Lost Your Charger? How to Charge Any Battery Survival-Style




Tim Anderson is the author of the "Heirloom Technology" column in Make Magazine. He is co-founde...

My camera shut down from low battery just when I needed to take some pictures.
The light was perfect. I suddenly remembered where I'd left the charger - 3000 miles away.
Everyone's had this experience, or the similar experience of spending one's vacation searching for a cellphone charger.

Here's how to charge any battery enough to keep doing the important stuff.

Fact 1: All past and future rechargeable batteries can be safely trickle charged if you don't overcharge them. Trickle means low current, like half an amp for an average camera or phone battery.
Fact 2: Small incandescent bulbs such as flashlight bulbs and christmast tree mini lights make great current regulators.

This is the battery to my Canon S30. It's got three terminals, labeled "+", "-", and "T".
I've clipped alligator-clips onto the two obvious ones.
You don't need clips, you can just hold wires on it for as long as it takes to charge, that's probably better anyway, so you can tell if anything is going wrong.

Warning! Wear eye protection and if anything weird happens don't breathe the fumes!

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: Hands on Charging

What was that other terminal marked "T" for? TIMMY of course, just like in the nursery rhyme!
It's for Timmy to to hold his finger on the battery. If it gets hot you're doing something wrong.
Actually it's "Thermistor" or similar names. Temperature sensor. Some batteries use that to regulate charging current, some just for a safety feature.

"Digital sensor, huh?" would be a running joke back when Americans knew Greek and knew that digits are fingers. But then numbers got so much use we forgot about counting on fingers, and now people barely know how to do anything with their hands.

Anyway, here's a finger used as a digital thermometer. Which reminds me of the joke about the doctor...

Step 2: Charge From Car Battery With Lightbulb Regulators

SAFETY WARNING: scroll to the bottom if you want to read safety warnings.

Here I am charging my camera's battery from my car battery. I'm using three christmas lightbulbs at once as a current regulator to get half an amp to flow into the camera battery.
Here's how it works:
As the current through a lightbulb increases, the filament gets hot. That increases the resistance, which limits the current.

For example, here's my test of one of these mini christmas lightbulbs hooked up to a bench DC supply:

Volts Amperes
.5 .05
1 .07
1.5 .08
2 .09
3 .11
4 .13
5 .15
6 .16
7 .17
8 .18
9 .18
10 dead. The filament burned out.

I tested two bulbs and the data was the same.

Since my car battery is at ~13 volts and the camera battery is at ~7 volts, there will be 6 volts across the bulb. So I put three bulbs in parallel to get about 0.5 amperes to flow into the battery.
I'm guaranteed that less than 0.6 amps will flow, because that would burn out the bulbs.
That's some protection against reverse-charging, but do make sure you connect the plus terminals and minus terminals correctly.

Now just stand there for fifteen minutes or so until your battery is charged enough to take pictures again, you can make calls on your phone or whatever.

Do not leave this unattended, and don't attempt to fully charge the battery.
If you charge too long and your camera battery gets over 8 volts, bad things could happen.
"Bad things" include possibly catching on fire.
Repeat: you can safely trickle charge any rechargeable battery part way.
But it is NOT SAFE to fully charge a battery without fully understanding the rules for that specific type of battery.
Car Batteries can produce a mixture of oxygen and hydrogen which can be ignited by a spark.
The resulting explosion sprays sulfuric acid everywhere. Don't let that happen to you. For simplicity's sake this photo shows me working right on the battery, but you could just as easily get your battery voltage from the cigarette lighter inside the car, far from the explosion hazard.
Also, don't electrocute yourself. I haven't heard of anyone being electrocuted by a car battery. I have heard of the other accidents described here actually happening.

Step 3: Charging From Any DC Source and Resistor

Hopefully you own an electrical meter.
Find a source of DC electricity. Look at the voltage of your source, the voltage of your battery, and use Ohm's law to figure out what kind of resistor you need to put between the two to get the right current to flow. Then go find that resistor. A piece of a heating element from a toaster or hair dryer can work. That clothes iron in the photo worked for a certain battery and a certain DC source.

As seen in the previous step, the best improvised current regulator is usually a small lightbulb. The bulb is great because the resistance goes way up when the filament gets hot and limits the current. And you can only put an amp or so through one without burning it out, so it acts as its own fuse. And they're free gifts from garbage Santa.

Here I'm using two bulbs in series to limit the current going into a gelcell. Use your multimeter to measure what current goes through your bulb at different voltages. Or you can just cowboy it and use one tiny bulb for a few minutes, while feeling to see if the battery gets hot or not.

Wear eye protection and don't breathe the fumes!

Step 4: Current and Voltage From a Bench Supply

This step uses a bench power supply to charge a battery. A lot of us have those sitting around even if we can't find our cell charger. Or we know where to find an electronics guy who has one. Now what?

Want to be totally mystified? Look up "lithium battery charge control".
All that stuff is important if you want to fully charge your battery, make it survive many charge cycles, or avoid lawsuits.

But we just want to take pictures or talk on the phone. So here's what we do:
Turn the current and voltage knobs all the way to the left. Turn on the powersupply and
hook it up to your battery, bearing in mind that red and black wires can possibly be plugged into the wrong sockets. Read the labels and unplug it if it seems to be arcwelding on your battery.

Turn up the voltage and current limit knobs until 500 milliamps (0.5 amps) is flowing into your battery. If you feel like being careful look up how much current the experts use.
But half an amp for a few minutes won't damage any battery that's big enough for a modern camera or phone.

My electronics guy told me to set my voltage limit to 8 volts for my 7.2v li-ion battery. Usually facts like that are luxuries. The point of this method is to trickle charge anything without being able to know much about the properties of the battery.

Step 5: Time Limit

If you really have to do something else while charging your battery, you better hook up something to disconnect it after a few minutes.

The whole point of this instructable is that we're not going to fully charge the battery because we don't know how, and we're in a hurry.
We're just going to charge it enough to go back to our regular jobs. The phone or camera will tell us roughly how charged the battery is after we start using it.

So we're only going to charge it for ten minutes or so.

My camera battery has 1200 milliamp/hours of capacity. So if it's fully discharged and we charge it at 500 milliamps, it would take more than two hours to fully charge it.

Some types of batteries can be badly damaged by overcharging. Some are just damaged a little.
This picture shows how my golf cart avoids overcharging. There's a built in appliance timer that turns off the charger automatically. It won't let you charge for more than 24 hours.

That's it! Enjoy cautiously!
To make your own external battery pack, check out AT's splefty booster pack.



  • Indoor Lighting Contest

    Indoor Lighting Contest
  • Metal Contest

    Metal Contest
  • Make It Fly Challenge

    Make It Fly Challenge

112 Discussions


3 years ago

How many bulbs would I need for resistance if I was charging a 3.7v lithium ion cell? Also could I cut the end off a spare charger and attach the wires to the lithium ion terminals? I understand that it's not easy knowing which of the wires is which in these mobile phone chargers, but my problem you see is the charging port! Tried to desolder the old one and I think I've stuffed it up! So instead of throwing the tablet away I thought might try fix it or in this case break it further and then ask expert whether it's salvageable?


3 years ago

Can anyone tell me I can charge an 18v lithium battery with a 21v lithium charger?? Or would it cause me heartache? (It's for a cordless drill)


3 years ago

can you recharge a rechargeable battery in the refregrator?


3 years ago

can you charge a battery in the refregrator?


4 years ago on Introduction

or just buy a couple extra batteries and maybe two battery chargers, solar charger, and a car charging station. If your a field freelance and assignment photographer I feel like maybe that is the solution. Learn once, feel bad for a little that you were not prepared and then correct and move on.


4 years ago on Introduction

I didn't have a phone so I connected the charger to the battery directly. Using a formula I found out the time to charge the battery completely. It took three hours. Mine was a BL-5C battery (1020 mAh). Thanks for the help!


4 years ago on Introduction

You can use a unregulated power brick pretty successfully too. Find a DC wall adaptor with a close voltage and amperage setting to the battery. Because it's unregulated, the voltage will drop to match that of the battery, but the current will increase. You can check to see if it's unregulated by measuring the voltage from the adapter, if it's higher than what it's rated for with no load, it's probably unregulated. As long as you don't go too far from what the adapter is rated current wise, should you be safe. Li-ion batteries have a unique charging profile, where the voltage and current needed to charge depends on how far along the charge is. As your battery charges, the voltage will climb, and the current will drop. This is the basic charging profile for Li-ion. You don't want to overcharge Li-ion batteries. Once the voltage reaches around 4/4.1 for a single cell, I'd call it quits. Remember you're trickle charging. You're only trying to put on a small percentage of current compared to the battery capacity. When I've done this in the past, I use a multimeter to measure the voltage, and one to measure current. I wouldn't do this without one to at least measure voltage. If you don't have one to measure current, make sure the wall adapter isn't getting too hot. Don't stray too far from your operation either.(don't leave home) Disclaimer: Intended for informational purposes only


4 years ago on Introduction

Thanks! I have a great cordless drill and the charger died. They don't make the battery anymore so there aren't any replacement chargers that fit it. I will explore options using my trickle charger.

himanshi sharma

4 years ago

What to do if we had no charger for battery of samsung galaxy fir


11 years ago on Step 5

I think Tim does a decent job of explaining the risks involved. I think I would give it a shot if I were in a jam. I would only use a very low amp source to do this though. Doing it with a car battery is too shaky for me.

1 reply

Reply 5 years ago on Step 5

Indeed the risks involved are well explained. It is true that a car battery is capable of supplying many, many amps, should something be wrong so using a low amp source would indeed make this much safer.

In fact, if you're stuck with a battery that is a single-cell lithium, voltage from a USB port should be sufficient (5V, single cell is 3.7V) without further current regulation (a 'normal' computer port is already regulated at 500mA at most; modern dedicated charging port can go over this specification and often reach 1A). If however your battery is multicell (often, 2 cells in series, 7.4V) then I guess you could fall back to a 12V battery or low current adapter.


11 years ago on Step 5

Thats a pretty dangerous way to charge a lithium battery. Ever seen one burst (explode!!) into flames. Any way, if it doesn't self destruct it certainly won't have a very long life using this method of charging. DONT ATTEMPT THIS

2 replies

Reply 5 years ago on Step 5

I believe the danger of explosion from lithium batteries comes from their very low internal resistance, which may permit a ridiculous current to flow if the proper conditions are met (e.g not regulating the current and applying a higher voltage to the cell, or short-circuiting the cell).

In this case, lightbulbs are used as a crude way to limit current. OP stated he used at most 0.6A for a limited time, while checking on the temperature, which is all reasonable. Modern smartphones batteries will today charge commonly at 1A and sometimes 2A (larger phones or tablets).

Of course this method will not permit you to fully charge a lithium battery because, as stated, lithium batteries need a specific charging profile to be fully charged (which is clearly not applied here). However this method works for charging a cell to a reasonable level which will allow a good level of use, perhaps even 80% if one checks the time and monitors the temperature.


6 years ago on Step 2

Some electronics do not have removable batts. I wonder if there is a way to safeguard the electronics if you need to do this with an e-phone, kindle, etc.

Ideas on this?

1 reply

Reply 6 years ago on Introduction

if its a kindle fire, it can be popped apart quite easily. as for an iphone, dont buy crapple, get an android :D


6 years ago on Step 2

Nice idea. Did you check the current flow on your setup with a meter to confirm your math?