# DIY emergency battery charger

#### vick5821

Jan 22, 2012
700
Hey guys, I just did a emergency charger for my nokia handphone. I am just applying the simple concept of using the circuit of 7805 voltage regulator and use the old nokia charger connector to make one. My external battery source is a 9V battery and step down to 5V and use to charge handphone. Is this a good idea ? Will that harm my handphone battery ( for eg shorten the hp battery life ?) To make thing clear, I just ake this as an emergency one to use as my hp always run out of batt when I need to use them ><

Aug 13, 2011
1,114
It's safe but inefficient, especially using a 9V battery. How about using an 8-cell holder full of AA cells?

#### vick5821

Jan 22, 2012
700
u mean by using 12V input? It still step down to 5v.mayb just the battery can last longer

Aug 13, 2011
1,114
You can increase efficiency by using a 5 V switch mode regulator instead of the 7805. You can buy pin-compatible replacements e.g. this: http://www.ledsales.com.au/catalog/index.php?main_page=product_info&cPath=142_146&products_id=840 or another similar product.

That Australian site seems to be down. This is the same or similar product.

http://www.dimensionengineering.com/products/de-sw050

The input can be 6.3V-30V. An 8-pack of AA cells, even rechargeables would work well with the switcher.

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
u mean by using 12V input? It still step down to 5v.mayb just the battery can last longer

You need to understand the difference between a switching regulator and a linear regulator.

A linear regulator's input current is the same (actually slightly more) than the output current. It wastes as heat the difference between the input voltage * current and output voltage * current. Increasing the input voltage decreases efficiency.

A switching regulator converts the power in to the power out so that voltage in * current in. For a regulator that reduces the voltage (switchmode regulators can increase the output voltage, something that linear regulators can't), the input current will be lower than the output current by almost the ratio of the voltages.

The advantage for you is that you can transfer far more of the energy in a battery to the load if you use a switchmode regulator than if you use a linear regulator.

In your case, efficiency really means a lot. It also means that you can use batteries that have more energy for a given size/cost. In your case, a pair of AA cells may be a lot cheaper and have equivalent (or higher) power to a 9V battery. (Clearly in this case you need to use a boost switchmode regulator).

#### vick5821

Jan 22, 2012
700
Yea, I need to use a DC-DC booster and then a switchmode regulator. What is the code for that ?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
There is no code.

One form of DC-DC "booster" is a boost switch mode regulator, so you're saying the same thing twice.

#### vick5821

Jan 22, 2012
700
There is no code.

One form of DC-DC "booster" is a boost switch mode regulator, so you're saying the same thing twice.

I mean which IC is DC DC booter ? I need to increase the current as it charged so slow

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
There are literally thousands.

Go to a site like Digikey and look up DC-DC switching regulators

They list over 15,000.

Use the parametric search to narrow down the list to something sensible and evaluate them.

Aug 13, 2011
1,114

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