How do I step voltage down with low losses, weight and size?

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How do I step voltage down with low losses, weight and size?

Post by John in CR » Feb 03 2020 5:40pm

Now that I'm switching all of my ebikes to Nucular controllers, I want to take advantage of the controller's ability to handle charging using the motor phase coils with any DC supply below pack voltage, with setting in the controller for charge profile, current and voltage limits. I'd like to simply rectify AC current from any 110V wall plug. While rectification gives me AC voltage/sqrt2 as the DC output, that cuts it a bit too close in terms of voltage limits with a 21s pack, since I don't trust the power company here for a stable 110V AC supply.

Is there an easy way to step down either AC or DC voltage in a cheap, small, light, and dead on reliable way that doesn't create too much heat. Whether it's a fixed voltage reduction of say 10V or a % reduction doesn't matter to me. I just want something that's small and resistant to vibration that I can add to a 4 diode rectifier giving me an onboard charger, so I only have to carry an extension cord, but still have a 10A-15A onboard charger.

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Re: How do I step voltage down with low losses, weight and size?

Post by thepronghorn » Feb 03 2020 6:53pm

You need to worry about the peak of the AC waveform, 110*sqrt(2)=156V, being under your battery voltage, so you need a bit more voltage step down than you think. I also doubt the controller will be very happy with pure rectified AC as its voltage source, so you will need to add some capacitance on the output of your rectifier.

With this non-isolated charger topology, I would strongly recommend some isolation monitoring to help make sure you do not electrocute yourself.

I don't believe the Nucular controllers really enable what you are looking for at this time. While charging direct from rectified AC is possible, you need a buck converter for battery voltages below the peak of the AC waveform like what farfle was working on a couple years back.

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Re: How do I step voltage down with low losses, weight and size?

Post by billvon » Feb 03 2020 8:33pm

John in CR wrote:
Feb 03 2020 5:40pm
Is there an easy way to step down either AC or DC voltage in a cheap, small, light, and dead on reliable way that doesn't create too much heat.
Fixed ratio resonant switchmode controller (Vicor has these)
Charge pump divider (very simple and efficient, but no isolation)
--bill von

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Re: How do I step voltage down with low losses, weight and size?

Post by john61ct » Feb 03 2020 9:58pm

Yes I'd look at Vicor 100-200V input series,

do you think 48V output would be OK?

maybe VI-254

stacked with VI-B54s to get the amps you want

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Re: How do I step voltage down with low losses, weight and size?

Post by John in CR » Feb 04 2020 4:48am

billvon wrote:
Feb 03 2020 8:33pm
Fixed ratio resonant switchmode controller (Vicor has these)
Charge pump divider (very simple and efficient, but no isolation)
john61ct wrote:
Feb 03 2020 9:58pm
Yes I'd look at Vicor 100-200V input series,
do you think 48V output would be OK?
maybe VI-254
stacked with VI-B54s to get the amps you want
I think you guys are missing what I'm trying to do. I just need to supply less than pack voltage for my controller to act as a charger. I'm unclear still if it's pack voltage before charge or max pack voltage. It's a 21s pack and I usually charge to 85V and rarely for long trips to 88V. Assuming pack charge cutoff voltage is my upper DC supply voltage limit for the system, then it seems like I can just take the standard 110V AC and using just 4 power diodes I get 78V DC, AC voltage / sqrt2 = DC voltage. If I could trust the power company here that would be fine, but I've seen 117V AC before, so no telling what the max might be. That makes me think I should use some kind of voltage regulator as protection. eg I see a tiny multi-kw regulators on Ebay for $25 or less, but I'd like something even more simple and sure to be vibration resistant.

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Re: How do I step voltage down with low losses, weight and size?

Post by serious_sam » Feb 04 2020 5:29am

The AC voltage is the RMS voltage, and the peak voltage is what you will see if you rectify to DC. So the rectified DC will be:

110VAC x sqrt(2) = 110 x 1.41 = 155.56VDC

(multiply, not divide by sqrt2).

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Re: How do I step voltage down with low losses, weight and size?

Post by John in CR » Feb 04 2020 5:52am

serious_sam wrote:
Feb 04 2020 5:29am
If your AC is 110V, then that is the RMS voltage, and the peak to peak voltage is what you will see if you rectify to DC. So the rectified DC will be:

110VAC x sqrt(2) = 110 x 1.41 = 155.56VDC

(multiply not divide by sqrt2).
I had to look it up, but you're half right and half incorrect. It is in fact 110V RMS, so that sucks. RMS is the DC equivalent, so what comes from the wall will rectify to 110V DC.

Maybe there's some way to do a simple voltage divider since I run dual controllers into a 6 phase motor that has 2 electrically separate sets of 3 phases.

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Re: How do I step voltage down with low losses, weight and size?

Post by serious_sam » Feb 04 2020 7:21am

John in CR wrote:
Feb 04 2020 5:52am
I had to look it up, but you're half right and half incorrect. It is in fact 110V RMS, so that sucks. RMS is the DC equivalent, so what comes from the wall will rectify to 110V DC.
No mate.

AC RMS is equivalent to the DC voltage that would produce the same average power dissipation. But it is not the peak DC voltage that you will see.

What we're talking about here is the maximum voltage that you can feed into the Nucular controller to charge the battery pack. The peak DC voltage that all components must be sized to handle. The peak DC voltage that you will see is VAC RMS x 1.41.
John in CR wrote:
Feb 03 2020 5:40pm
While rectification gives me AC voltage/sqrt2 as the DC output,
That was from your first post. You were on the right path, but divided instead of multiplied.

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Re: How do I step voltage down with low losses, weight and size?

Post by Punx0r » Feb 04 2020 10:25am

John in CR wrote:
Feb 04 2020 5:52am

Maybe there's some way to do a simple voltage divider since I run dual controllers into a 6 phase motor that has 2 electrically separate sets of 3 phases.
This is seriously ghetto, but if you put a single dioide across the mains (half wave rectification) you will get (110 x 1.414)/2 = 77.8VDC

...but it will be lumpy as hell (smoothing caps required) and you'll only get 50% of the possible power rating of the mains supply.

85VDC for a (hopefully) worst-case 120VAC input (less ~1V for drop across the diode).

I suspect you might be able to feed each controller half of the rectified AC wave and then parallel the outputs, as you suggest above, to give ~78VDC at full outlet power.

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Re: How do I step voltage down with low losses, weight and size?

Post by fechter » Feb 04 2020 10:41am

I would look for a 48v server power supply. There are many sizes to choose from.
"One test is worth a thousand opinions"

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Re: How do I step voltage down with low losses, weight and size?

Post by John in CR » Feb 04 2020 11:38am

Punx0r wrote:
Feb 04 2020 10:25am
John in CR wrote:
Feb 04 2020 5:52am

Maybe there's some way to do a simple voltage divider since I run dual controllers into a 6 phase motor that has 2 electrically separate sets of 3 phases.
This is seriously ghetto, but if you put a single dioide across the mains (half wave rectification) you will get (110 x 1.414)/2 = 77.8VDC

...but it will be lumpy as hell (smoothing caps required) and you'll only get 50% of the possible power rating of the mains supply.

85VDC for a (hopefully) worst-case 120VAC input (less ~1V for drop across the diode).

I suspect you might be able to feed each controller half of the rectified AC wave and then parallel the outputs, as you suggest above, to give ~78VDC at full outlet power.
The controllers regulate the current and cutoff, so the idea isn't ghetto to me, since I'm not trying to get a maximum current charge. I just want something that takes up almost no space and is vibration proof.

I think the divider thing and both controllers might work. I'd just have to ensure I turn both on before plugging to the wall. I believe the biggest challenge would be that they both cut off at exactly the same time, so no way one could see high voltage.

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Re: How do I step voltage down with low losses, weight and size?

Post by John in CR » Feb 04 2020 11:46am

fechter wrote:
Feb 04 2020 10:41am
I would look for a 48v server power supply. There are many sizes to choose from.
You have a lot more faith in those being vibration and weather resistant than I do. Plus to me cost and size are show stoppers compared to some diodes , resistors, and capacitors.

Could Punxor's smoothing caps be small and cheap and reliable, so I could just feed one or both controllers with a half bridge rectifier to ensure I keep voltage low enough? Since the DC gets fed through 2 phases of the motor, won't the inductance of those coils help slow the flow and smooth it out too? How do I compute what caps I need with plenty of safety margin for durability?

Help me think outside the box here guys, and pick up some slack where my electronics knowledge fall woefully short. I gotta believe there's something cheap small and reliable to pull this off, since turning AC into DC is so simple, I'm not too far off in voltage, and I have 2 things I can put into use.

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Re: How do I step voltage down with low losses, weight and size?

Post by john61ct » Feb 04 2020 12:48pm

Your component-level skillz far surpass mine, to the point that, until an expert demonstrates success, and only after thorough testing by multiple idjit noobs, it all looks ghetto to me.

I'm not saying the Vicor converters are the only way, nor the best and certainly not the cheapest

but they are very small / dense, professionally designed to do exactly what you're asking for, vibration rated and very well engineered to milspec / aerospace standards.

The MW HLG series would be my other reco, but yes, including the rectification makes for a bit lower density.

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Re: How do I step voltage down with low losses, weight and size?

Post by fechter » Feb 04 2020 3:28pm

Whether you are using a full wave bridge or half wave rectifier, the peak voltage will be the same. A capacitor across the output will charge to the same level when there is no load. The power delivered will be half, but not the peak voltage.

A line frequency transformer is simple and robust but huge and heavy. I can totally understand not wanting to use one of those.

Most other "bad boy" charger schemes involve using an AC capacitor in series to limit the current but what you need is something to limit the voltage.

What about a triac light dimmer feeding a bridge rectifier and capacitor? You could get fancy and use a feedback circuit to regulate the voltage. The ripple would be horrible though. To get a relatively smooth DC, the capacitor would need to be huge.

How much charging power are you trying to get?
"One test is worth a thousand opinions"

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Re: How do I step voltage down with low losses, weight and size?

Post by John in CR » Feb 04 2020 8:56pm

Oh well, thanks guys. Now I don't see much use in this charge through the controller thing, except once I do a home based solar system with the storage with probably a 20s battery bank with an 80V or so top of charge voltage.

For a supervised opportunity charge deal, I'll go with a full bridge rectifier and whatever couple of components the DoctorBass came up with as simple super dangerous charger some years ago and posted here in the technical section. Maybe I'll include an ammeter and a cutoff timer, so I can estimate the time to full and set the cutoff timer before that for a safety measure.

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Re: How do I step voltage down with low losses, weight and size?

Post by john61ct » Feb 04 2020 9:35pm

Please document each project best you can, to help the rest of us learn.

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Re: How do I step voltage down with low losses, weight and size?

Post by Punx0r » Feb 05 2020 5:30am

fechter wrote:
Feb 04 2020 3:28pm
Whether you are using a full wave bridge or half wave rectifier, the peak voltage will be the same. A capacitor across the output will charge to the same level when there is no load. The power delivered will be half, but not the peak voltage.
Apologies, you are right: I was looking at the *average* DC voltage. I was also wrong anyway: Vdc (Av.) = 0.45Vrms = 0.45x110 = 49.5

Sorry john, electrical theory was a long time ago...

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Re: How do I step voltage down with low losses, weight and size?

Post by John in CR » Feb 05 2020 7:17am

Punx0r wrote:
Feb 05 2020 5:30am
Sorry john, electrical theory was a long time ago...
Better than me with 0 electrical theory other than Endless-Sphere, and I chose to treat controller and chargers as black boxes to be supplied by the electron gurus...at least anything beyond how they directly affect my ebike motors and batteries. In hindsight that was a bad decision, since surely the 12 years under my belt could have had me designing and building my own stuff. I figured advances in the necessary electronics would have been far faster than the inexcusable slowness that has taken place. Now the Chinese just need to ramp up production to drive down the costs of the expensive bits in controllers and copy good stuff, so we can have $100 high power FOC controllers with variable regen braking and finely tunable torque throttle.

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Re: How do I step voltage down with low losses, weight and size?

Post by billvon » Feb 05 2020 11:14am

John in CR wrote:
Feb 04 2020 4:48am
I think you guys are missing what I'm trying to do. I just need to supply less than pack voltage for my controller to act as a charger. I'm unclear still if it's pack voltage before charge or max pack voltage. It's a 21s pack and I usually charge to 85V and rarely for long trips to 88V. Assuming pack charge cutoff voltage is my upper DC supply voltage limit for the system, then it seems like I can just take the standard 110V AC and using just 4 power diodes I get 78V DC, AC voltage / sqrt2 = DC voltage. If I could trust the power company here that would be fine, but I've seen 117V AC before, so no telling what the max might be.
As others have mentioned, rectifying AC will give you 160 volts, not 78 volts. Most 110V lines are really between 98 and 132 volts (that's what most power supplies design to) so you're not going to be able to rely on that.

For simplest/cheapest thing you could do (with some degree of safety) I would do a full wave bridge rectifier with a nonisolated buck. You'd have to build it using a controller IC, an inductor and a FET.

For even cheaper, do a full wave bridge with a big resistor after it. Then use a relay to cut off charge when you hit 88 volts. You are going to need a very big resistor, like a hairdryer coil. But if cheap is your goal - that's pretty cheap.

Or just do what everyone else does and get two used 48V Meanwell supplies and series them.
--bill von

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Re: How do I step voltage down with low losses, weight and size?

Post by John in CR » Feb 05 2020 12:38pm

billvon wrote:
Feb 05 2020 11:14am
...Or just do what everyone else does and get two used 48V Meanwell supplies and series them.
Not for an onboard charger, no space and won't hold up, and I can't get myself to pay well over $100 for an onboard charger, since my controller can be a charger too. Someone has to come up with something small, cheap, and durable to rectify and cut voltage say in half (if easier) without necessarily a fixed voltage DC supply. It's gotta be possible.

I don't know why you guys are saying DC would be higher voltage than the AC after rectification, since it's equal to the RMS AC voltage. 160 may very well be peak of AC, but not RMS.

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Re: How do I step voltage down with low losses, weight and size?

Post by Addy » Feb 05 2020 12:54pm

John in CR wrote:
Feb 05 2020 12:38pm
I don't know why you guys are saying DC would be higher voltage than the AC after rectification, since it's equal to the RMS AC voltage. 160 may very well be peak of AC, but not RMS.
biseh[1].gif
biseh[1].gif (94.19 KiB) Viewed 771 times

Even if you're not filtering the bridge rectifier output with a capacitor, the output is going to reach that peak voltage momentarily and whatever is connected to that output needs to be rated for that peak voltage.

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Re: How do I step voltage down with low losses, weight and size?

Post by fechter » Feb 05 2020 1:56pm

John in CR wrote:
Feb 05 2020 12:38pm
Someone has to come up with something small, cheap, and durable to rectify and cut voltage say in half (if easier) without necessarily a fixed voltage DC supply. It's gotta be possible.
Light dimmer + bridge rectifier + capacitor.
But the ripple will be severe and the capacitor needs to be rated for it. Something like a motor run capacitor.
You might ask the nuclear guy what happens if the DC input has a lot of ripple. Maybe it will be OK.
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Re: How do I step voltage down with low losses, weight and size?

Post by billvon » Feb 05 2020 5:03pm

John in CR wrote:
Feb 05 2020 12:38pm
Not for an onboard charger, no space and won't hold up, and I can't get myself to pay well over $100 for an onboard charger, since my controller can be a charger too.
So do that. It will likely take months, and will take a lot of changes to your charger to handle the higher voltages, and you'll have to make some significant changes to your bike as well (like locking the wheel while charging.) But if you have the skill, go for it.
Someone has to come up with something small, cheap, and durable to rectify and cut voltage say in half (if easier) without necessarily a fixed voltage DC supply. It's gotta be possible.
Sure, a halver is quite possible - but is going to take a little while to design and build, since they are not off the shelf items. It would give you DC voltages from 69 to 92 volts after the rectifier. Will that work for your battery?
I don't know why you guys are saying DC would be higher voltage than the AC after rectification, since it's equal to the RMS AC voltage. 160 may very well be peak of AC, but not RMS.
A 120 volt RMS sine wave has a peak of 160 volts. Rectifiers conduct ALL of the forward waveform, including the peak. Then the very same rectifiers prevent the 160 volts from discharging back to the mains supply. Which is why it's called peak rectification.
--bill von

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Re: How do I step voltage down with low losses, weight and size?

Post by amberwolf » Feb 05 2020 5:26pm

John in CR wrote:
Feb 05 2020 12:38pm
billvon wrote:
Feb 05 2020 11:14am
...Or just do what everyone else does and get two used 48V Meanwell supplies and series them.
Not for an onboard charger, no space and won't hold up,
oh, the meanwells will definitely hold up, if you use the sealed/potted versions like the hlg. been using one on the bottom of the trike exposed to everything (including at least two complete immersions in flash floods during my commutes) for years now as my built in charger.

but they *are* big and heavy, about the size of a hardback book and about 7lbs for the 600h (600-700w version).
Someone has to come up with something small, cheap, and durable to rectify and cut voltage say in half (if easier) without necessarily a fixed voltage DC supply. It's gotta be possible.

if it helps, here's a discussion about the basic "bad boy" charger, which is essentially what you're talking about. this type of charger has been "done to death" over the years, so there's lots of discussion out there about them, and a number of basic ev car conversions have used them, to whatever degree of success each has had (you'd have to look them up, like on evalbum or diyelectriccar forums and the like).

https://www.electric-cars-are-for-girls ... matic.html

in case it goes away someday, i'll quote it below.

Battery Charger Schematic
bonn_charger.gif
bonn_charger.gif (4.04 KiB) Viewed 745 times
Interview with Lee Hart
bonn charger schematic

I'm talking to Lee Hart, a well-respected electrical systems adviser in the EV community, about his "bad boy" battery charger schematic, who recommends teaching your bad boy a few manners before you turn him loose on your expensive batteries.

Lynne: I'm looking at this battery charger schematic you contributed to the EV discussion list.

Lee: That's the "Bonn charger" schematic, invented by Don Bonn.


What's a bad boy charger, anyway?
extension cord

The classic "bad boy" charger is a bridge rectifier and enough extension cords to serve as resistor to limit the current so it won't melt cords or trip circuit breakers. From the 120vac line, they are generally useful for 96v to 132v packs. At 96v or less, the current is so high that it destroys things. At 132v or above, it won't quite fully charge the batteries.

Extension cords? You mean you just add a bunch of 100 ft. extension cords for resistors?

That's what they do. Understand, this is not my idea; it's just what some people have done. Ever seen the "Red Green" Show? It's their style.

How many extension cords do you think would do the trick?

Don't do this, okay...the resistance is completely unpredictable. For example, if you coil up the cord, it can melt. Getting the picture? Too many things can go wrong for this to be practical!

I know I shouldn't, but what can I say? I have this well-known weakness for bad boys.

I'm not saying you can't have a long, happy relationship with a bad boy. I'm just saying that if he's gonna be a useful charger, you're going to have to civilize him a little bit. This means adding:
fuses

A fuse or circuit breaker so you don't fry something;
A timer to automatically turn it off;
A voltmeter and an amp gauge so you can see what you're doing;
Some kind of "controllable impedance" so you can set the maximum charging current.

Elaborate on that last one, please?


Types of "controllable impedance" you can use in your charger

An "impedance" is anything that acts like a resistor. It can be an actual resistor, an inductor, a capacitor, a transformer or autotransformer, a phase-controlled rectifier, or any combination of these.

A resistor can certainly act as a resistor; ) It's the cheapest, and also the least efficient. It converts the voltage difference between the AC line and your car's battery pack voltage into heat...so the result is that it gets hot!

That might be a drawback.

But it would work, if you could deal with that issue.

Next are inductors and capacitors, which limit the current by shifting the phase between the AC voltage and current. They go in series with the AC input to the bridge rectifier. These don't get nearly as hot, but they make the power factor worse.

Meaning?

You can't draw as much charging current from the AC outlet. Let's take these one at a time:

An inductor is a fist-sized coil of wire on a transformer-type E-I core. As I said before, it goes in series with the AC input to the bridge rectifier. Inductances of about 0.1 to 2 millihenries are about right. If you're making one, includes some taps so you can easily adjust the inductance to control the current. You can also adjust the inductance by stacking all the core's E's in one stack, all the I's in another, and adjusting the size of the gap between them with paper or cardboard shims.


A capacitor is a big AC-rated metal can of about 10-100 microfarads. They are generally AC motor "run" capacitors rated at 360vac or more. Word of caution: Don't use an "electrolytic" or "starting" capacitor!

Why not?

The capacitor will burn up. It might even explode.

There are two kinds of big capacitors: electrolytic "starting capacitors", which are only used for a few seconds at a time to start the motor, and "run" capacitors (usually plastic film, or an oil-filled metal can with paper and foil). You want a "run" capacitor rather than a "starting capacitor".

Capacitors will be bigger and cost more than inductors, but they're easier to find used or surplus. They can't be adjusted, so you generally need a collection of capacitors that you can wire in series/parallel combinations to get the desired current.


Then there's autotransformers.
variac undressed

Autotransformer...is that the same thing as a variac?

Yes, a variac is a type of autotransformer that happens to be adjustable with a knob instead of my switching between taps.

An autotransformer is somewhat like an inductor, but has a better power factor and can step the voltage up or down. With the proper one, any battery pack voltage can be charged from any AC line voltage. These are sometimes called "third world" chargers.

"Third world" chargers?

"Third world" battery chargers are just simple chargers that actually work reasonably well, but are built using old or cheap technology.

I thought they were called that because of the ability to switch back and forth between incoming voltage lines.

That too.

Hey, that sounds pretty useful!

Yeah. On any autotransformer, you can switch back and forth between 110 and 220v lines by picking the taps on the autotransformer.

What's "picking the taps" mean?

Choosing which taps you want to use. A "variac" is an adjustable autotransformer that makes this very easy.

Oh. I get it, now. Finally. Okay, so that's it for autotransformers...

Yes. Remember on these that the efficiency and performance are good, but the downside is that the autotransformer is big and heavy.

How big and heavy are we talking, here?

About 10 pounds per kilowatt.

Hm.

The output is not isolated on an autotransformer, one side of it is connected directly to the AC line. Regular transformers have an isolated output. They weigh more like 20 lbs. per kilowatt.


And transformers...
transformer rectifier

Which brings us to transformers.

Right. Next step up is to use a transformer instead of an autotransformer in your battery charger. Now you have a conventional transformer-rectifier charger, by far the most common kind commercially. They work as well as the autotransformer, but, as I said, they have the added benefit that the output is isolated. But heavy!

I read this at Wikipedia: "A failure of the insulation or the windings of an autotransformer can result in full input voltage to be applied to the output." Sounds like a battery barbecue waiting to happen! Is this a reasonable concern, or do these things give you plenty of warning before they fail?

It's possible for them to fail, but transformers and autotransformers are normally very reliable (aside from cheap junk or user abuse).

If a winding does open, it will probably give no warning. An open winding normally means less charging current, which is a safe direction. Or, a winding can short, which will generally blow a fuse or circuit breaker (assuming your "bad boy" has these).

If reliability is vital, there is a special kind of transformer called a "constant voltage transformer". They have a semi-regulated output voltage, intrinsic current limiting, and have no failure modes that cause an excess output voltage or current. Lester (for one) uses them in their battery chargers, which are very common in the golf cart industry.

And finally, you can use a phase controller for controllable impedance. This is a simple electronic circuit with SCRs or triacs. A light dimmer is a common (low power) example of a phase controller.

That sounds like a cheap, easy solution.

Yes, they are cheap and efficient, but have a terrible power factor and generate a lot of noise (radio interference).


Use a combination for better results

So is there a "best" solution?

Yes - you can combine them. Combining the above devices often produces better results than any one by itself.

For instance, an effective combination is to use the 120v-to-12v transformer out of an old 12v battery charger, a home dining room light dimmer, and a capacitor. Use the light dimmer to control the primary voltage of the transformer; it will adjust its output from 0 to 12vac. Now wire this isolated 12vac in series with the 120vac line itself. Depending on the polarity, you now have an adjustable 120 + 12 = 132vac source, or a 120 - 12 = 108vac source. Use this to power your bridge rectifier. Wire the capacitor directly across the AC line to correct the power factor somewhat. With some fiddling, this will charge your 132v pack adequately.

Awesome! Thank you, Lee.

That's all there is to it! You can see the Bonn battery charger schematic here.

Lee emphasized that he does not recommend any kind of bad boy charger for beginners, that they are a very good way to get shocked and/or to fry some very expensive equipment.

If you want to talk to Lee about his battery charger schematic, you're welcome to join the EV discussion list.



from the link of the schematic
Last Update: 3/15/06

Lee Hart wrote:

The Bonn charger (designed by Don Bonn) is a bad-boy charger with a few manners. The key addition is a big series inductor, in series with the input to the bridge rectifier. The inductor filters and limits the peak current, so it won't burn up cords. The inductor also improves the power factor, so you get more charging current out of a given AC outlet. It also has a GFCI for safety, an ammeter so you know what you're doing, and a timer to automatically shut it off.

GFCI = Ground Fault Circuit Interruptor. This is the same gadget you find in your kitchen, bathroom, or outside outlets. It is there to prevent shocks if you touch your batteries while the charger is operating.

S1 = Circuit breaker. I used a 120vac 12amp breaker which looks like an oversized toggle switch. This provides an easy way to turn the charger on/off. I used a 12amp breaker so it will trip *before* the 15amp breaker that protects the circuit, so you don't have to run down to the basement or get someone with a key to reset a tripped breaker.

S2 = Intermatic 240vac 20amp 12-hour timer. This is a mechanical timer with a knob that you can set for 0-12 hours. Set it to the maximum charging time to automatically shut off when done.

S3 = SPDT toggle switch, 120vac 15amp minimum. This switch selects between taps on the inductor, to control the charging rate. If you can find one, a multi-position rotary switch can be used instead to select more than 2 taps.

Fan = 120vac 5-10watt fan, about 3"-4" square. It's needed because D1 and L1 get hot.

C1 = 5-10uf 220vac or more motor run capacitor. This must be a film or paper/oil capacitor, not an electrolytic! It improves the power factor so you can charge at a higher current without tripping the breaker.

J1 = normal 120vac 15amp receptacle. This is a "convenience" outlet for plugging in a 12v accessory battery charger, heater, or other gadget you might need. As shown it is a switched outlet. If your GFCI includes such a receptacle, you can use it.

J2 = Anderson PowerPole connector, 15/30/45 amp size (all use the same housing). These are DC rated to 600v and the industry standard for battery connectors.

D1 = bridge rectifier, 400v 35amp minimum. If you skimp on the ratings, it will fail sooner or later!

F1 = 250vdc 15amp fuse, Bussman ABC or Littelfuse 3AB 1.25" x 0.25" ceramic-body fuse. Must be DC rated!

M1 = 0-15amp DC ammeter. Inexpensive ones are found in almost any old 12v battery charger.

L1 = inductor. The Bonn inductor is about 3.5" on a side and weighs about 5 lbs. It is a single winding of #12 wire with six connections 1-6. Inductance is as follows:

1-2 = 0.15mH
1-3 = 0.64mH
1-4 = 3mH
1-5 = 7mH
1-6 = 13mH

This provides lots of connection options. Here are the ones I've figured out (with #1 being the input):

96v pack: high=5, low=6
108v pack: high=4, low=5
120v pack: high=3, low=4
132v pack: high=2, low=3

You'll probably have to make this inductor yourself. Find a 60hz transformer of about the right weight, and take it apart. Rewind it with as much #12 wire as will fit, bringing out taps every layer or so. Reassemble it with all the "E" laminations in one stack, and all the "I" in another. Put a thin paper shim between the E and I stacks (this stabilizes the inductance so it won't change so much with current). The thickness of this paper shim will adjust the inductance, too.

You also need a good accurate voltmeter. The charging current alone is meaningless unless you also know the voltage! I use the E-meter already in my EV. You could add a cheap digital multimeter onto the charger, powering it with a "wall wart" so you don't have to keep replacing the battery.

Lee A. Hart
plenty of other bad boy charger designs and discussions out there, too
https://www.google.com/search?q=full+wa ... oy+charger

Ohbse   10 kW

10 kW
Posts: 880
Joined: Dec 26 2013 5:15pm
Location: Auckland, New Zealand

Re: How do I step voltage down with low losses, weight and size?

Post by Ohbse » Feb 05 2020 5:33pm

While I totally understand where you're coming from with wanting to roll your own, it's really, really hard to beat a second hand telecom rectifier for power density, safety, thermal throttling, longevity and bang for buck. There's a great deal of engineering in these that you can buy for pennies, relative to engineering your own without most of the safety features.

Eaton APR48
Eltek Flatpack2 HE or Flatpack S (Combine with This)
Lineage Power CP2000
Emerson/Vertiv R48-2000e3

All of these are *very* compact, 40mm high, 80-100mm wide and 250-300mm long and rated at a real 2000w of output across a huge range of input voltages, temperatures etc. They will run at rated maximums for extended periods. They weight 1.5-2kg and in my experience tolerate vibration etc pretty well. Briefly looking now I found a bunch for ~$70 usd. This combined with the Nucular to define how quickly you want to charge can provide an excellent solution, easily delivering a modest charge for everyday use but burst to the circuit limits to quick charge when required. I bought a bunch of Lineage power 1800w units for $20ea at some point for use on my Adaptto setups, long since surplus to requirements.

Some require some hackery to get them fired up, some just work right out the box.

Off topic, but after years of improving my charging - faster/smaller/lighter I completely changed direction. Now days I charge with a high voltage cycle satiator, generally at only 180w, running overnight. I have ~3.5kwh onboard the Surron and so don't require topups outside of home unless I was doing some sort of crazy road trip.

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