Battery charging

Factual Questions
1

After seeing the car battery post, and the wrong DC voltage post I feel compelled to to ask about a situation I was in a little while ago.

The situation: Due to an interior light that was left on my car battery was run flat (I don’t drive it much, and the battery is in pretty bad shape).

As I’m a computer student I have lots of spare computer parts. These parts include a power box which has a 12V rail (I’m currently using it to charge my mobile phone which I lost the wall charger for, but for which I still have the car charger)

I think you can see where this is leading.

As far as I can tell most battery chargers just apply voltage and current to the battery with some sort of regulator to prevent overcharge. I figured, if cheap charges can get away without this regulator, why can’t I? After all, the power supply doesn’t supply all that many amps, and I figured (with help from some webpages about proper chargers) that I’d need to charge it overnight. Unfortunately I chickened out worried that I’d blow my last AT power supply.

I signed up to the RAC and got them to give me a jump start, with the added benifit that I can call for help next time my car wont start even if I’m not at home.

2

A little knowledge can be a dangerous thing…
You can make a pretty simple charger, but not easily out of an AT power supply. First of all, in an AT supply all the beef is on the 5 volt line. The 12 volt line can’t supply all that much current. The second problem you face is that lead acid batteries run at higher than 12 volts (unless it’s really dead), which means the battery is going to try and power the power supply, not the other way around. Depending on how the power supply is designed, this could easily make smoke. Your next mistake was that PC power supplies use a switching regulator, and these things don’t work well without a minimum load (typically about 10 percent of their rated max). Usually the power supply will just shut itself off, but some of the really badly designed el-cheapo ones can actually damage themselves if not loaded properly. The way most of them are designed, you need to load the 5 volt line, and the rest of the voltages regulate off of that, but there is no standard PC power supply design so yours might not work that way.

Lucky for you that you chickened out.

If you were an engineering studend I might advise otherwise, but you’ve already demonstrated just enough knowledge to be deadly. Go to ye ol local auto shop and buy yourself a battery charger. It will cost you all of about 20 or 30 bucks tops, and won’t burn down your dorm. If you absolutely insist on playing with fire, get yourself a voltage supply that puts out closer to 14 volts and use a series resistor for current limiting. If you can’t figure out what size resistor to use then you shouldn’t be trying this yourself.

3

Phew, knew there was something more to this charging thing. Scary thing is my dad said I should go ahead and try it (Lucky I don’t trust him with anything electrical)
As for electronics I know enough to follow instructions, and add the “make sure the smoke alarm works” step. With all the resistors, transistors, diodes and capacitors how can you not get confused?
I’ve got RAC membership, so I don’t need a charger - I’ll just call for a jump start each time I leave the lights on :rolleyes: hell, I don’t know why I even need a battery! Hows that for an anti-theft device?

Oh, and I’ll assume that the mobile phone thing is OK as a permanent tempory solution?

4

I once tried connecting a 12-volt 2-amp power supply to an SLA (sealed lead-acid) battery, to see if it would charge at all. (I must have been about 13 yrs old.) I don’t have a clear memory of what exactly happened, but it involved smoke, horrible smell of burning insulation and perhaps a flash of light. Apparently the battery has a very low internal resistance, causing lots of current to pass throug the thin wire I used. The wire heated up and burnt off the insulation. So you see, a voltage supply does not make a good battery charger. You need a current limiting circuit at the very least.

5

What about those solar powered trickle chargers you can put on your roof or whatever, to slowly charge up a car battery? I don’t think they will charge a discharged battery (they might given enough time?) but they will keep one ‘topped off.’ They don’t look like much, but they have to have some sort of zener diode and capacitors or somesuch?

A fully charged automobile battery, iirc, is 12.6 volts, if you check it immediately after charging it might read around 13 volts until the ‘surface’ charge is bled off.

6

If the battery won’t hold a charge then you need a new battery and there’s no way of getting around that. If you need a one time recharge because the battery was drained then it is fairly easy to do if you know what you are doing. I have been in that situation a few times. Pretty much any source is usable if you know how to do it.

If you have a DC source greater than 12 V you just have to make sure the Amps are limited to a figure which is safe for the battery and the supply. A resistor should do nicely.

I have done the following on several ocassions but the disclaimer first: Don’t do it unless you absolutely know what you are doing. Hell, don’t do it even if you think you know what you are doing. If you screw up you will burn the car, the house and the entire neighborhood. Ok, here’s the deal:

I run an extension cord out to the car. Connect the (white) neutral to the negative of the battery. Connect the live wire through a lightbulb (to limit the current) and a diode (to rectify) in series to the positive of the battery. Over a few hours it will give enough charge to start the car.

You have to know how to connect the diode, how to size it, how to understand the whole thing. I warned you: don’t try this at home. But what it comes down to is that a 50 cent diode is all you need to recharge a battery.

7

That circuit actually works very nicely, and a (correctly sized) light bulb is the ideal current limiter due to its positive temperature coefficient.

8

To answer the OP.

I have used a switching power supply to charge a lead acid battery.

A little background: A couple of years ago I designed the electronics for a “multipowered” refrigerator. Here’s a link to it ‘case you’re interested:

http://www.vaxicool.com/

But let me tell you something: It wasn’t easy.

The first thing I needed was a charge controller. I chose to use a commercial unit. The charge controller basically senses the battery voltage and passes current to it via a series MOSFET using pulse width modulation. (Nice, efficient technique.) The duty cycle depends on the battery voltage. I adjusted the control voltage to 13.2V.

I next needed a power supply. The specs called for 120/240 AC input over a fairly wide frequency range, so I had to use a switched-mode power supply.

So I purchased a run-of-the-mill 100 watt, 15VDC switching power supply, hooked up the charge controller & lead acid battery, and proceeded to blow up the power supply. I tried a couple other power supplies and they blew up, too. (I didn’t want to use an external current limiter for various reasons.)

Why did they blow up? Well, when you’re charging a battery from a power supply, the battery voltage “rules.” In other words, when the MOSFET in the charge controller was in the “on” state, the battery was basically connected directly to the output of the power supply. So now there’s a battle going on: The power supply vs. the battery. The power supply wants to be at 15 V, and the battery is at a voltage determined by how much charge it has. So who wins this battle? Answer: The battery.

So what happens to a power supply when you force its output to be something it doesn’t want to be at?

It depends on the output circuitry of the power supply, in particular how it “deals” with an overload condition. (I found this out the hard way.)

You see, when a regulated power supply is adjusted for 15 V, and you hook a beefy lead acid battery to it that is at (for example) 11 V, you basically have an overload/overcurrent from the power supply’s point of view. So how does the power supply handle this? Most (good) linear supplies go into “constant current” mode when you do this. In other words, most linear supplies will “drop their voltage” (i.e. limit the current) to match the battery’s voltage and deliver the maximum current that it is capable of delivering. This is a good thing.

Switching supplies don’t tend to do this, simply because they’re usually never asked to do this. They tend to either shutdown or blow up.

So what I needed was a switching supply that behaved like a linear supply.

I spent weeks calling every power supply manufacturer and distributor in the U.S. to ask if they had a switching power supply that would not blow up or shutdown in overload/overcurrent condition. Only one manufacturer said that their switching power supplies “behaved like linear supplies when in overload/overcurrent condition.” I responded, “Oh yea? Well I’m going to hook one up to a lead acid battery. How do you like that! So are you certain your power supply won’t fry when I do this?” They said, “We’re certain. We’ll even guarantee it.”

And they were right; their switching power supply worked perfectly when charging a lead acid battery.

Moral of the story: You can use a switching power supply to directly charge a battery, provided you have the right kind of supply and the right kind of charge controller. But is your computer’s power supply the right kind? I wouldn’t bet on it.

9

I figured since this thread was already discussing charging car batteries, I might be able to sneak in another question:

When “jumping” another person’s car, I was always taught that one of the cables should be connected to a metallic part of the car not touching the battery. This is to ground it, I guess.

However, without fail, everytime I’m helping someone out & I proceed to attach one of the cables to a metal part, someone always “corrects” me. I’ve been told so many times that all four ends of the cables should be connected to the battery terminals. Now this does actually work, and the car does start, but what are the dangers of doing it this way?

10

If I understand it correctly, you connect the last cable to the car frame to prevent igniting and flamable vapours that may be coming from the battery. Batteries release hydrogen under the right condintions, charging I think, not sure.

I used to use the car frame for the last connection, but have gotten lazy and now just connect to the battery. I’ll let you know if I get blown up.

11

Yes, you want the last connection to be a frame/chassis connection.

A spark usually occurs when the last connection is made. Because this spark occurs at the location of the last connection, you want this connection to be away from the battery. For obvious reasons.

12

My first experience of trying to charge my car battery involved a lapse of conversation, a smell of burining and a large spanner-shaped burn mark in my desk’s surface.

13

You should be afraid when hooking up battery cables. Usually the positive + connector cable is red, but don’t believe it. Double check that it had the + and connect those two first.

Usually the negative - is black and usually it is directly attached to the metal of the car. Therefore, you can hook frame to frame (or bumper to bumper).

When hooking up correctly, you will often see a spark. When hooked up backward, you often see a bigger spark. There can be hydrogen around a battery that can explode If you’re unlucky enough, hooking it up backward will cause the battery to boil inside and explode. Explosions tend to blow boiling sulfuric acid everywhere. Many blindings have occurred.

Avoid hooking directly to the battery. If it explodes, you’ll be somewhere else. Search www.cartalk.com ‘jumper cables’ for more info.

14

I should have included this when referring to cartalk.com