Recently bought a piece of tech and it can only be charged via USB socket. Great if I'm at home AND the laptop is on, I can plug into that.

 

But am looking for options when out and about in the car, so an in USB socket, and also a USB wall charger. Ideally the wall charger needs to have a cable attached as the USB plug swings out from the item.

 

Can anyone suggest anything? Wall charger not urgent but would like to try and get in car one over the weekend, so a high street retailer would be good.

 

Cheers

There are plenty of USB wall chargers.  Most mobiles come with USB chargers.  You should be looking around £5-10.  Car chargers, B&Q do a dual charger for £4

 

Just match the wall charger or slightly exceed the mA rating of your devices battery otherwise it'll charge slowly.

I'm always intrigued with the mA current rating thing on USB chargers. USB is powered via the 2 outside connections on that 4 pin socket at 5v DC. The middle 2 connections are data and not needed for power or charging.

Therefore as long as a regulated 5v DC is on those pins the device can't charge any faster or slower. I can understand if you had an underrated PSU that could supply little current that the 5v would drop under load and the device would either charge slower, never reach full charge or not charge at all. But as long as the PSU was providing 5v when under load then it could do no more.

Even if the USB power supply (and I'm purposely not using the word charger here as really the device itself contains the constant current charging circuit) was capable of supplying millions of amps at 5v it makes no difference as the device itself takes from that 5v feed as it needs.

I seem to remember the USB spec is 500mA maximum load so any adapter should give that at the very least.

Batteries are measured in milliamp hours or Amp Hours for bigger ones like in cars. That is the amount the battery can give in 1 hour. So a 2000mAH battery could in theory give 2A for an hour or 1A for 2 hours etc. But in reality it would not be designed for such a high continuous current draw. That would damage the battery. Even a car battery would overheat its plates and they would warp if cranking current was continually drawn until the battery was dead.

NiCads used to charge as a rule 10% of their mAH rating so a 2AH battery would charge at 200mA for 10-14 hours. There are fast charging methods that up the current at the beginning, monitor battery temperature and voltage and drop the current to a trickle at the end.

All this charging circuitry as said is contained in the USB device, just give the device its required 5v and let it handle the charging itself.

I'm a electronic engineer btw.

Edited 13 July, 201312 yr by aubrey

I think chargers over 500ma conform to usb2 standards which can deliver up to 2500ma.

Part of the spec is that the data pins can be used to work out how much it can pull (500mA or more).

 

This can be done in a number of ways, but basically, if you make sure the charger can supply at least 1A, then your devices will charge at a reasonable speed. If they only want 500mA, they'll only take that, but if they can take more they will.

Interesting, I never knew about that inquiring for current limits. Thanks

 

Looking on the web Wikipedia has this :

 

Charging ports exist in two flavors: charging downstream ports (CDP), supporting data transfers as well, and dedicated charging ports (DCP), without data support. A portable device can recognize the type of USB port from the way the D+ and D- pins are connected. For example, on a dedicated charging port, the D+ and D- pins are shorted. With charging downstream ports, current passing through the thin ground wire may interfere with high-speed data signals. Therefore, current draw may not exceed 900 mA during high-speed data transfer. A dedicated charge port may have a rated current between 500 and 1,500 mA. There is no upper limit for the rated current of a charging downstream port, as long as the connector can handle the current (standard USB 2.0 A-connectors are rated at 1.5 A).

Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's iPodand iPhone chargers indicate the available current by voltages on the D- and D+ lines. When D+ = D- = 2 V, the device may pull up to 500 mA. When D+ = 2.0 V and D- = 2.8 V, the device may pull up to 1 A of current.^[54] When D+ = 2.8 V and D- = 2.0 V, the device may pull up to 2 A of current.^[55]

Dedicated charging ports can be found on USB power adapters that convert utility power or another power source (e.g., a car's electrical system) to run attached devices and battery packs. On a host (such as a laptop computer) with both standard and charging USB ports, the charging ports should be labeled as such.^[53]

 

So it looks like on just a USB Socket for a charger you link the 2 data connections (pins 2 and 3) and have a 5v source capable of 1.5A

If still looking, Staples have a dual usb charger by Cygnett for £14.

Head out to a local Texaco garage. On offer is a bullet charger that plugs into car socket ( somewhere around £2.99) ,and an Ipod charge lead ( around £4.99).