Efficient festival phonecharger using a DC/DC circuit

Why build a phone charger

chargingCharging with a laptop battery

When going to a festival, you obviously have to bring your phone in order to find your friend. Also, more and more festivals make specialised festival-apps making it a bare nesessity.

Modern day smartphones do, however, need charging every second day so one needs to find a clever way of keeping the phone topped up. Either you can pay for it, or you can make your own solution. I chosed the latter, and found a pretty highly effiecient way of doing so.

Economic justification

roskilde-festival-logo-02These guys charge you WAY too much

During the latter years, festivals have found that charging your phone is a convenient way of robbing you of your money.

For instance the Roskilde Festival charges *20 DKR* to charge it for you.

This is an outrageous price since the powercompanys price for 1kWh (1000 watts in 1 hour) is at max 2,5 DKR.

Now, a modern smartphone will charge with a maximum of 0,03 kW per hour. (including vampire power from charger and charging circuit ineffency).

One might explain the blatant overprice as the cost of the employees, but at a festival people work for free and only get a ticket in return. As a ticket  to the festvial cost around 1800 DKK and you have to work for 32 hours, the wages are around 57 DKK per hour. Ofcourse these employees get a little food and beer as well, so lets set it at 100DKK/h. Also the charging station has to be build, but the price is still WAY to high!!!

If we are counting on at 50% of the 70.000 guests will charge their phones every second day, we will get an income of 140.000 DKK per DAY.

That is in my opinion an outrage. To charge so much for a service that could basically be free if the festival desided to.

...In the festivals defence, all earnings go to charity. I am very much pro-charity - but not in this fashion. As the festival actively supports people bringing smartphones, this is a hidden extra cost you have no way avoiding - and that is so NOT OK!

Putting together an efficient but easy charger.

Now, the previous years, My camp has brought shitloads of carbatteries. They will support the stereo and charge the phones (Roskilde, charges 150 DKK to charge one of these, by the way)

As most phones charge from USB at 5V, i simply used a voltage regulator (LM7805) - which is basically a chip that converts the voltage down to 5V, dissipating the surplus voltage as heat. ... Now, dissipating 7V and only using 5 is an extreme waste. Also these regulators have the problem of not nessesarily supporting enough mAh to the phone. Most Apple products has this problem, but also product bought by sane people might have troubles.

Full View

ubecRecieved DC/DC converter ubec chinaFull view of the DC/DC converter

Therefore, this year I desided to buy a tiny switch mode DC/DC converter this year that I soldered to an USB plug. Especially i picked an UBEC (fancy name for the a DC converter used in RC helicopters, which are higly effecient and noiseless) It cost me $12 and has the following specs:

  • Effectivity of 90%
  • Voltage input range 5,5-26V @3A
  • Voltage output 5/6V, selectable

As I would like something lighter than the car battery, I will be using the Lithium batteries from my IBM laptop, of which I have two in good condition.

  • 10,1V @ 4400 mAh
  • 10,1V @ 6800 mAh

Total batterypower = 10,1V @ 4400+6800=11200mAh = 11,2A

I know that my normal phone charger has an output of maximum 5V@1500mA = 7,5w/h, so this is the maximum at which the phone will charge.

Using normal Ohms law: P=U*I, we have 10,1V*11,2A = We have 113,12W at disposal.

We will, however convert to 5V @ 90% effencicy, leaving us with  (113,12W/5V)*0.9=20.4A

As the phonecharger uses at max 1,5A/h we can get 20.4A/1,5A=13,6 hours of charging.

A full charge takes around 2 hours, so I can charge my phone to its fullest extend almost 7 times.


Old-Guy-Thumbs-UpSuccess! It works just fine.
chargingComplete circuit with usb and laptop battery

Now the expected charge times are only aproximate. I consider it doubtfull that the converter is actually 90% efficient, and the batteries are probably not is excelent condition. The phone does, however, not charge at 1.5A the whole time as this is controlled by an internal microcontroller. In fact it uses much less.

Fact is, however that I have a cheap and fairly efficient charger that will give me enough charge to avoid to visit the expensive charging stands. In fact, though the DC/DC converter set me back $12, i have still saved money - and the charge will probably last many festivals as soon as it gets a decent enclosure for it.