I was looking into building a battery powered USB charger for camping. It occurred to me that car chargers are pre-built and run off 12 V. If I made a battery pack out of 8x 1.5 V batteries in series and wired them to the electronics in a car USB charger would that work as a simple cheap and effective solution?

Also can I calculate the charges from the pack by summing the mAh of the 8 batteries and dividing by the mAh of the unit to be charged or is the efficiency of the device non negligible?

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    \$\begingroup\$ Geesh, if you're going camping leave the phone, MP3 player, tablet, gameboy, and all that other nonsense home already! Are you going to have pizza delivered when you get hungry too? \$\endgroup\$ – Olin Lathrop May 2 '12 at 12:09
  • \$\begingroup\$ Not sure about the original author, but we go "Car Camping" to save the cost of a hotel room. With Smart phones and most RV parks having free WiFi having a way to charge a phone at night is useful. \$\endgroup\$ – John F. Miller Aug 10 '14 at 2:24

You can use existing commercial 12V to USB (= 12V to 5V effectively) converters or ANY "X" Volt to USB converter already available.

Some 12v-USB units use a linear regulator so have efficiency of 5V/12V ~= 40% :-(.
Many use some form of switching regulator.
These are typically around 80 - 85% efficient but more or less is also possible. I have seen some which use the venerable MC34063, and there are many other choices.
MC34063 datasheet

Energy wasted:

At 80% you waste 20% of the energy.
At 40% see above) you waste 60%.
So a not overly efficient smps wastes 3 x less energy than a linear regulator.

Power input

For 1 unit of power output (say 1 Watt)
A 40% efficient unit needs 1W/0.40 = 2.5 Watts.
An 80% efficient unit needs 1W/0.8 = 1.25 W.
So a linear regulator needs 2.5 / 1.25 = twice the wattage

Note that power needed and energy wasted are related but not in the same ratio between linear and smps supplies.

If you want to try do-it-yourself the venerable MC34063 is not as efficient as some modern ICs but is reasonably close and is cheap and very flexible.
DIY MC34063 examples at end.

Lead acid battery: If you have complete control over the design then a good choice can be a 6V sealed lad acid gel cell. These give good energy per $ and can last quite well if treated well.
Even with a linear regulator a 6V battery will give and efficiency of 5V/6V ~= 83%- so you can do better with a smps but not vastly so in many cases.
eg a 90% efficient smps uses 83/90 =!~ 8% less Wattage than a linear supply from 6V. As voltage drops under 6V the linear regulator efficiency increases.
To use a 6V battery for 5V output you need a low low low dropout regulator. This can be achieved with a P Channel MOSFET and one opamp section.

Lead Acid discharge voltage: The curves below are indicative only of voltages expected from a lead acid battery. These were for one brand of large deep discharge cells and small gel cells will be different BUT it gives an idea of what can be expected. If you use a 6V 7Ah battery then if the USB output is 1A (= charge a 1000 mAh cellphone battery at max allowed rate. then the peak discharge rate of 1A = 1A/7 Ah = C/7.
End point voltage for a 6V battery would be ~= 5.25V = ample.
If you charge EyeFones or EyePadz with battery capacities of about 1600 mAh or even 3000 mAh then you may ant 1.6A and 3A peak. Even 3A is 3A/7Ah ~= C/2.5.
At C/2.5 battery voltage would be about 10.5V at about 70% of capacity expended - still very useful and it helps lifetime to not discharge deeply.
At 3Ah from a 7Ah battery you could notionally charge 2 x 3Ah batteries. Or maybe 5 or 6 x 1000 mAh cellphone batteries.

For lead acid, battery brand matters. Yuasa are excellent and actually have several grades aimed at different uses and price ranges and good data is available.
Panasonic are good. Just about anything made by Panasonic is good :-) (I have zero involvement with them). Some others are good but cheap batteries may be very bad. Or not.

enter image description here

MC34063: Datasheet & application guide {again}

Example below is 25 V in, 5V 500 mA out. Would take 12V in as is except halve Rsc and ensure inductor suitable core size. Efficiency of ~84% may be slightly lower at 12V in.
Count the external parts!.
While there may be a very few more than in some minimalist designs the same IC can do Boost, buck boost and anything else wanted.

PCB layouts - see data sheet for boost and inverting circuits.

enter image description here

enter image description here

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  • \$\begingroup\$ Thank you. I would give you an upvote but I'm new :( You have confirmed my suspicion that a cigarette lighter USB charger would not be efficient. I know next to nothing about electronics though so most of this went over my head. Just to confirm: soldering 8 C cell batteries in series to a cigarette lighter USB charger will work? \$\endgroup\$ – Lunix May 2 '12 at 15:24
  • \$\begingroup\$ @Lunix - I did not say that NO igarette lighter chargers are bad - jus that som are: ie "Some 12v-USB units use a linear regulator so have efficiency of 5V/12V ~= 40% :-(. Many use some form of switching regulator." \$\endgroup\$ – Russell McMahon May 2 '12 at 16:17
  • \$\begingroup\$ Lead-acid is the heaviest possible option for carrying in a backpack. \$\endgroup\$ – Vorac Sep 19 '13 at 14:20
  • \$\begingroup\$ @Vorac - You can get heavier batteries if you wish :-) (NiFe, ...) but lead acid is probably tolerable. As he said camping and not hiking (or tramping or backpacking) I'd assume weight and size are important but not crucial. An eg 6V 7Ah battery would not be an utter imposition unless feather-eight hiking, and at about 40 Watt hours energy capacity would be immensely useful. A 40 Wh battery buck converted to 5V can be highly efficient. Even with a linear regulator wort case efficiency for 5V out at 6V in = 5/6 = 83%. So 40Wh gives about say 30 Wh into 5V USB or 6Ah = 3+ x iPhone charge. \$\endgroup\$ – Russell McMahon Sep 22 '13 at 9:36
  • \$\begingroup\$ I just want to make it explicitly clear that Lead-acid -> cheep, safe. Lithium-polymer -> light. Also this site. Other than that, I find your answer excellent and very practical. \$\endgroup\$ – Vorac Sep 23 '13 at 8:56

You can make USB charges out of many different types of batteries. You can use voltages higher than USB voltage and then regulate them down to USB level. You can even use voltages lower than 5 volts and use a DC to DC boost converter to raise it to a usable level.

To calculate how long your device can charge something you need to find the current consumption of the device and the capacity of the battery. Most batteries are going to have a mAH rating (Milliamp Hour). If you have a device that uses 500mA (the most current a standard USB port can output) and a battery that has a capacity of 1000mAH you can charge this device for around 2 hours.

This kind of device is a popular DIY project and plenty of documentation exists. For a really famous variant check out Adafruit's Minty Boost. It comes with full documentation and parts list.

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  • \$\begingroup\$ Btw, if you're using batteries, make sure to use a switching regulator if you don't want nearly 50% of your batteries' power to be wasted by heat \$\endgroup\$ – Earlz May 2 '12 at 7:00
  • \$\begingroup\$ Wont the electronics from the cigarette lighter USB charger eliminate the need for the more involved DIY portion which I feel I am not quite up to yet as I just learned to solder? \$\endgroup\$ – Lunix May 2 '12 at 15:16

If you're car camping, I would recommend getting a solar panel to float charge your car battery.

If you were talking about alkaline C batteries, I would recommend 10 NiMH AA batteries instead. Lighter, about the same capacity & operates at a relatively steady 1.2V. Voltage range of 12-14V (NiMH) vs 10-12V (alkaline).

Nowadays you can also look into LiIon usb chargers that are lighter than NiMH. Most can be charged with a solar panel too!

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  • \$\begingroup\$ +1 for the important research start points :"float charge", "LiIon usb chargers are lighter". \$\endgroup\$ – Vorac Sep 23 '13 at 8:59

Turns out that sealed, weatherproof 12-24V to 5V DC-DC buck regulators are pretty cheap these days and can handle enough current to power 5 2A USB outlets. Neat!

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  • \$\begingroup\$ Yup. Amazon and Ebay are good options for the US. For Europe I would look into the chinese sites with free delivery - e.g. dx.com. \$\endgroup\$ – Vorac Sep 23 '13 at 9:01

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