usb fast charger question

Lenovo: A10-70F

Not the cable. The voltage is negotiated between the device and the charger. Before that, you only have "1 Load unit" available.

USB power delivery section, too.

There is very limited voltage range to negotiate anyway.

The 3A rating is mainly to protect the cable and connector, so that you don't drive 3A through a 2A device.

As for the battery, nothing wrong with 1.5C charging, although 1C is the optimized rate.

I am not following. How does the charging block know what voltage and current is allowed? Is the charging block a constant current device that clips at 5V?

if it is regular USB it is just 5V an the device will draw as much current as it needs up to the 2A USB charging limit

if it is USB-C the device can ask for higher voltages and will be told how much current it is allowed to draw

The charging current is limited by the internal resistance of the battery. So, it's OK to hook up a 2A device to a 3A charger.

On Wednesday, 17 November 2021 at 17:18:47 UTC-8, T wrote: ...

With USB-C there there is a wire in the cable that is used to communicate between the power supply and the device. They negotiate what is the optimum voltage and current to use that is within both of their capabilities.

The power supply then provides that voltage and the device manages the charing current into its battery to not overload the power supply or overcharge the battery.

With the older USB there was a protocol for a device to negotiate up from 100mA to the maximum 500mA but that didn't suit charging so there were various other approaches either using resistors at the power supply end or a short between the two data lines that would give the device the information about how much power was available.

Again, the device would then manage the battery charging.

kw

On Wednesday, 17 November 2021 at 18:11:48 UTC-8, Ed Lee wrote: ...

That is wrong.

the charger within the device will manage the charging current to not exceed the capability of the power supply.

kw

On Wednesday, 17 November 2021 at 18:46:47 UTC-8, ke...@... wrote: ...

Excellent explanation! Thank you!

The charger limits current to 3A. The device might not draw more than 2A due to internal resistance. A smart phone would likely have current limiting circuit anyway.

On Wednesday, 17 November 2021 at 18:55:44 UTC-8, Ed Lee wrote: ..

I would expect that all devices with lithium-ion batteries will actively manage the charging current. It won't be limited by "internal resistance".

Strictly speaking the charger is within the device, the unit that plugs into the wall is a power supply.

For USB-C devices it will function as I described where there device will determine the maximum current that is allowed and not go over that limit. For simpler USB 1 or 2 devices there are simpler, less capable arrangements but in all cases the charger within the phone, tablet etc will control the current into the battery. It is not just plain "current limiting" either it will usually follow a charging profile.

kw

I am pumping 2A into a 2000mAhr cell at 4.3V when connected, and 3.8V open circuit. The 0.5V difference is limited by the internal resistance. When the cell is near empty (2.5V), it's likely 3A to 5A. But it's quickly charging up to 3.7V. The only thing limiting my charging rate is the internal resistance.

I can charge from 20% to 60% in 15 minutes.

Langwadt Christensen snipped-for-privacy@fonz.dk wrote in snipped-for-privacy@googlegroups.com:

Old USB is limited to 500mA and does not communicate.

en.wikipedia.org wiki USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and ... USB hardware · USB 3.0 · USB-C · USB4 Max. voltage: 5.25 V (USB 1.0 to 3.1 pre 2014); 5.50 V (USB 2.0 and later, post 2014); 6.50 V (USB BC 1.1); 6.00 V (USB BC 1.2); ... Max. current: 0.5 A (USB 2.0); 0.9 A (USB 3.0); 1.5 A (USB BC 1.2); 3.0 A (USB-C); 5.0 A (USB PD) Cable: 4 wires plus shield; 9 wires plus shield (SuperSpeed) Max. devices: 127

This is pretty much how level 2 EVSE work. The car shows it's presence and the power supply indicates how much current it can provide via a PWM signal. The car then proceeds to draw as much current as it needs up to the EVSE limit as indicated. I think the different systems work very similarly so it is not hard to convert an input to a different type of output. J1772 is directly compatible with Tesla, for example, other than the connector body. In this case there is no setting of the voltage. The car has to work with the voltage provided or hang up the phone. I know in the US this voltage can vary between a nominal 208 VAC to 240 VAC and wider as real examples.

WRONG! The device being charged is responsible for supplying the right voltage to the battery by either a series dropper or a switching converter. As has been described in this forum many times, typical charging profiles use an initial constant current until reaching some voltage level followed by a constant voltage charge with the current tapering off. In the case of lead-acid batteries once the current drops to some level the voltage is lowered a bit supplying a "float" charge at a low, continuous current without harming the battery. Lithium cells aren't provided with a float current.

That is correct, but not for the reason you gave.

Whatever the voltage is supplied, it's still limited by the internal resistance. At 60% SOC, most of the cells i tested have around 1/4 ohms at 4.3V voltage source. My voltage source supplies a constant voltage of 4.3V, but the cell won't draw more than 2A.

I am using constant voltage source of 4.3V with up to 5A. My charging profile is:

20% to 30%, 5A in 2 minutes 30% to 40%, 5A in 2 minutes 40% to 50%, 3A in 7 minutes 50% to 60%, 2A in 15 minutes

I am only using 40% (20% to 60% SOC) of the capabilities, but that's OK.

I am hooking up a 2A device to a 5A charger, no other limitations except with internal resistance.

On Thursday, 18 November 2021 at 06:53:33 UTC-8, Ed Lee wrote: ...

But that's not the way the chargers work in devices such as tablets, phones, and cars.

They do as Rick says, constant current controlled charging until the target voltage is reached then they switch to constant voltage. By then the current has dropped to a fairly low value, maybe 0.5C. They are never in a condition that the battery resistance limits the current.

kw

Not for slow chargers. But even DC fast chargers for car are limited by internal resistance. Beyond 60%, you can't pump any more reasonable voltage into the battery. Charging rate is dictated by internal resistance.

Because the effective internal resistance is so high. Current has to be lowered.

Yes it is, when SOC is over 60%.