Well I told her to wait till I got home and I will charge the camper batteries, my wife just took my nice 120v 30a car charger and using an uk to us adapter plugged the 120v AC charger into a 220vac.

Needless to say it blew out the transformer, full wave bridge rectifier, and the capacitor. She said it sounded like a shotgun blast and scared the *(*^$ out of her. I believe that it was the capacitor but I am unsure.

So I am out a battery charger, I went to the local "auto supply store" where I am currently working, (overseas) and they of course don't sell a 30A battery charger.

They do have transformers, full wave bridge rectifiers capacitors and many other bits and doodads.

The battery charger was an older one (no short circuit protection nor digital crap) So I think I should be able to rebuild it.
The battery charger was 110VAC in, the output was 6VDC, 12VDC and 24VDC at 12v it had 30A It had a 3A 10A 30A charging rates.
I know from the manual that it was 12VDC@30A I believe that there was 4 coils 1 primary, (1) 6v, (1) 12v, and (1) 24v coils. I do not need the 6v or 24v never used them in 30 years.

The rating paper that was on the transformer is nothing but ash so I do not know what are the ratings of the transformer,
the rectifier is rated at 60A (is this right)???
and (what I believe is a) capacitor is gone, just metal shell.

So my question are
1. what are the ratings / type of transformer do I need to get? I don't need the 6v or the 24v, The shop has various 2 coil transformers of 120X 12v, but what rating do I need to get??
2. what is the correct rating of the full bridge rectifier?? (is 60A correct??)
3. and last but not least what size and type of capacitor should be used.

Last question is if I got a transformer that was 220vac X 12vdc could I run 110 v directly in it?? would this work? so as not to blow the transformer and items again???

Thank you for your help

 

Well I told her to wait till I got home and I will charge the camper batteries, my wife just took my nice 120v 30a car charger and using an uk to us adapter plugged the 120v AC charger into a 220vac.

Needless to say it blew out the transformer, full wave bridge rectifier, and the capacitor. She said it sounded like a shotgun blast and scared the *(*^$ out of her. I believe that it was the capacitor but I am unsure.

So I am out a battery charger, I went to the local "auto supply store" where I am currently working, (overseas) and they of course don't sell a 30A battery charger.

They do have transformers, full wave bridge rectifiers capacitors and many other bits and doodads.

The battery charger was an older one (no short circuit protection nor digital crap) So I think I should be able to rebuild it.
The battery charger was 110VAC in, the output was 6VDC, 12VDC and 24VDC at 12v it had 30A It had a 3A 10A 30A charging rates.
I know from the manual that it was 12VDC@30A I believe that there was 4 coils 1 primary, (1) 6v, (1) 12v, and (1) 24v coils. I do not need the 6v or 24v never used them in 30 years.

The rating paper that was on the transformer is nothing but ash so I do not know what are the ratings of the transformer,
the rectifier is rated at 60A (is this right)???
and (what I believe is a) capacitor is gone, just metal shell.

So my question are
1. what are the ratings / type of transformer do I need to get? I don't need the 6v or the 24v, The shop has various 2 coil transformers of 120X 12v, but what rating do I need to get??
2. what is the correct rating of the full bridge rectifier?? (is 60A correct??)
3. and last but not least what size and type of capacitor should be used.

Last question is if I got a transformer that was 220vac X 12vdc could I run 110 v directly in it?? would this work? so as not to blow the transformer and items again???

Thank you for your help

The "primary" (input voltage) of the transformer should be rated for your wall power voltage, if it is 220vAC, then pick out a 220vAC primary. If 120, then get 120.
Note, if you put 2x the voltage into a transformer, you get 2x the voltage out.

The bridge rectifier rating is the maximum You can put through it. you don't want your rectifier to be your weakest link, your fuse should be the weakest link.

Your best option may be to find a used charger at a pawn shop, second hand store or eBay. I am not sure you are up for the rebuild and if it will be cheaper than buying a used one.

 

Hey Gopher,
Being cheaper than if I was in the states very true, However the only one I can find NEW here is a 40A and it is one of the wheelie jobs, here it cost over $476 USD. having one shipped from Ebay is cost of item and $100 just for shipping. Pawn shops here, hell we are lucky that we have running water sometime. There is one place that had a used one but it was over $260 and it is over 30 years old...

I can get new transformers here for between $3 and $35. They have a 120vac X 24v 200A transformer new for $45. so they are cheap here.

I have wired up transformers before, and rectifiers WHEN I know what the items should be, so this is not new to me.

As far as the rectifier goes does 60A sound correct my question is if the rectifier is over the amps of the source will this affect the function??? i.e. power output 30A but rectifier at 60A will this work the same as a 30A and a 30A

What rating capacity/type of capacitor would you recommend???

with the wall rating I understand that it should be 120 for 120 and 240 for 240.
What I am asking is if the coil is for 240, can I plug in 120v? something is saying yes but the secondary coils would be outputting 1/2 the power. I.e. 240VAC with 12v would output 6v if hooked up to 120v. Thus I would have to use the 240vac secondary winding at 24v out, to get 12v out on 120v in

The "primary" (input voltage) of the transformer should be rated for your wall power voltage, if it is 220vAC, then pick out a 220vAC primary. If 120, then get 120.
Note, if you put 2x the voltage into a transformer, you get 2x the voltage out.

The bridge rectifier rating is the maximum You can put through it. you don't want your rectifier to be your weakest link, your fuse should be the weakest link.

Your best option may be to find a used charger at a pawn shop, second hand store or eBay. I am not sure you are up for the rebuild and if it will be cheaper than buying a used one.

 

As far as the rectifier goes does 60A sound correct my question is if the rectifier is over the amps of the source will this affect the function??? i.e. power output 30A but rectifier at 60A will this work the same as a 30A and a 30A

60A is the maximum of the rectifier, as a matter of normal design most electronics are used at around half of there maximum rating. This is to keep them from overheating in use. Diodes(your rectifier), capacitors, transistors etc, are always chosen for an double or in some cases more than double of their maximum value needed in a circuit. Makes the circuit more robust and live longer.

Why would you need 30A for a battery charger? Unless it's used as a starting booster, ~5 or 6 amps is plenty to charge a car battery, any more and you risk boiling out the cells, especially if it isn't a regulating charger.

 

Thanks for clarifying about the rectifier. Short Bus.
What about the transformer and the capacitor??

We are running 2 of this type batteries in the camper
http://www.powerstridebattery.com/t...s-coach-batteries/maintenance-free-8d-battery
They are massive batteries. They give us excess of 8 - 16 hours of reserve depending on what we are using. There is no electricity readily available here outside of the cities. (even in the cities it is iffy)
It takes a large 30A charger for them.

60A is the maximum of the rectifier, as a matter of normal design most electronics are used at around half of there maximum rating. This is to keep them from overheating in use. Diodes(your rectifier), capacitors, transistors etc, are always chosen for an double or in some cases more than double of their maximum value needed in a circuit. Makes the circuit more robust and live longer.

Why would you need 30A for a battery charger? Unless it's used as a starting booster, ~5 or 6 amps is plenty to charge a car battery, any more and you risk boiling out the cells, especially if it isn't a regulating charger.

 

Hi,

Yeah i was going to say shoot for 10 amps. 30 amps is a very fast charger and you probably dont need that. I was going to build one a while back that did 30 amps but with 10 amps i just wait a little longer.

Dont short change on the diodes. If you use the highest rating they should last forever.

Diodes are often selected based on the RMS current. 30 amps DC means 30 amps RMS, but it will be less in the bridge because each diode conducts for only 1/2 the total time. That may not come out to as low as 15 amps RMS, but it certainly should not be over 30 amps.

Dont put a 24v mode on it if you dont need that. That alone could blow out the circuit if switched on by accident.

You dont really need capacitors if you are just charging a battery and there is no regulation. The pulsing rectified DC will charge the battery just fine, and there are ideas floating around that this is actually better for the battery because it sees a higher peak current for part of the time.

If transformers are cheap you might even consider a variac so that you can adjust the current as needed. The variac goes on the front end of the transformer. You should still use a transformer too though for the isolation.

 

Suprised that the primary is open and a fuse didn't protect it? Sometimes devices are "hidden" in the transformer that decects overloads. You should check for one. A lot of times, stuff breaks at the weakest link, like the external posts. For giggles, can you post a pic.

 

Thanks for clarifying about the rectifier. Short Bus.
What about the transformer and the capacitor??

We are running 2 of this type batteries in the camper
http://www.powerstridebattery.com/t...s-coach-batteries/maintenance-free-8d-battery
They are massive batteries. They give us excess of 8 - 16 hours of reserve depending on what we are using. There is no electricity readily available here outside of the cities. (even in the cities it is iffy)
It takes a large 30A charger for them.

Maybe you can give us a story about where in the world you are and what conditions you are living in.

Also, maximum loads and typical/average loads should be considered.

You want to charge a 1300 CCA at 20 amps and be charged in about 6 hours.
You should have a transformer of about 30 amps so the output voltage doesn't "sag" if you are approaching its max rating. I am suspicious of a high current transformer selling for the price you have so voltage sag is likely with a discount transformer.

Next, you need a rectifier, 2x or more over sized (by current for safety). "Over-sizing" the rectifier is just like putting only 20 psi of water pressure into a pipe that is rated to withstand 200 psi. No problem.

The capacitor should, ideally, be able to keep the rectified voltage from outputting a string of camel humps to the battery. That is, 50Hz input means you should have a full cycle every 0.02 seconds and, once the negative halfwaves are rectified, you have a hump every 0.01 seconds. Then you need to keep your 16 or so volts from falling under 12v when discharging 20 amps.
HOWEVER, you are just charging a battery and your bridge rectifier will prevent the battery from discharging as the voltage falls below the battery voltage. So, in theory, you can skip the difficulties of a capacitor.

The problems with a capacitor are heat. A cap that will keep voltage within 2v of your supply will be about 70,000 to 100,000 uF at 20 amps. you'll need a cap with a very low "equivalent series resistance" ESR to avoid those 20 amps from heating up the cap.

Remember, where in the world are you?

 

So my question are
1. what are the ratings / type of transformer do I need to get? I don't need the 6v or the 24v, The shop has various 2 coil transformers of 120X 12v, but what rating do I need to get??
2. what is the correct rating of the full bridge rectifier?? (is 60A correct??)
3. and last but not least what size and type of capacitor should be used.

Unfortunately picking a transformer for building a battery charger is not quite as simple as matching voltages and current ratings and here's the short list of why.

First off a 1:1 ratio of transformer output to battery voltage does not work and will lead to a overcharged battery.

A 12 volt battery actually charges at 14 - 15 volts DC whereas 12 volts AC has a peak voltage of ~ 17 volts which is way too high.

Next the rectifiers that turned the AC voltage into DC have a forward voltage drop of about .6 - .8 volts and some even higher at their rated current and depending on what rectification method use there will either be a single diode forward voltage drop as in a half bridge configuration or two forward drops in a full bridge rectifier configuration.

Beyond that having or not having a filter capacitor after the rectifiers makes for some peculiarities in what the actual peak charging voltage will settle in at. With a large capacitor in the circuit the transformer AC output voltage rating will need to be about 7 - 10% less than without it due to the PWM effect unfiltered rectified AC has over it being more filtered and stable.

Once you have all that figured out only then can you decide on what actual AC output voltage you need from your transformer which in a basic 12 volt battery charging application that might mean needing a transformer that only needs to put out ~ 10.5 VAC or on that is up close the the full 12 VAC or be a ~21 - 24 VAC center tapped unit depending on what rectifier configuration and filter capacitor size (if any) being used.

After all of that there is still the whole current capacity ratings you want as well. Personally I tend to shoot for a full charge in 3 - 5 hours or less based on a 150% of actual battery capacity rating. Which for a larger set like a 12 volts 100Ah group 31 type deep cycle that would require a continuous current capacity of at least 50 amps continuous duty which in transformer selection would require something in the 800 - 1000 VA rating.
To many they will throw fits over putting a 50 amp charge into battery yet in reality those types of batteries are standard application in larger vehicle and commercial equipment applications that might have alternators that can put out 100 - 200 amps so giving them 50+ amps off a battery charger is really no big deal.

My general rule of charging batteries is to let them have whatever current they want and just limit the charging voltage even if that means letting a 100 AH battery try and take 200 - 300 maps for the first 20 - 30 seconds provided the AC supply line can keep up with it without too much voltage drop, which most can't and thusly that voltage drop will work as automatic current limiter at the beginning.

Next is the realistic peak currents your system may see while cranking an engine over which can be well above the average as calculated values by a long shot being a well made transformer can easily handle peak loads of 4 - 5+ times its continuous rating for short periods.

Over the years I have built many custom high capacity battery chargers for people based around old industrial linear and unregulated power supply transformers in the 1000 - 2000 VA capacity range and surplus arc welder rectifier diodes capable of 300 - 500 amps and they have all worked well.
The bigger ones were capable of starting most I4 and small V6 engines with stone dead batteries simply due to their extremely high peak current capacities, assuming the AC supply could provide the power of course.

Last question is if I got a transformer that was 220vac X 12vdc could I run 110 v directly in it?? would this work? so as not to blow the transformer and items again???

Unfortunately no. It would burn out. You can go below a transformers input voltage rating but not much above it which for a dual range output you could use a dual primary transformer like a 115:230 VAC on a 120 VAC inpt to make a 6/12 volt or a 12/24 volt unit by switching between series and parallel on the primary side. The only way you would get a three range 6 - 12 - 24 is to use that method plus a combination of center tapped half bridge rectified secondary plus full wave rectified.
Which to honest, given all the small but important details involved if you want to keep the 6 - 12 - 24 capability you would be best off just buying a new properly built transformer designed for such a battery charger application that is wound to work with the local supply voltages. Its that or settle for a limited 2 or single range redesign of what you have.

 

Hey Gopher I am currently teaching in Addis Ababa Ethiopia.
automotive parts and cars are expensive as HE"£. A 67 VW beetle is selling for over $6000 USD when it is a bucket of bolts. The taxes here on anything automotive are not to believe. I was going to bring in my 2000 Ford windstar, valued at about $3000 USD, the taxes are 150% of the new value of the van even though it is 17 years old. Just the import tax was going to be $18,000 + shipping and shipping tax, and engine size tax. said and done over $25,000 in taxes on a 2000 ford windstar.

Thankfully I found a truck from one of the embassays, and a camper (not a motor home) from a expat cheap (tax free status) so this is why I got this.
The battery manufacture recommends 30A@4 hours X 2 batteries in parallel so it is 8 hours @ 30A

Maybe you can give us a story about where in the world you are and what conditions you are living in.



Remember, where in the world are you?

 

Hey Gopher I am currently teaching in Addis Ababa Ethiopia.
automotive parts and cars are expensive as HE"£. A 67 VW beetle is selling for over $6000 USD when it is a bucket of bolts. The taxes here on anything automotive are not to believe. I was going to bring in my 2000 Ford windstar, valued at about $3000 USD, the taxes are 150% of the new value of the van even though it is 17 years old. Just the import tax was going to be $18,000 + shipping and shipping tax, and engine size tax. said and done over $25,000 in taxes on a 2000 ford windstar.

Thankfully I found a truck from one of the embassays, and a camper (not a motor home) from a expat cheap (tax free status) so this is why I got this.
The battery manufacture recommends 30A@4 hours X 2 batteries in parallel so it is 8 hours @ 30A

Anthony Bourdain (CNN "Parts Unknown" show) just had an episode about the food of Ethiopia- along with his little history and culture lesson. Interesting place.

 

Hey keep it simple,
Sorry no photos, the neighbour who is a local took it to a "shop" to see if they can get the parts.
As far as the breaker primary fuse, I checked the breaker, it was jammed in the closed (on) position and fuesed closed probably did not trip due to gunk and sand in the housing. The contacts are actually welded closed.

Suprised that the primary is open and a fuse didn't protect it? Sometimes devices are "hidden" in the transformer that decects overloads. You should check for one. A lot of times, stuff breaks at the weakest link, like the external posts. For giggles, can you post a pic.

 

The thing is gentlemen I have to use what is available. half the time they are used parts (in working order) I do not have the luxury of running out to the "automotive store, snickering " or have a wide variety speciality electronics here. This is why I ask so many questions on how to get things to work.

 

Is that not what I am doing? if the primary coil is rated for 220v and I put in 110v, that is below the input voltage rating is it not?? so would it not work?? wouldn't the dc output be 1/2 then??
ie. a 220v primary with a 24v secondary, if 120v is applied to the primary wouldn't the 24v secondary out put at 12v???

Unfortunately no. It would burn out. You can go below a transformers input voltage rating but not much above it which for a dual range output you could use a dual primary transformer like a 115:230 VAC on a 120 VAC inpt to make a 6/12 volt or a 12/24 volt unit by switching between series and parallel on the primary side. The only way you would get a three range 6 - 12 - 24 is to use that method plus a combination of center tapped half bridge rectified secondary plus full wave rectified.
Which to honest, given all the small but important details involved if you want to keep the 6 - 12 - 24 capability you would be best off just buying a new properly built transformer designed for such a battery charger application that is wound to work with the local supply voltages. Its that or settle for a limited 2 or single range redesign of what you have.

 

Is that not what I am doing? if the primary coil is rated for 220v and I put in 110v, that is below the input voltage rating is it not?? so would it not work?? wouldn't the dc output be 1/2 then??
ie. a 220v primary with a 24v secondary, if 120v is applied to the primary wouldn't the 24v secondary out put at 12v???

Yes running it at half it rated input voltage will give you half the output voltage. The concern is trying to go the other way which I have come the find is a oddly familiar theme that comes up in these types of online discussions and my daily life while fixing stuff for others.

The other thing with running at half voltage is you are also limited to half the transformers VA capacity as well. The amp limits stay the same but at half the voltage so thusly half the capable wattage or VA capacity.

 

Hey TMCtech
I am guessing this is where the 1.4 conversion rating comes to play.

So what would be your parts list that you would use to rebuild a charger with 12vdc, @30A
What would be the ratings for your transformer
I have access to the 60A, 80A ,100A, 120A 150A 200A full wave rectifiers here
What size capacitor would you need?

What would be your best parts recommendations?

Unfortunately picking a transformer for building a battery charger is not quite as simple as matching voltages and current ratings and here's the short list of why.

First off a 1:1 ratio of transformer output to battery voltage does not work and will lead to a overcharged battery.

A 12 volt battery actually charges at 14 - 15 volts DC whereas 12 volts AC has a peak voltage of ~ 17 volts which is way too high.

Next the rectifiers that turned the AC voltage into DC have a forward voltage drop of about .6 - .8 volts and some even higher at their rated current and depending on what rectification method use there will either be a single diode forward voltage drop as in a half bridge configuration or two forward drops in a full bridge rectifier configuration.

Beyond that having or not having a filter capacitor after the rectifiers makes for some peculiarities in what the actual peak charging voltage will settle in at. With a large capacitor in the circuit the transformer AC output voltage rating will need to be about 7 - 10% less than without it due to the PWM effect unfiltered rectified AC has over it being more filtered and stable.

Once you have all that figured out only then can you decide on what actual AC output voltage you need from your transformer which in a basic 12 volt battery charging application that might mean needing a transformer that only needs to put out ~ 10.5 VAC or on that is up close the the full 12 VAC or be a ~21 - 24 VAC center tapped unit depending on what rectifier configuration and filter capacitor size (if any) being used.

After all of that there is still the whole current capacity ratings you want as well. Personally I tend to shoot for a full charge in 3 - 5 hours or less based on a 150% of actual battery capacity rating. Which for a larger set like a 12 volts 100Ah group 31 type deep cycle that would require a continuous current capacity of at least 50 amps continuous duty which in transformer selection would require something in the 800 - 1000 VA rating.
To many they will throw fits over putting a 50 amp charge into battery yet in reality those types of batteries are standard application in larger vehicle and commercial equipment applications that might have alternators that can put out 100 - 200 amps so giving them 50+ amps off a battery charger is really no big deal.

My general rule of charging batteries is to let them have whatever current they want and just limit the charging voltage even if that means letting a 100 AH battery try and take 200 - 300 maps for the first 20 - 30 seconds provided the AC supply line can keep up with it without too much voltage drop, which most can't and thusly that voltage drop will work as automatic current limiter at the beginning.

Next is the realistic peak currents your system may see while cranking an engine over which can be well above the average as calculated values by a long shot being a well made transformer can easily handle peak loads of 4 - 5+ times its continuous rating for short periods.

Over the years I have built many custom high capacity battery chargers for people based around old industrial linear and unregulated power supply transformers in the 1000 - 2000 VA capacity range and surplus arc welder rectifier diodes capable of 300 - 500 amps and they have all worked well.
The bigger ones were capable of starting most I4 and small V6 engines with stone dead batteries simply due to their extremely high peak current capacities, assuming the AC supply could provide the power of course.



Unfortunately no. It would burn out. You can go below a transformers input voltage rating but not much above it which for a dual range output you could use a dual primary transformer like a 115:230 VAC on a 120 VAC inpt to make a 6/12 volt or a 12/24 volt unit by switching between series and parallel on the primary side. The only way you would get a three range 6 - 12 - 24 is to use that method plus a combination of center tapped half bridge rectified secondary plus full wave rectified.
Which to honest, given all the small but important details involved if you want to keep the 6 - 12 - 24 capability you would be best off just buying a new properly built transformer designed for such a battery charger application that is wound to work with the local supply voltages. Its that or settle for a limited 2 or single range redesign of what you have.

 

What I was thinking was to purchase a 220v coil with a 12 and 24v secondary windings.
This way if I have 220v wall power I would use the 12v coil
Then If I had 120v via a switch, I would switch up to the I would use the 24v coil.
This way I still would have 12v on either way. And wire in a LED to tell me which voltage/ coil I am on.

Would this work?

So instead of 30A like on 220v I would be getting 15A at 110v is what you are saying
That would work too. for peak charging the batteries or for charging longer. 16 instead of 8 that would be ok too

Yes running it at half it rated input voltage will give you half the output voltage. The concern is trying to go the other way which I have come the find is a oddly familiar theme that comes up in these types of online discussions and my daily life while fixing stuff for others.

The other thing with running at half voltage is you are also limited to half the transformers VA capacity as well. The amp limits stay the same but at half the voltage so thusly half the capable wattage or VA capacity.

 

Tcmtech
What parts would you recommend if you were rebuilding this unit?

I know now the reason for the capacitor on the charger. it is for the Camper electronics if you are charging the battery at the same time as using the charger. it is to smooth the spikes (wave) so as not to damage the electronics on the camper.

Unfortunately picking a transformer for building a battery charger is not quite as simple as matching voltages and current ratings and here's the short list of why.

First off a 1:1 ratio of transformer output to battery voltage does not work and will lead to a overcharged battery.

A 12 volt battery actually charges at 14 - 15 volts DC whereas 12 volts AC has a peak voltage of ~ 17 volts which is way too high.

Next the rectifiers that turned the AC voltage into DC have a forward voltage drop of about .6 - .8 volts and some even higher at their rated current and depending on what rectification method use there will either be a single diode forward voltage drop as in a half bridge configuration or two forward drops in a full bridge rectifier configuration.

Beyond that having or not having a filter capacitor after the rectifiers makes for some peculiarities in what the actual peak charging voltage will settle in at. With a large capacitor in the circuit the transformer AC output voltage rating will need to be about 7 - 10% less than without it due to the PWM effect unfiltered rectified AC has over it being more filtered and stable.

Once you have all that figured out only then can you decide on what actual AC output voltage you need from your transformer which in a basic 12 volt battery charging application that might mean needing a transformer that only needs to put out ~ 10.5 VAC or on that is up close the the full 12 VAC or be a ~21 - 24 VAC center tapped unit depending on what rectifier configuration and filter capacitor size (if any) being used.

After all of that there is still the whole current capacity ratings you want as well. Personally I tend to shoot for a full charge in 3 - 5 hours or less based on a 150% of actual battery capacity rating. Which for a larger set like a 12 volts 100Ah group 31 type deep cycle that would require a continuous current capacity of at least 50 amps continuous duty which in transformer selection would require something in the 800 - 1000 VA rating.
To many they will throw fits over putting a 50 amp charge into battery yet in reality those types of batteries are standard application in larger vehicle and commercial equipment applications that might have alternators that can put out 100 - 200 amps so giving them 50+ amps off a battery charger is really no big deal.

My general rule of charging batteries is to let them have whatever current they want and just limit the charging voltage even if that means letting a 100 AH battery try and take 200 - 300 maps for the first 20 - 30 seconds provided the AC supply line can keep up with it without too much voltage drop, which most can't and thusly that voltage drop will work as automatic current limiter at the beginning.

Next is the realistic peak currents your system may see while cranking an engine over which can be well above the average as calculated values by a long shot being a well made transformer can easily handle peak loads of 4 - 5+ times its continuous rating for short periods.

Over the years I have built many custom high capacity battery chargers for people based around old industrial linear and unregulated power supply transformers in the 1000 - 2000 VA capacity range and surplus arc welder rectifier diodes capable of 300 - 500 amps and they have all worked well.
The bigger ones were capable of starting most I4 and small V6 engines with stone dead batteries simply due to their extremely high peak current capacities, assuming the AC supply could provide the power of course.



Unfortunately no. It would burn out. You can go below a transformers input voltage rating but not much above it which for a dual range output you could use a dual primary transformer like a 115:230 VAC on a 120 VAC inpt to make a 6/12 volt or a 12/24 volt unit by switching between series and parallel on the primary side. The only way you would get a three range 6 - 12 - 24 is to use that method plus a combination of center tapped half bridge rectified secondary plus full wave rectified.
Which to honest, given all the small but important details involved if you want to keep the 6 - 12 - 24 capability you would be best off just buying a new properly built transformer designed for such a battery charger application that is wound to work with the local supply voltages. Its that or settle for a limited 2 or single range redesign of what you have.

 

Regarding the Filter Capacitor I just got word back from a motor home forum. The reason that the charger has a filter capacitor is to try and smooth the signal/ wave and prevent damage to the Camper electronics ( i.e. power distribution circuits, heater, refridg, radio ETS...) if the battery charger is plugged in and charging the batteries WHILE at the same time using the electronics in the camper.

If there is not the filter capacitor it "can" blow the circuit boards in the camper.

They said that the capacitor "should" be large enough to smooth the rated output amps.

 

I forgot to mention there is what appears to be a voltage regulator bolted to the side of the charger. a silver box with 4 wires out of it at the bottom of the box. Would this be a 30A voltage regulator and why???

This charger came with the camper and in 5 years has never let me down I like it and would like to see it up and running again.