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Mobile charging from Car or bike power source

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Junior Member level 1
Dec 29, 2010
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How safe is it to use a 7805 or LM 315 based circuit to charge Mobile phone from Car or Bike battery (12V Lead Acid) while on drive? I mean how safe will this be for the mobile battery? I am sure some of our senior members must have tried this before. Is there any suggestible circuit? I intend to put my Samsung Galaxy in charge from my Bike or Car on the move.

Though readymade China made chargers are available for this purpose, I am not very confident on their quality.
Thanks to all in advance-

You are better off buying ready made chargers, I certinaly would not trust a home made version to charge my exspensive phone. I would have more trust in any charger that is made by a company even if it was made in china I've never had any problems in all the years I've used them well this is my thoughts
It is always better to buy a charger that is available ready made, because if you try to make one by yourself you have to test it by plugging it into the phone and if there was some mistake in the circuit results could be catastrophic for your expensive phone.
I made some chargers for my friends all were based on MC34063 chip.MC34063 is very robust and easy to use chip.The advantages of this chip over linear regulators is in the way that the power loss is minimum and there will be almost no heat to be dissipated but it is not a case with linear regulators.You can download the datasheet and see the sample circuits there for step up and step down regulations with this chip.Its a buck/boost/inverting switching regulator, a real beauty,i love this chip.

Also up-to this day i found that all the car phone charges available in local market whether they are made by Nokia or China, they all have the same circuitry inside based on MC34063.The difference is only in price but they perform equally.
The cigarette lighter chargers are inexpensive 13v to 5v switching power supplies. You can open them and use the PCB if you want. Buy one with USB socket instead of dedicated cord. You can order a separate USB cable to connect to what ever variety of plug the phone uses.

The charge current on a smartphone is 700mA to 1200 mA's. A LM317 linear regulator would have to dissipate 7 to 9 watts. It would not produce the required current because it would cut back due to the high input to output voltage differential.
Thanks a lot. But, when on the run, the battery of car or bike receives around 18V. As RCinFLA wrote, the readymade cigarrette lighters are 13 to 5 V one. Won't it get too hot If I keep them on use while the car or bike engine is running?

Cigarette lighter adaptors can accept a wide input voltage range, 10-20 vdc.

Yes its true cigarette lighter adapters are wide input voltage step down switching supplies.While the bike is running the generator produces around 18 volts but all of it will never reach the adapter , The battery of the bike will itself act as filter and utilize some of the excess voltage produced by the generator. So the voltage at cigarette lighter adaptor may reach 13.9 to 14.5 or little above depending on the battery charge state.It will be safe to use.
While making those chargers the makers keep in mind such things. And the mc34063 has a supply voltage in the range of 3-40 volts, so the over voltage even if it is 18v in you bike or car is absolutely safe.

One more thing that i always observe is those supplies are designed to output a 6v+/-0.5v without any load or phone connected to their output so its a marginal voltage to balance the output voltage to supply appropriate voltage when a phone is connected with totally depleted battery or a call has to be made during charging.Most of the modern phone specially Chinese phones will not accept a charger with a voltage lower than around 6.5v and will stop charging after a while.

However it is not the case with usb port at the phone.A charger that has a usb cable to be attached to the phone or if the phone is connected to pc usb port the voltage at the usb port will be 5v-5.2v its a universal standard for usb operation and the port interface inside the phone is designed to work with 5v only.
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I appreciate the elaborate answer. Things are clear to even my brain- novice to electronics. Now I think it will be better to depend on the readymade chargers than assembling something on 7805.

Please clarify one more thing. All the modern phones are with Micro USB port for charging as well as data transfer. They need 5V for charging. I have cigarette lighter connector with 5V USB output for Ipod charging. **broken link removed**

Can that be used to charge my Samsung Galaxy 5570 Cell phone safely? It has a provision for USB charging and comes with a standard accessory cable for the purpose.

It looks like it can be used to charge your phone,you will be using the usb to micro usb adapter cable that came with your phone. The description of this item on e-bay says it can be used to charge cell phone, GPS, iPod, and PDA.Looks good to me.I don't have a samsung phone so i can't be 100% sure.
Ask a question at e-bay to confirm it.

Look at your AC adaptor for the phone to see what its current rating is. My Samsung Galaxy S phone charger is rated for 700 mA.

Most cigarette lighter adaptors put out up to 1 amp so they are fine. I have several different manufactures' cigarette lighter adaptors and they all work fine on my Galaxy S phone. Typically, I see an unloaded voltage of about 5.5 volts on these adaptors. This allows for some cable and connector voltage drop. Drawing 1 amp though a micro USB connector is a stretch.

Some phones (like Motorola) rely on external charger module for current limiting. This reduces the power dissipation in the phone's internal charger regulator by allowing the phones internal series pass device to go into fully ON switching mode. It is not absolutely necessary but the phone will back off on charging periodically to avoid too much heat build up in the phone if phone regulator has to operate in linear mode. It will just take a bit longer to charge the battery.

Also, phones must be happy if the USB 5v source limits the current to 500 mA since this is the maximum spec on USB devices. Some USB ports, like on laptops, or even phones with USB On-The-Go capability, limit sourcing to 100 mA from their USB output. Most desktops allow more then the 500 mA limit.

Some portable cellular data modems, and external portable disk drives come with a USB cord with two USB connectors. Since these devices need more then 500 mA to operate, they put two connectors to theoretically get 1 amp supply by using two USB ports. Some laptops will not allow two ports to simultaneously draw 500 mA each, also there is no guaranty that the each port might have independent 5 volt regulators that have a little different voltage due to regulator tolerance. This causes the slightly higher voltage port to take most of the load. Some laptops will shut down the port if an overcurrent limit is detected.
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there is also discussion .
Just search before posting

Thanks for the link of previous discussion.
Actually I tried it earlier but could not fathom down to any conclusion due to lack of my knowledge in electronics. And that prompted me to post a new thread to have better idea from senior members.

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