Battery chargers

brianl703 · Nov 8, 2004

quote:

Originally posted by 1 FMF:
A battery's resistance is very small, so by just applying 13.8V to it, the battery will draw as much current as it can like you said.

The amount of current it draws at 13.8V is determined by the size of the battery and the depth of discharge--it's not going to draw an unlimited amount of current.

You can make it draw more current, and therefore charge faster, by increasing the voltage. Sealed lead acid batteries can be charged at voltages up to 14.6V, and gassing is particularly fatal to those batteries because water cannot be added to them.

Amperage settings on a lead-acid battery charger are kind of questionable because this a battery type that works best with voltage regulated, not current regulated, chargers.

brianl703 · Nov 8, 2004

quote:

Originally posted by 1 FMF:
what do you mean about the alternator?

The 130-amp alternator in my car will happily put 130 amps @ 13.8V into the car battery if the car battery asks for it.

But the car battery never does.

The charging system in a vehicle is a voltage regulated system.

There's nothing making sure that the battery only gets x amount of current when it's fully charged, other than the battery itself.

As I said before, stick a 12V lead acid battery across a 13.8V power supply (current capacity does not matter) and if it's not charged it'll draw what current it needs to charge up and once it's charged it'll stop drawing current and the 13.8V will provide a float charge that won't damage the battery.

brianl703 · Nov 8, 2004

quote:

Originally posted by 1 FMF:

My point was in order to maintain a battery, you need a charger that is designed to do that, and they will reduce current to 1-amp or less, with voltage in the 13.2-13.8 range.

A fully charged 12V lead acid battery will draw almost no current at 13.8V. The battery itself will reduce the current as it charges up, there's no need for the battery charger to do it unless it's using a higher charge voltage.

In that case the charger SHOULD drop the charge voltage to 13.8V when the battery has fully charged, or you had best disconnect it to prevent damage to the battery.

labman · Nov 16, 2004

I have four 250 VA 120/12 volt transformers setting on my work bench, wall warts not. I see any one of them as being the heart of a serious battery charger, especially if they put out more like 14 volts. However run through a single diode, I only measure 6 volts DC. Do I need a more complicated circuit?

Ugly3 · Nov 16, 2004

brianl703 - The 130-amp alternator in my car will happily put 130 amps @ 13.8V into the car battery if the car battery asks for it.

I thought there was a limit of maybe 30 amps going to the battery.

Ugly3 · Nov 16, 2004

This is all I could find.

With the charge rate selector increased to HIGH, the charger is delivering a 40 amp current flow to the battery and voltage at the battery rises to 14.2volts. This is the rate that a powerful alternator would recharge a powerful model battery, at peak charge rate. But in the car, the voltage regulator would limit system voltage to about 14.2volts, the amps would taper off as the battery became charged.

XS650 · Nov 16, 2004

quote:

Originally posted by Ugly3:
brianl703 - The 130-amp alternator in my car will happily put 130 amps @ 13.8V into the car battery if the car battery asks for it.

I thought there was a limit of maybe 30 amps going to the battery.

There is no limit other than the maximum capacity of the alternator.

What happens is that the output voltage of the alternator is regulated to some desired charging voltage. When the battery is at the same voltage as the alternator voltage, no current flows.

When the battery is at some lower voltage, a current flows:

Current = (alt voltage - batt voltage)/ batt internal resistance.

With a battery in decent condition, that charge rate is usually reasonable. Voltage will increase fairly rapidly due to surface charge and cut down the charge rate to reasonable levels, even on a severly discharged battery.

The hundreds of millions of cars running around with voltage regulated alternators are testimony to just how well that works.

XS650 · Nov 16, 2004

quote:

Originally posted by labman:
I have four 250 VA 120/12 volt transformers setting on my work bench, wall warts not. I see any one of them as being the heart of a serious battery charger, especially if they put out more like 14 volts. However run through a single diode, I only measure 6 volts DC. Do I need a more complicated circuit?

It might not be a reasonable thing to do. If you have accuratly measured the 6 volts with a single diode, a full wave diode network will only get you 12 volts, a couple of volts shy of what you need. I suspect any circuit to get you the voltage you need would cost as much as a store bought battery charger.

brianl703 · Nov 16, 2004

quote:

Originally posted by XS650:
The hundreds of millions of cars running around with voltage regulated alternators are testimony to just how well that works.

You can also make a 13.8V UPS by taking a 13.8V regulated power supply, connecting a 12V lead-acid battery in parallel with it, and connecting your load in parallel with that.

Now, if the power fails, the load operates off of the battery. When the power comes back on, the load operates off of the 13.8V power supply which also charges the battery at the same time. This same setup can also be used when you have, say, a high-power transciever that operates from 13.8V but your power supply isn't capable of supplying the intermittent high current that it needs--in which case the battery will fill in what the power supply can't.

In fact, I do believe that telephone central offices (which use 48V lead-acid batteries) are wired up in exactly this manner, except the power supply is about 56V, and if you measure the on-hook voltage of your phone line you will find that it is indeed close to 56V, unless your phone line isn't a standard phone line (like it's on a PBX or a pairgain box) (13.8V * 4 = 55.2V)

jsharp · Nov 16, 2004

quote:

Originally posted by Gary Allan:

quote:

really? i always wonder if this would work?? will battery demand a stronger current and overload and kill the transformer?

Click to expand...

It should be a self-biased regulated power supply. it only provides whatever the ma rating is at the spec'd VDC. I think you can put a short across it (although I've never tried).

We use thousands of assorted wall warts on our products. Most of these things are just a transformer with a single diode and a small filter capacitor. They typically, but not always have a one time thermal cutout that will open if the maximum temperature for the transformer is exceeded. Easy to do if you try to get too much current from the transformer.

They're that simple, that's how they can sell them for $2...

Gary Allan · Nov 16, 2004

quote:

quote:Originally posted by Gary Allan:

quote: really? i always wonder if this would work?? will battery demand a stronger current and overload and kill the transformer?

It should be a self-biased regulated power supply. it only provides whatever the ma rating is at the spec'd VDC. I think you can put a short across it (although I've never tried).

We use thousands of assorted wall warts on our products. Most of these things are just a transformer with a single diode and a small filter capacitor. They typically, but not always have a one time thermal cutout that will open if the maximum temperature for the transformer is exceeded. Easy to do if you try to get too much current from the transformer.

They're that simple, that's how they can sell them for $2...

Not to disagree with you per se`, but I'll point out that the length and the gauge of the wire employed in these transformers basically prohibits any "over draw" of current from the demand side of the circuit. The internal fuseable link (in whatever modality they choose to employ it) is for internal short/failures ..not from demand side current draw. Otherwise why would radio shack have one of those universal wall Xformers with all those exposed multi-application jack ends ..just swinging in the breeze for the oportunity to blow the thing?? I think this is the same case when you accidentally hook up to the wrong terminals on a relay used for something like fog lamps. You can put the control wires on the load terminals ...see 12-13vdc ...yet the fog lamps will not light ...and the wire will not smoke. The main contact on the load terminal should have no internal resistance. That's the whole idea of using very thin wire for control circuits ..you don't care if they hit ground under the dash ..

This is just an assumption based on observed collected data. I'll paw through my junk and see if I can blow one of these things with a dead short. I would think that I've had plenty of opportunity to do so in the past with the way I've handled the ends over decades (especially with the "tip and ring" type ends) to have conincidentally provide a short ..yet I've never had one fail. Now it's totally possible that I've gone this far without "bumping the walls" on this thing ..and I could very well be totally wrong. If so, I'll be a more educated person after the learning.

[ November 17, 2004, 01:10 PM: Message edited by: Gary Allan ]

XS650 · Nov 16, 2004

quote:

Originally posted by Gary Allan:

This is just an assumption based on observed collected data. I'll paw through my junk and see if I can blow one of these things with a dead short. I would think that I've had plenty of opportunity to do so in the past with the way I've handled the ends over decades (especially with the "tip and ring" type ends) to have conincidentally provide a short ..yet I've never had one fail. Now it's totally possible that I've gone this far without "bumping the walls" on this thing ..and I could very well be totally wrong. If so, I'll be a more educated person after the learning.

It's a matter of the wire size in the transformer, as you saie, and the amount and quality of iron in the transformer core.

If you are going to smoke a tranformer in the interest of science....

Measure the output voltage with a couple of different loads on the tranformer and use that to figure out the internal resistance of the tranformer. If you use light bulbs, you will need to meaure current and voltage because the filament resistance changes drastically with temperature.

brianl703 · Nov 16, 2004

Might also want to make sure that the transformer doesn't overheat since they tend to get warm or even hot if you draw more than the rated amount of current for any significant amount of time.

jsharp · Nov 17, 2004

quote:

Originally posted by Gary Allan:

quote:

quote:Originally posted by Gary Allan:

quote: really? i always wonder if this would work?? will battery demand a stronger current and overload and kill the transformer?

It should be a self-biased regulated power supply. it only provides whatever the ma rating is at the spec'd VDC. I think you can put a short across it (although I've never tried).

Click to expand...

quote:

quote:Originally posted by JSharp:

We use thousands of assorted wall warts on our products. Most of these things are just a transformer with a single diode and a small filter capacitor. They typically, but not always have a one time thermal cutout that will open if the maximum temperature for the transformer is exceeded. Easy to do if you try to get too much current from the transformer.

They're that simple, that's how they can sell them for $2...

Click to expand...

Not to disagree with you per se`, but I'll point out that the length and the gauge of the wire employed in these transformers basically prohibits any "over draw" of current from the demand side of the circuit. The internal fuseable link (in whatever modality they choose to employ it) is for internal short/failures ..not from demand side current draw. Otherwise why would radio shack have one of those universal wall Xformers with all those exposed multi-application jack ends ..just swinging in the breeze for the oportunity to blow the thing?? I think this is the same case when you accidentally hook up to the wrong terminals on a relay used for something like fog lamps. You can put the control wires on the load terminals ...see 12-13vdc ...yet the fog lamps will not light ...and the wire will not smoke. The main contact on the load terminal should have no internal resistance. That's the whole idea of using very thin wire for control circuits ..you don't care if they hit ground under the dash ..

This is just an assumption based on observed collected data. I'll paw through my junk and see if I can blow one of these things with a dead short. I would think that I've had plenty of opportunity to do so in the past with the way I've handled the ends over decades (especially with the "tip and ring" type ends) to have conincidentally provide a short ..yet I've never had one fail. Now it's totally possible that I've gone this far without "bumping the walls" on this thing ..and I could very well be totally wrong. If so, I'll be a more educated person after the learning.

These small transformers are relatively safe but there have been a number of cases where the thermal links failed to open and caused the transformer case to melt and in rare cases caused fires. The thermal link is there to prevent these types of catastrophic failures, not to protect the device from failure due to overcurrent since most of the links are one time cutouts. It is possible to do it with simple overcurrent though. I've done it.

So you may or may not be able to destroy one of the small wall warts you have with a short. It depends on the device, ambient temperature, the time the output spends shorted, line voltage, the location of the short on the output cable, etc., etc. since the internal protection link is most often thermal. Of course the length of wire attached to the device will be another factor. Make it long enough and it becomes what is known as "impedance protected" and it's impossible to get enough internal heating to open the cutout with a short at the end of the output cable.

I have submitted about a half dozen of my power supply designs to UL for testing and recognition over the years so it's something I'm familiar with. In one case I had to add a thermal cutout wound into the transformer winding because one the the things UL did as part of their testing was disable my output current limiting circuitry and short the output cable. The temperature rise of the supply then exceeded the maximum allowed yet this is something I've never been able to simulate in real life...

Gary Allan · Nov 17, 2004

So using one, with your intimate knowledge of their inner workings, as a battery charger (at least large lead acid batteries ..since wall warts are often part of a NiCad recharger) is probably something you would never do. This is in spite of you never being able to wreck one in your own failure analysis.

That is, you would never recommend it ...even though it is quite possible it could serve in that capacity ..and do it well (we have drifted off of the main theme here= rewind).

Sorta like the unknowns of home brews. These people appear to like to out guess the engineers as well. "You can try it ...but I can't recommend it." type thing..

Thank you for this insight

brianl703 · Nov 17, 2004

I've seen a couple of products where a wall-wart supply was used as a lead acid battery charger, but only for small, sealed lead acid batteries in the 1.2 amp/hour to 4 amp/hour range.

In both products, leaving the battery connected to the wall-wart supply beyond the point at which the battery had fully charged was not good for the battery because the output voltage increased to as high as 15V due to not having much of a load because the battery had charged.

I once used an unregulated 12V power supply to charge a 7 amp/hour battery, and ruined the battery. There was electrolyte seeping out of the vent holes on top. At that point the 12V power supply was putting about 15V into that poor battery, but when I started charging the battery it was only putting about 13V into the battery.

Conclusion...voltage regulation is important. I have a regulated 13.8V power supply, 5 amps continuous, 7 amps intermittent, and I've used to to charge all sorts of lead-acid batteries (including car batteries) with no problem at all. (Under heavy current draw that exceeds the rated limits, this regulated power supply will reduce the output voltage, making it ideal for a battery charger since it won't kill itself trying to charge a dead battery).

jsharp · Nov 18, 2004

quote:

Originally posted by Gary Allan:
So using one, with your intimate knowledge of their inner workings, as a battery charger (at least large lead acid batteries ..since wall warts are often part of a NiCad recharger) is probably something you would never do. This is in spite of you never being able to wreck one in your own failure analysis.

That is, you would never recommend it ...even though it is quite possible it could serve in that capacity ..and do it well (we have drifted off of the main theme here= rewind).

Sorta like the unknowns of home brews. These people appear to like to out guess the engineers as well. "You can try it ...but I can't recommend it." type thing..

Thank you for this insight

Sorry Gary. I think I got off the track when we started talking about shorting the outputs of these small devices and am too long winded.

The small battery chargers are nothing more than simple "wall warts," usually even simpler because they don't have any output filter capacitor. I've got a couple around here, 100ma-500ma types that I bought years ago before I had access to them in bulk that were sold as "trickle chargers" or "battery maintainers." They work fine and there's no reason you couldn't use a generic one that was rated for the proper voltage to charge batteries.

But, the point was that they're not always safe if you get far from what they were designed for. You seldom have to worry about a dead short with battery becasue even one that we consider "dead" probably has some cell voltage that will reduce the circuit current enough to protect the supply. And as you mentioned there is the cable resistance that also helps limit the current, plus, as the battery charges the current will be reduced right up until the charger voltage and the battery voltage become equal.

There is the worry about overheating the transformer, or overcharging the battery, but I was more concened about someone trying the "make your own charger cheap" project and using a 2 or 3 amp or more supply, possibly of a higher voltage than is correct, then attaching it to a battery with a shorted cell and walking away.

We then might have a battery that's charging and possibly releasing hydrogen gas from the good cells and a nearby "charger" that's on the verge of overheating. And "that guy on BITOG said it would work" Why the H$!! did I burn my garage down...

We won't even go into what happens when someone hooks the unit up after the little internal diode and some of the windings have failed shorted and we put a 1/2 charged automotive battery that can output hundreds of amps across the secondary winding of the little transformer.

So sure, you can use those things for chargers. It pays to be careful though. That's all I'm saying...

[ November 19, 2004, 12:58 AM: Message edited by: jsharp ]

Battery chargers

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