View Full Version : Why aren't all USB chargers

Paul Pless
02-05-2017, 09:21 AM
'Fast chargers'???

02-05-2017, 09:49 AM
because it impacts battery life

02-05-2017, 10:07 AM
because it impacts battery life

As I recall, a Li-ion battery can be charged very quickly without damage.

However, one must be watchful that one does not charge to more than ~95% of capacity.

Also, one must not draw the charge down below ~10%.

With any luck, The Donnster will be along to show just how wrong I am about this, 'cause The Donnster, he actually knows.

Norman Bernstein
02-05-2017, 10:18 AM
USB chargers are actually small power supplies whose design is of the 'switcher' variety.... and they have an output current capability commensurate with the design. A 1 amp output charger is cheaper to build than a 2 amp charger.... we're talking pennies of difference here, by the way, because these things have paper thin profit margins.

Batteries have a certain 'acceptance rate', i.e., the amount of current they can 'accept' during a charge cycle. When the battery is low, the acceptance is at it's maximum, and the terminal voltage of the battery will be lower that it will be when fully charged. As the battery charges, the terminal voltage rises, and the charger will try to maintain the voltage at the specified level (in the case of USB, it's +5.0 volts). Virtually all chargers work on a 'CCCV' principle, that is, a constant current until the terminal voltage reaches +5.0 volts, followed by a constant voltage, which becomes the 'float' level.

Much depends on how the device being charged is designed. Since lithium-based batteries have a terminal voltage below +5.0 volts, the device (or battery pack) will commonly have some circuitry that limits the charge rate to a safe level. Therefore, if you've got a small device capable of accepting only a 1 amp (standard) charge rate, using a high power charger with 2 amp capability won't do you any additional good (unless it's badly designed, without any current limiting... and without protection circuitry!). Conversely, if your device or battery back is capable of accepting a 2 amp charge rate, but your charger is only capable of 1 amp, you'll simply recharge more slowly.

02-06-2017, 12:49 AM
Isn't a USB port simply a power source? The charging rate is determined by the circuitry in the device being charged.

If I'm wrong, Norman will tell me how.

Too Little Time
02-06-2017, 09:17 AM
'Fast chargers'???
Some legacy devices will not be charged by the fast chargers.

Norman Bernstein
02-06-2017, 09:32 AM
Isn't a USB port simply a power source? The charging rate is determined by the circuitry in the device being charged.

If I'm wrong, Norman will tell me how.

More or less. The original USB 2.0 spec called for a 1 amp 5 volt output, and the circuitry inside is designed to limit the current to 1 amp. Down the road, manufacturers came out with 2 amp chargers... but once again, circuitry in the charger does limit the current.

However, Li-Po or lithium ion batteries have a terminal voltage in the range of 3.1 to 4.2 volts... so the devices usually contain circuitry to implement the well-known 'CCCV' function: the charging current remains constant until the terminal voltage reaches a pre-determined setpoint, at which time, the current is throttled back so the battery doesn't overcharge. There are endless variations on this theme; some more sophisticated circuits can detect a very slight 'drop' in the terminal voltage when the battery is fully charged, and then drop the current to a bare trickle. There are many integrated circuits available that implement the charging function; for example, here's the description of one of them:

The bq2409x is a highly-integrated family of single cell Li-Ion and Li-Pol chargers. The charger can be used tocharge a battery, power a system or both. The charger has three phases of charging: Pre-charge to recover afully discharged battery, fast-charge constant current to supply the buck charge safely and voltage regulation tosafely reach full capacity. The charger is very flexible, allowing programming of the fast-charge current and Precharge/TerminationCurrent. This charger is designed to work with a USB connection or Adaptor (DC out). Thecharger also checks to see if a battery is present.The charger also comes with a full set of safety features: JEITA Temperature Standard, Over-Voltage Protection,DPM-IN, Safety Timers, and ISET short protection. All of these features and more are described in detail below.The charger is designed for a single power path from the input to the output to charge a single cell Li-Ion or LiPolbattery pack. Upon application of a 5VDC power source the ISET and OUT short checks are performed toassure a proper charge cycle.If the battery voltage is below the LOWV threshold, the battery is considered discharged and a preconditioningcycle begins. The amount of precharge current can be programmed using the PRE-TERM pin which programs apercent of fast charge current (10 to 100%) as the precharge current. This feature is useful when the system loadis connected across the battery “stealing” the battery current. The precharge current can be set higher to accountfor the system loading while allowing the battery to be properly conditioned. The PRE-TERM pin is a dualfunction pin which sets the precharge current level and the termination threshold level. The termination "currentthreshold" is always half of the precharge programmed current level.Once the battery voltage has charged to the VLOWV threshold, fast charge is initiated and the fast charge currentis applied. The fast charge constant current is programmed using the ISET pin. The constant current provides thebulk of the charge. Power dissipation in the IC is greatest in fast charge with a lower battery voltage. If the ICreaches 125°C the IC enters thermal regulation, slows the timer clock by half and reduce the charge current asneeded to keep the temperature from rising any further. Figure 14 shows the charging profile with thermalregulation. Typically under normal operating conditions, the IC’s junction temperature is less than 125°C andthermal regulation is not entered.Once the cell has charged to the regulation voltage the voltage loop takes control and holds the battery at theregulation voltage until the current tapers to the termination threshold. The termination can be disabled if desired.The CHG pin is low (LED on) during the first charge cycle only and turns off once the termination threshold isreached, regardless if termination, for charge current, is enabled or disabled.

As you can see, these things are amazingly sophisticated, with all sorts of protection functions to prevent overheating, explosion, and fire.

Some years back, I worked on a project for Analog Devices which was a multicell battery charger.... it used temperature sensors for each cell in a multicell pack, to detect any overheating. I came up with a clever (well, *I* thought it was clever) flexible circuit which used an array of temperature sensors, and could be folded in a myriad of ways so that a temperature sensor would be in contact with each battery cell.