but particularly Li-Ion ones.

Fast charging not only reduces battery longevity, it also reduces the amount of charge you can squeeze in the battery - so while relatively 'gentle' charging can charge to 100% (or near enough) fast charging might only be able to reach 90% or less.

Fast charging is also much more dangerous, with more potential for catching on fire - which is probably the main reason for limiting how much charge you actually try and fit in the battery.

The Samsung phones catching fire fiasco was caused by trying to fast charge to too high a level, and was 'cured' by updating the software to limit how far they charged.

However, certainly for an electric vehicle, you NEED fast charging - and limiting battery life, and only charging to a percentage of capacity, are essential evils.

How can i control the charging rate of a phone? I mean, is it possible to switch between a fast charging and over night charging on a single AC to DC 5v adapter?

rjenkinsgb I know that lot of the devices designed these days have better capabilities when it comes to safeguarding itself while charging. However, as i earlier said how can i make sure that a adapter i use will be good enough for a particular phone, considering that it didn't come with the original make.

So Nigel Goodwin , do you mean to say that never charge your phone to the fullest when you are charging your phone? This also holds good for vehicles or any other battery for that matter. I heard that this was opposite for Magnesium ion batteries, which always required the battery to be charged to the fullest (memory effect). Correct me if i am wrong.