Keyring Phone Charger




About: Innovative Projects, Diy's, Life Hacks

In this Instructable, I made a simple keyring charger.

It can charge your phone's battery with a standard 9v battery.

If you ever travel and find that your battery is dying, just get a 9v battery and use your keyring to charge your phone.

For this project you'll need:

  • Car's USB charger
  • Micro USB to USB adapter
  • 2x 9v batteries or battery connectors
  • Sugru

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Step 1:

I removed the connector from a 9 v battery.

Then, I soldered cables to it.

Alternatively you can buy a battery connector.

Step 2:

I took apart car's USB charger and soldered battery connectors to it.

Step 3:

I could have used USB adapter without opening it, but I wanted to make it smaller, so I did open it.

Step 4:

I removed the LED from the charger.

Then, I covered everything with Sugru.

I also made a pattern with a screwdriver.

Once I was happy with the result, I half-plugged the charger in the phone and let it dry.

Step 5:

I also made a protective cap for the connector.

Finally, I attached another connector from a 9V battery to my keys.

Now the charger can be easily attached to the keys.

If you need to give your phone a boost, take off the keyring and use a 9V battery to charger your phone.

Charging speed and efficiency will depend on your phone, cars USB charger and the battery you will use.

Smaller phones with a high capacity 9V battery will charge better than big phones.

2 People Made This Project!


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34 Discussions

Gomi no Sensei

3 years ago

Most smart phone batteries are 1500-3300mah.

A 9V battery has 50-400mah and at 500mw load has less than 300mah capacity.

You're also losing 20% of that 300mah thru the circuit as heat.

Just exactly HOW much does this remaining 240mah charge your phone ???

Ref :

blakeghxMark AJA

Reply 3 years ago

It's usually available in several different colors at Lowe's, Home Depot, etc! At my Lowe's it's hanging next to the masking tape and paint supplies by the paint counter! If you wanna save some money search for the Oogoo (DIY Sugru) 'able!

Flash635Mark AJA

Reply 3 years ago

It's silicone rubber paste. It's sets like RTV silicone.


4 years ago on Introduction

Whenever I see a "Power" application of a 9V Block battery I cringe and think: The Moderator shall give them an "environmental unfriendly"(to say it nicely) label.

10 to 15 years ago it had been acceptable to use a 9v Battery for an 5 V microprocessor board using some 30mA.

But today in the age of cheap Chinese boost switch boards it is no longer acceptable to use a construct of 6 tiny 1,5 V cells(the 9V Block) for such an purpose. It is symply a waste of resources. Even here on "Instructables" symply type "18650" in the search bar and you will find a lot of Ideas to use scavenged liion cells from thrown away laptop batteries. Go to alibaba or other chinese markets and you will even find 0.8 - 5V boosters to 5 V USB, or buy a ready made one for an AA cell(like some one else here already suggested).

The 9V Battery Block should be reserved for some sub-milliamp applications where you need a free floating power supply with no noise, like pre amplifiars for the stage, E-guitars, microphones and such things.

I did a test with a normal 9v battery and "Alcatel One Touch X'Pop" phone.

I got 30% charger exactly.

It went from 13% - 33% in first hour and another 10% in next hour.

So it depends on your phone.

For emergencies is more than enough at least for my phone.


4 years ago on Introduction

I don't know why people are still stuck with these 9volt batteries... they can only keep ypur phone alive for a while and will not charge it. A lot of 9 volt batteries are only 200 mAh and considering a fact that the voltage converters are not 100% efficient they will "charge" your phone only few percent... batteries in modern phones are at least 2000mAh and capacity of a 9 volt battery is only 10% of phone battery capacity. Considering a fact that the efficiency of the voltage converter is around 70% it will give you total of 140 mAh

6 replies

Reply 4 years ago on Introduction

If the phone battery voltage differs from the source charging battery you shouldn't compare them in terms of Ah. You should do it in terms of Wh instead, which is the energy stored. So, the correct comparison is:

- 9V batt energy = 9 V * 565 mAh = 5085 mWh

- Phone batt energy = 3.7 V * 1600 mAh = 5920 mWh

Taken into account a 70% efficient dc/dc converter, it would charge a 60% of the phone battery.

I got the 9V battery stored charge value from wikipedia ( and the phone battery charge from my own smart phone.

Remeber, the energy stored in a battery is equal to its charge (Ah) multiplied by its nominal voltage (V).


Reply 4 years ago on Introduction

I recently found out that AA & AAA batteries have *Way* more power than a 9v battery.

AAA 1,200 mah

AA 2700 mah

C 8000 mah

D 12000 mah

9V 565 mah

6v lantern 26000 mah

I have a cheap phone charger that is basically a pipe shaped holder that holds a AA battery and has several tips that screw on. It seems to charge a phone a bit.


Reply 4 years ago on Introduction

mAh is not a measure of power. If you want to compare the amount of energy stored in each type of battery, you will have to multiply mAh by the voltage (9, 6, and 1.5 for the others).

You are right.

It's not going to charge your phone to 100%, but it will give you some extra juice to run it a bit longer.

I might do some tests and post results.



4 years ago on Introduction

This is great, but if you are carrying around a 9V you should be careful with it. Make sure one or both of the leads are covered because any piece of conductive material connecting those leads will burn you or potentially blow up the battery. I know someone who had a 9V in his pocket and some loose change bridged the leads and left him with a nasty burn.