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

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.

SAFETY WARNINGS:
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.

Comments

[harlyquin]

no one's killed themselves from a car battery because it's DC and won't stop your heart. The only part of a car that can kill you with electricity is the alternator (the current has to pass through a bridge rectifier to switch to DC, so it can be used by the accessories and engine)
Don't get me wrong, you really REALLY don't want to ground out a coil pack or a distributor. It'll feel like having your arm yanked on while being covered in a shower of angle grinder sparks.

[amchugh]

harlyquin, you might want to youtube search for bush welding with car batteries... 90+ A/hours to some truck batteries with is well and truly enough to stop a heart (considering it takes 0.1Amp across the heart to do so)

[lukeyj15]

I have been unlucky enough to take a brief spark straight out of a coil. Uncomfortable but not deadly.

[ericcox]

@harlyquin: DC current can most certainly stop your heart, just like AC. But not usually at the 12 volts that cars use. The resistance of your skin is fairly high (around 500K ohms if memory serves), so 12 volts isn't enough to cause much current to flow. But if your skin is wet or has other substances on it, the resistance can drop severely, to 1000 Ohms or less. And that can cause much more current to flow.

But it is NOT the fact that it's DC that makes it less dangerous - plenty of people have been electrocuted by high voltage DC. It's the fact that the battery voltage is so low. Coil packs can generate 50,000 volts or more - that's why they are more dangerous than the battery.

Just want to make sure that no one thinks that DC current is "safe", that's the way people get careless, and killed.

[balisticsquirel]

Tru dat

Unfortunately it's not that simple either.
The characteristic of a power supply that makes it dangerous is actually the.. um ... power rating. And that's best determined from the current (Amps) rating. High voltage IS what will let the power supply (battery / whatever) overcome the high resistance (low conductivity) to get to your vital organs, but current gives it the power to do something when it gets there.
So any high current situation is dangerous, PARTICULARLY at high voltage.
And your car battery has plenty power to do serious damage under the right circumstances.

[ericcox]

I disagree. The power rating on a car battery would be around 7200 watts for one with 600 cranking amps. Much bigger than, say, the PS on an old CRT or a PC or a refrigerator. Yet compared to any of those things, the car battery is much less of an electrocution risk, because the voltage is only 12 volts.

The amount of current that can kill you is so small (anything over about 100 milliamps - that's tiny!) , that almost any power supply can kill you if it has enough voltage to push the current through your skin.

A 1000V power supply that can only deliver .2 amps is just as deadly - at least from electrocution - as a 1000V power supply that can deliver 100 Amps. The converse: A 12V battery that deliver 600 amps is not any safer than the same battery that can only deliver 5 amps.

[balisticsquirel]

Which part do you disagree with? The bit where i agree with you, or the bit where i back up your comment that DC isn't necessarily safe (see that i said "under the right circumstances")?

The rest is semantic argument and probably not best suited for this thread. But i'd be glad to learn more if you want to message me.

[seabeepirate]

Wouldn't it be better to keep the current low?(as in, just one bulb) It would take longer to charge, but it would reduce the risk of fire or damaging the battery too wouldn't it?

[lilpepsikraker]

actually to make the current lower, you add more bulbs.

[BC-45]

um yah aboute these idea i treid it and the battery blew up on me good thing i was away left it there for 1 min the i hear a boom im like WTF and i see the battery in have like as if i burn or something and half of it i was not able to find but it was cool lol