For a simple brute force battery charger I would go for 12 volts AC at the secondary. Adding a current limiting resistor, will make it perform a little more like a current source than a voltage source. This charger would not be one that you would leave a battery connected to. Consider adding a timer so the battery won't be over charged.

This would make a good charger to recover sulfated batteries.

Yes, I agree, the transformer size is no where near 3000 VA. Still it is a good size for a battery charger.

 

Yes, I agree, the transformer size is no where near 3000 VA. Still it is a good size for a battery charger.

Just for reference, here is a 1500 VA Toroidal transformer to which I added taps so that I have a wide selection of voltages available, so i agree with Lestraveled and also doubt the assumed 3K VA rating.

For those who may not know, adding taps in the way I have, considerably de-rates Toroidal transformers. To be truly Toroidal, the winding should occupy the full 360 degrees of the core. I take this into consideration when I'm using it only for experiments.
"Proper" center tapped toroidals, have two identical windings each that use the whole core, not just a tap halfway through the winding.

 

Yea if a person wants to build a hammer down 12 volt battery charger with off the shelf parts 12/24 VAC output commercial buck/boost transformers and some heavy diodes are the way to go!

You can find those transformers in sizes from a few tens of VA up into the multi KVA range rather cheap.
I see them up into the 5+ KVA range on eBay all the time for around $200 - $300 but to be honest being they are designed for line side voltage correction systems they can take substantial overloads without issue which would allow for a person to make a200 - 400+ amp capable charger out of a 1 - 1.5 KVA unit easily enough.

 

Yes, I agree, the transformer size is no where near 3000 VA. Still it is a good size for a battery charger.

I too have to agree, I just finished using a 240/40V 1000VA toroidal, it was 7 inches across and weighed 7 kilos.

 

@recklessrog
@Marcus2012
To my transformer rewinding brothers across the pond, here is my 1500VA toroidal transformer that I retapped. I used half a role of electricians tape making it look pretty. I bought it for $25.

 

@recklessrog
@Marcus2012
To my transformer rewinding brothers across the pond, here is my 1500VA toroidal transformer that I retapped. I used half a role of electricians tape making it look pretty. I bought it for $25.

Nice that must of taken you some time to do, great bargain though less than twice what I paid for mine

 

So if I post a picture of my toroid core that I can fit my fat head through the center of would that just be showing off?

It used to be core of a reactor coil (massive high current inductor) for a 1200 amp submerged arc welder built back in the 60 - or 70's.

 

I showed you mine, you show me yours.

 

I will have to dig to see if I still have it. I may have scrapped it being I pulled the copper off it years ago.

It's inside dia would be about the same as yours is on OD and the OD was maybe just a bit larger than a 5 gallon bucket top and roughly 6 - 7 inches tall. I think I got about 50 pounds of copper off of it and core was maybe double that.

Initially I had wanted to wind a 240 volt coil set over it to make huge high current low voltage spot welding transformer out of it but it never worked out being it was designed to have very lossy inductance for arc stabilization that gave it a horrible 60 HZ AC operation characteristics. Basically it's inductive properties for AC operation were about as good as an old truck rim.

If I don't have it the next time I am down at my buddies scrap yard I will snag one of his scrap high line CT transformers. He has a few that once the HV pass throughs were knocked out are at least 18" in dia and weigh around 150 pounds bare!

Now they could make for some huge toroid transformers!

 

WOW! You're all helping this kid design something without knowing all the facts. All we've been told is that it's 3000 watts and 25 volts output. FIRST, how are those numbers derived? From a label? OK, more information from the label would be helpful. SECOND, how is it wound? Single primary? Double Primary? And its output; Single or Double?

I've read some comments about batteries drawing 200 to 300 amps from dead. Not sure I believe that, but I am not the Amazing Krespen, I don't know everything. Still, it makes sense that if a battery has an internal resistance of 2 ohms (when dead) at 12 volts it's going to draw 24 amps. I have NO IDEA where 200 comes from. Strictly speaking, a nearly completely dead battery isn't going to draw very much power. And the typical resistive characteristic of a battery is that its internal resistance goes UP when the state of charge goes DOWN. In other words, a "Stone Cold Dead" battery should have even higher resistance, resulting in an even LOWER charge rate.

I don't care if you have 1,000,000 amps available, the battery is going to draw ONLY what OHMS LAW says it will. Can you turn that transformer into a battery charger? Yes, you can. Is it practical? Just my opinion but I think not. Still, when I was a kid a very long time ago I tried to make a go-cart out of a bottle of compressed gas. I THOUGHT it'd be fast. It hardly moved at all. OH, IT MOVED! But wasn't worth the time and wasted resources.

IF the transformer in question has a double primary and a double secondary (which had been the most common form of transformer construction for a very very long time) then it would be easy enough to wire it for a 120 VAC input, and likely (I SAID "LIKELY") 12.5 VAC (all RMS values).

And someone said you have to get the voltage down to 11 or 12 volts. Yeah, that'd work. But if you're going to "Let nature take its course" (as was stated somewhere above) then to get a peak 14.4 volts AC you'd need an RMS value of 10.18224 VAC. But then you'd have a roll-off of power as the battery approached 14.4 volts. In other words, the charge rate would roll off to a trickle.

Can this be done? Absolutely. But is it worth your time? Well, maybe, if you learn something from it. But in order to learn something you need good information. Just because I built a rocket a couple times doesn't make me a rocket scientist. Just a hobbyest. But I learned something from my endeavor.

I laughed till I nearly wet myself when someone said "Get a buck converter". Yeah, like that's going to push 120 amps. Incidentally, 120 amps at 12 volts is going to be 1440 watts. Go grab a 60 watt bulb (incandescent) when it's been operating for 10 minutes. SIXTY WATTS PEOPLE. That's a lot of heat. You're talking 24 times the heat. Know what THAT will do to efficiency?

Good luck. I hope you find the answers you seek. But I'd put more trust in a professor with a PhD on the subject. Someone here may just get you burned. I hope it's not my fault.

 

One followup on what I said about internal resistance and state of charge: I've tried to recharge the "Stone Cold Dead" battery. The voltage will almost immediately jump to your Peak to Peak value. But current will be nearly nothing. As the state of charge increases the internal resistance will drop. THAT is when it begins to take more current. I can't draw you a "Sign wave, but if you can imagine, I think I can describe it: Imagine a sign wave starting at the bottom most peak and sweeping to the upper most peak. At either end of those peaks the current is almost zero because the wave is not changing. But during the change from bottom peak to top peak, in the middle is where there is the greatest change in state of charge. THAT is when the battery is drawing the most current. NOT at start up.

Just as a voltage sign wave changes states the current mirrors that change but at a 90˚ shift. A "Dead" battery will NOT draw massive current.

Oh, and just so we're clear, a battery at 80% of nominal charge is considered "Dead". So a 12 volt battery is considered dead at 9.6 volts. And a lead acid car battery - if it's "Stone Cold Dead" it's dead for life. It will NEVER recharge. EVER!

 

Uh.... what?

There's so many things wrong with those last two posts I don't know where to begin.

 

Yea, and he doesn't sound too casual to me. Anyway.....

@Casual Observer welcome to the forum. Have you thought about switching to decaf?

 

One followup on what I said about internal resistance and state of charge: I've tried to recharge the "Stone Cold Dead" battery. The voltage will almost immediately jump to your Peak to Peak value. But current will be nearly nothing. As the state of charge increases the internal resistance will drop. THAT is when it begins to take more current. I can't draw you a "Sign wave, but if you can imagine, I think I can describe it: Imagine a sign wave starting at the bottom most peak and sweeping to the upper most peak. At either end of those peaks the current is almost zero because the wave is not changing. But during the change from bottom peak to top peak, in the middle is where there is the greatest change in state of charge. THAT is when the battery is drawing the most current. NOT at start up.

Just as a voltage sign wave changes states the current mirrors that change but at a 90˚ shift. A "Dead" battery will NOT draw massive current.

Oh, and just so we're clear, a battery at 80% of nominal charge is considered "Dead". So a 12 volt battery is considered dead at 9.6 volts. And a lead acid car battery - if it's "Stone Cold Dead" it's dead for life. It will NEVER recharge. EVER!

Welcome, just was wondering what a SIGN wave is??? some sort of semaphore perhaps?

 

Casual Observer,
I don't know why you nearly wet yourself when someone mentioned a buck converter. Do you really think a dead flat battery should be allowed to charge at 120amps?

A buck converter is the most efficient way to go, and one like this will supply 15A (adjustable max current limit) at the correct float-charge voltage, and I think that's plenty for a battery charger.

http://www.ebay.co.uk/itm/BUCK-DC-D...033404?hash=item58cb2fdbfc:g:zXkAAOSwT6pV0xnY

 

WHATEVER! Y'ALL don't amaze me with brilliance. I suppose for your next project you're going to turn a 318 Diesel Cummins into a Prius. Or a Nissan Leaf. About the same effect.

Oh, and SINE. Sorry for my immense stupidity. Didn't realize there were grammar Nazi's here too. This web site (or is it sight) is bogus.

 

WHATEVER! Y'ALL don't amaze me with brilliance. I suppose for your next project you're going to turn a 318 Diesel Cummins into a Prius. Or a Nissan Leaf. About the same effect.

Oh, and SINE. Sorry for my immense stupidity. Didn't realize there were grammar Nazi's here too. This web site (or is it sight) is bogus.

I have a feeling we've seen this character before.
FYI That's not grammar pal it's vocabulary and is part of our lexicon. Oh wait.....I'm using big words sorry.

 

WHATEVER! Y'ALL don't amaze me with brilliance.
Oh, and SINE. Sorry for my immense stupidity. Didn't realize there were grammar Nazi's here too.

Well there was also this,

Still, it makes sense that if a battery has an internal resistance of 2 ohms (when dead) at 12 volts it's going to draw 24 amps.

Ohms law fail.

And this,

Strictly speaking, a nearly completely dead battery isn't going to draw very much power. And the typical resistive characteristic of a battery is that its internal resistance goes UP when the state of charge goes DOWN. In other words, a "Stone Cold Dead" battery should have even higher resistance, resulting in an even LOWER charge rate.

I don't care if you have 1,000,000 amps available, the battery is going to draw ONLY what OHMS LAW says it will.

Electrochemical reactions are not governed by Ohms law. Fail again. If Ohms law dictates a batteries charge rate then what dictates its discharge rate as in how can a common 10 Ah lead acid battery put out over 100 amps while still maintaining 11 volts or more? Double Ohms law application fail.

And this,

when I was a kid a very long time ago I tried to make a go-cart out of a bottle of compressed gas. I THOUGHT it'd be fast. It hardly moved at all. OH, IT MOVED! But wasn't worth the time and wasted resources.

Basic High School physics and math application fail.

And this,

And someone said you have to get the voltage down to 11 or 12 volts. Yeah, that'd work. But if you're going to "Let nature take its course" (as was stated somewhere above) then to get a peak 14.4 volts AC you'd need an RMS value of 10.18224 VAC. But then you'd have a roll-off of power as the battery approached 14.4 volts. In other words, the charge rate would roll off to a trickle.

Theoretically yes 10.18 volts AC would give a 14.4 volt DC output but once the additional rectifier forward voltage drops of .6 - 1 volt, and there being 2 - 4 of them, are added in plus realistic line and other resistance losses that's where a 11 - 12 volt AC input is needed to get an output that rolls off to a nice neat trickle charge by 14.4 volts is needed.

And then this mess,

I laughed till I nearly wet myself when someone said "Get a buck converter". Yeah, like that's going to push 120 amps. Incidentally, 120 amps at 12 volts is going to be 1440 watts. Go grab a 60 watt bulb (incandescent) when it's been operating for 10 minutes. SIXTY WATTS PEOPLE. That's a lot of heat. You're talking 24 times the heat. Know what THAT will do to efficiency?

Apparent no understanding of SMPS type applications and their relative efficiency of wich most senior tech people here could knock out a 1500 watt buck converter that has at least 90% efficiency and attach it to a large enough eat sink that it would not be 'As hot as a 60-watt bulb' either.

Thanks for playing! You sure showed us your intellectual and applied skills superiority there.

We are all very impressed.

 

WHATEVER! Y'ALL don't amaze me with brilliance.

You don't amaze me at all.

 

I showed you mine, you show me yours.

Looks like it disappeared. I got to thinking about it last night and I started thinking that back a few years ago a buddy of mine needed a roll of sheet steel for something and I told him about it so I suspect that either he came and got it or the two kids dad had hired for summer help that were hauling my scrap for me while I was in the oil field picked it up being that it was with a bunch my heavy chunks of specialty metals I told them not to take that disappeared in one of their scrap loads as well.

Oh well. Next time I go to work on my buddies scrap yard cranes or whatever else he broke I will pick up a few of the larger toroid CT cores he has and post a picture here!