Mobile phone charger - camping

[East Coast Adventurers]

We are talking here about Car chargers so DC low end voltages (less than 15v) so RMS and switchmodes don't factor into it..



Anyone who knows anything about electronics should know that

So I wonder how you step the voltage down from 13.8V (rough alternator charged battery output) to a smooth 3.7V or whatever your phone requires? I guess you have at least three options - 1. transformer*, 2. burn the difference in voltage with a resistor, 3. chop up the 13.8V then smooth it down again / convert it to AC, transform it then convert it back to DC.

Anyone who knows anything about electronics will know that transformers don't work with DC So I guess, like a cheap chinese charger you could burn the 10.1 volts with a resistor and soak up the heat with a heat sink

I think I would rather go for the third option and make sure my charger used a good DC-DC Converter or Switching power supply circuit which offers a higher efficiency than simply burning what power you don't want, still, I am posh like that

[Kymmy]

Just a simple voltage regulator will do the job.. it'll provide a stable voltage with little ripple and any such ripple can be smoothed though theirs no need.

The same devices are what you'll find inside of the mobile giving 1.8-1.5v (depending on the phone) to the cpu.

Voltage regulation in this manner has been used for years and is good proven technology.. It's only recently that switch mode has been small enough to put into the AC adaptors and replaced standard regulation but for in car it's the best way

[East Coast Adventurers]

Something like an old school 7805 in a T220 package - except this is 5V fixed, but I know what you're on about.

There is much more to charging a battery than simply applying a voltage though... things like the thermal characteristics, max charge current, pulse charging, shut off, charge rate all need to be taken into consideration for efficient and safe charging though I accept that on some phones this is taken care of inside the phone and your voltage regulator would be fine .

Phones that rely on the charger - to do the charging :-) Well they need to be well looked after...with some phones you need to provide the current limit that the phone battery requires for its maximum charge current rate. The reason for this is to avoid excessive charge requlator heating within the phone (you don't want your phone getting really hot when you hold your ear to it ).

As I understand.... it the charger transistor within these phone just switches ON or OFF, again relying on the external power pack to limit the totally ON state current.
In the case where the supply does not provide the current regulation then the phone internal MOSFET pulses in linear mode for short duty cycle to keep the device from overheating. It takes longer to recharge because it must keep the series pass MOSFET from overheating in linear mode....

The phone internal charge regulator circuitry handles the voltage management on the battery, determining when battery is fully charged.

So there is a lot more to charging a phone battery than you would think and also a lot of difference in the level of technology that you may run into when choosing a brand of charger...for this reason I always place safe and only buy the one designed for the phone by the company that designed the phone...

[Kymmy]

Even phones in the last 10 years that required a separate charging line from the PSU has always been regulated (both current and voltage) Long dead are the days when you could blow up a phone by attaching it to a non-current limited supply .. These days they simply would not make it through EU type approval if that was the case

As for DC-DC convertors I thought the whole point about your choice was to prevent ripple yet you want to use a simple buck convertor Also they don't tend to like the low voltages as found in modern mobiles. Just use a variable regulator with a low drop-out with a zenier as the referernce, clean simple, reliable and most importantly it works without trying to induce a high pitched whine down your car radio

One thing I learned a long time ago whilst I was being taught electronic was the KISS method.. Perhaps you should familiarise yourself with it..

[jklchravel]

I built a mobile mains device that uses 30 NIMH batteries in a water proof box. It is lighter, greener and works very well. 3 parallel banks of 10 x 2000mA AA rechargeable batteries. schottky diodes are used to protect the charge input to the battery pack, just in case the terminal is shorted. They are also at the output of each bank to stop runaway charging if a cell fails. A resettable fuse is placed on the output terminal also. The light weight battery is then fed into a small 150 watt (soft start) inverter that also has a USB output. The inverter only takes 280mA when on. The device can charge mobiles and even run a laptop. This means that I can bring my own mains supply without the need to start the car just to charge a mobile phone. Total battery power is 75 watts per hour. We charge the batteries using a powerful yet small 15 watt solar panel. it does not matter that the battery gets wet while on charge, you can simply bring it in and connect it to the inverter via a plug and have 240 volts mains. No need for adapters. just plug in your standard mains charger that you use in the house. I guess its not cheap but its the best way to do it. I will make a better battery pack one day by using Eneloop batteries.

We use it camping and while staying in youth hostels in wales that have no electricity. We did use lead acid batteries but they are very heavy and will die if left uncharged. The good thing about NIMH batteries is that they will last for many many years unlike conventional lithium ion that will only last three years.

[jklchravel]

To East Coast Adventurers. Mobil phones use lithium ion batteries. These batteries have advanced internal regulation and can not be over or under charged. The mobile will also have a regulator built in also. As long as you feed 5 Volts DC into the phone, the phone battery will charge and stop charging when its full. Simple! 12V solar panel connected to a cigarette car adapter allowing you to get USB is the cheapest option I thing. No need for cars then. Most mobiles will charge through a USB connection.