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!
Step 1: Hands on Charging
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
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:
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
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
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
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.