Using a car battery as energy source for projects

Thread Starter

ComplexDivinity

Joined Nov 10, 2016
4
I'm working on a school (early engineering) project. Our teacher told us a condition: "Do not use wall current". So I decided to use a car battery instead.
The problem: I connected the car battery to a pair of rails for testing a tungsten rod's electric properties and it sparked. Badly. I fear the same might happen to my trusty cure little 12vdc motor if I plug it trough a relay to try battery. Or to my Arduino. Or to me.
Then I remembered high school, the welding class. There I learned: The electric arc (sparks) feeds from current, not voltage. So I was wondering: how could I limit the amperage that is drawed from the battery, so my motor (or my Arduino) doesn't get roasted?
 

ian field

Joined Oct 27, 2012
6,539
I'm working on a school (early engineering) project. Our teacher told us a condition: "Do not use wall current". So I decided to use a car battery instead.
The problem: I connected the car battery to a pair of rails for testing a tungsten rod's electric properties and it sparked. Badly. I fear the same might happen to my trusty cure little 12vdc motor if I plug it trough a relay to try battery. Or to my Arduino. Or to me.
Then I remembered high school, the welding class. There I learned: The electric arc (sparks) feeds from current, not voltage. So I was wondering: how could I limit the amperage that is drawed from the battery, so my motor (or my Arduino) doesn't get roasted?
Sparks near a lead-acid battery are a really bad idea - they split the distilled water into hydrogen and oxygen while charging. The bang will make your ears ring and shower you with acid and bits of battery.

Sealed lead-acid batteries are becoming common on cars and motorcycles, and are much safer. A motorcycle battery is more compact and convenient for bench work, but you can get smaller "Dryfit" style batteries that were pretty much standard kit in fire alarm back up supplies.

Size for size - Ni-Cd are about equal on current draw capability with LA, but much lower Ah capacity.

Ni-Mh are up to 4x the Ah capacity of Ni-Cd, but are poor for high current draw.
 

Thread Starter

ComplexDivinity

Joined Nov 10, 2016
4
I've used 12v wall warts with Arduino. So, you guarantee me that if I plug the board to the battery directly it will NOT blow up?
What about my tiny electric motor?
 

crutschow

Joined Mar 14, 2008
25,634
I've used 12v wall warts with Arduino. So, you guarantee me that if I plug the board to the battery directly it will NOT blow up?
What about my tiny electric motor?
As long as they are all rated for 12V, they won't blow up.
These devices only take the current they need, independent of how much the battery can deliver.

But for safety you need to add a fuse or circuit breaker at the battery output, so an accidental short won't cause fireworks.
 

WBahn

Joined Mar 31, 2012
26,275
I'm working on a school (early engineering) project. Our teacher told us a condition: "Do not use wall current". So I decided to use a car battery instead.
The problem: I connected the car battery to a pair of rails for testing a tungsten rod's electric properties and it sparked. Badly. I fear the same might happen to my trusty cure little 12vdc motor if I plug it trough a relay to try battery. Or to my Arduino. Or to me.
Then I remembered high school, the welding class. There I learned: The electric arc (sparks) feeds from current, not voltage. So I was wondering: how could I limit the amperage that is drawed from the battery, so my motor (or my Arduino) doesn't get roasted?
Ask your teacher if their prohibition of "Do not use wall current," was meant to preclude you from using it at the full voltage. Ask them specifically if it would be okay to use a wall wart or other wall-powered device that provides an output in the 12 V range. If so, that makes things a LOT easier. Also a lot safer since most such supplies aren't going to produce more than a few amps if you short them (and most will either shut down or fail as an open if you do) while a car battery can deliver hundreds, if not thousands, of amps if it is shorted. In either case, you should include a fuse or breaker for protection.
 

Kermit2

Joined Feb 5, 2010
4,162
Get a safer battery. Car starter batteries are dangerous. You can turn a length of heavy gauge jumper cable to a glowing white hot snake of 2300 degree molten copper in an unbelievably short time.
They are specifically made with a huge number of thin lead plates.
This greatly increases how much current they can push into a short circuit.
In general. Any lead acid battery you need two hands to pick up is going to be dangerous without a special battery holder and insulated connections with fuses.

Here's a better idea.
http://www.batteryjunction.com/rayovac-918.html


You should buy a small number of these batteries with the screw terminal connections. Portable and much more safe.

Look around at the other batteries there as well.

If, for understandable reasons, you have no budget, scrounge your friends for old battery powered drill or similar that has a batterypack and charger. Still much safer and lighter than the car battery.
 
Last edited:

wayneh

Joined Sep 9, 2010
16,400
Remember it's a CAR battery.
And remember the immense power available at your wall outlet. The fact that is is available doesn't mean it will all get used by anything you plug into it. Picture a little nightlight plugged into the wall. It uses just a few watts or less. The wall could supply it 15A at 120V, or 1800W before your circuit breaker blows, but it doesn't.

For many (not all) devices, as long as you supply the right voltage, they will take the power they are designed for, and no more. (Some devices, such as an LED, require a fixed current supply instead of a fixed voltage.)
 

tcmtech

Joined Nov 4, 2013
2,867
Remember it's a CAR battery.
And?

Resistance of the circuit determines the current that flows through it not what type of voltage source it is.

I think you need to study up on voltage, amperage, ohms laws and how electrical circuits work in general before you go much further.
 

#12

Joined Nov 30, 2010
18,210
So, you guarantee me that if I plug the board to the battery directly it will NOT blow up?
No.
If you can't understand Ohm's Law, nobody can guarantee anything about electronics for you.
Ohm's Law is the first Law you learn. I wrote the simplest version I could. You still don't believe it. I can't fix that.

I agree with the others in the idea that you shouldn't be using a car battery when you might use a wall wart or a lawn mower battery.
That's like starting with a chain saw instead of a pocket knife. If you're afraid it's going to get loose and run amok, don't do that, and I guarantee a car battery can run amok the first time you make a mistake.
 

ian field

Joined Oct 27, 2012
6,539
I've used 12v wall warts with Arduino. So, you guarantee me that if I plug the board to the battery directly it will NOT blow up?
What about my tiny electric motor?
Most wall warts give up at about 1A - a typical car battery can supply as much as 200A.

Most electronic things will let the magic smoke out if you connect the supply wires backwards. Most wall warts the wrong polarity will just damage the device - with a car battery - the magic smoke usually lets you know you've blown it.

If you stick a hacksaw blade across car battery lugs; It'll glow red and maybe melt - imagine what'd happen to various PCB tracks if you do it wrong.................
 

wayneh

Joined Sep 9, 2010
16,400
I once dropped a wrench across the terminals of my tractor's battery.

Yeah, you don't want to do that. In my case it vaporized the terminal of the battery. Gone. Had to buy another.
 

takao21203

Joined Apr 28, 2012
3,695
A car battery is a health and safety risk in many regards.

-If theres no fuse a short is very likely to cause fire
-If theres unauthorized access the battery could fall down and acid may spill and cause damage

The maintenance free kinds are not as much of a risk. But even these may produce gas (its unlikely though).

So you must have fuses and check who may access the setup. Or you have a considerable health and safety risk.

Modules and circuits will behave differently. I had MT3608 and SX1306 converters just exploding when powered from a 12v battery.

For a car battery or any large rechargeable battery fusing is mandatory.
 

Thread Starter

ComplexDivinity

Joined Nov 10, 2016
4
Ask your teacher if their prohibition of "Do not use wall current," was meant to preclude you from using it at the full voltage. Ask them specifically if it would be okay to use a wall wart or other wall-powered device that provides an output in the 12 V range. If so, that makes things a LOT easier. Also a lot safer since most such supplies aren't going to produce more than a few amps if you short them (and most will either shut down or fail as an open if you do) while a car battery can deliver hundreds, if not thousands, of amps if it is shorted. In either ca
It was actually a challenge. She said "you must do this without plugging it to the wall. Use anything else. I don't care if it's a gas-powered generator, solar power, nuclear plant or car battery, just don't use a wall... You are not at kindergarten any more"
For me it's nonsense, but it's school.
 

WBahn

Joined Mar 31, 2012
26,275
It was actually a challenge. She said "you must do this without plugging it to the wall. Use anything else. I don't care if it's a gas-powered generator, solar power, nuclear plant or car battery, just don't use a wall... You are not at kindergarten any more"
For me it's nonsense, but it's school.
That makes the context a bit clearer. Whether it is a rational prohibition depends on what her objective really is. But it is what it is.

What are your current requirements and how long does the thing need to run at that current? If you can do it with a couple of 6 V lantern batteries, that is a MUCH safer way to go.
 

ian field

Joined Oct 27, 2012
6,539
A car battery is a health and safety risk in many regards.

-If theres no fuse a short is very likely to cause fire
-If theres unauthorized access the battery could fall down and acid may spill and cause damage

The maintenance free kinds are not as much of a risk. But even these may produce gas (its unlikely though).

.
Many maintenance free/dryfit type batteries (allegedly) have a chemical system to recombine the evolved gasses - but they're still quite capable of drying out.

Any escaped gas at any one time should be too insignificant to pose an explosion risk. And even if any gas did ignite, the vent caps should keep the flame out of the battery. The chances of getting showered with acid and bits of battery *should* be non existent.

There used to be a computer scrap warehouse near me, often with a big pile of scrap UPS boxes by the door. A useful source of high current power transistors/MOSFETs, but the SLA batteries took a bit more effort.

They almost always dry out so the H2SO4 becomes more concentrated, after that, any discharge rapidly sulphates the plates.
 
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