Poor man's 15 Amp battery charger?

I found the next schematic for making a simple and cheap constant current source. Adding a capacitor in the DC part smoothes out the current. This could make a cheap charger for all kind of battery-strings. The voltage automatically adapts to a value that provides the design current. Only things needed are a non-polar capacitor (15$), a DC capacitor (almost for free) and a bridge rectifier (1,5 $).
The only essential thing that is missing is a switch that cuts off the device when the threshold voltage is reached.
I am definitely not a specialist, so could someone post the schematics for such a high voltage cut off switch, adjustable would be nice? I could use a relais to break the circuit or a solid state relais.

The circuit is working. I made one with a 40 µF capacitor and it provides 2,5A. Shorting A en B brings the voltage close to zero. Opening A en B without any load brings the voltage to the maximum of 220V. I am currently using it to desulphate some old SLA-batteries. Initially the voltage goes up to 20+ V DC, but after some time it comes down to 15V. Capacity of those batteries has somewhat improved.

Yep, looks like a "Freddy" charger. I was thinking of doing a 30amp version for charging my 40AH LiFePO4 cells from the J1772 charging stations; it would cut my opportunity charge times by 70%. But I was concerned about the power factor and whether the J1772 would reject this ugly load. Instead of a motor-run cap (non-polar), i was looking at a inductor to limit current.

If you hook it up for 15Amp, I would be curious what the waveforms look like before the AC non-polar cap (i.e. how ugly is the PF) and at the load (I've heard these supplies create some huge current or voltage spikes).

For desulfating, it looks like you are running at 120Hz; whereas, most desulfators run at 1KZhz. Like these: http://www.batterytechsolutions.com/

Let me know how your charger works as a desulfator.

Good luck

There's a thread called something liek "dangerous lightweight charger" and antoher "dirty power stealing charger" that have some info about this kind of thing, too.

where does the 15A come from? i did not see a resistor anywhere. imagine when it gets to the point people will put something like this on a big lipo pack. then the laws will change. how cheap is that?

dnmun said:

where does the 15A come from? i did not see a resistor anywhere. imagine when it gets to the point people will put something like this on a big lipo pack. then the laws will change. how cheap is that?

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lol, yup

My guess is 15A comes from the breaker in the main box, or the outlet and wiring ratings. Or maybe the rectifier diodes if using one rated for that or less, so that "hopefully" one of those things will fail just before the battery gets hot enough to explode.

Also, I missed the part about "constant current source" in there before...I see nothing that will regulate the current to a constant level (which would require changing the voltage upward as the battery charges, in a matching curve). I'd be interested in seeing a chart of real-world voltage and current measurements while this is being used to charge something (but not in *my* house!).

If you have a Cycle Analyst, you could use it along with a serial port on a computer to log the data it outputs, and have it between the load and the charger. Just make sure that whatever voltage is being output by this charger is not higher than the 100V the standard CA can take, or use the HV CA version instead. Or modify the CA's input wiring so it does not take it's power from the shunt input, and use a separate external power source for it.

dnmun said:

where does the 15A come from? i did not see a resistor anywhere. imagine when it gets to the point people will put something like this on a big lipo pack. then the laws will change. how cheap is that?

Click to expand...

I think they are suggesting that the 250 uf capacitor would limit the current to about 15 amps. (based on it's reactance of ~11 ohms, I get about 22 amps) and that the 40 uf would allow around 2-3 amps. I would not even consider this circuit, especially as it runs on 220 VAC. The capacitors have to be non polarized and would be expensive at these current and voltage ratings.

dnmun said:

where does the 15A come from? i did not see a resistor anywhere. imagine when it gets to the point people will put something like this on a big lipo pack. then the laws will change. how cheap is that?

Click to expand...

The 15A is limited by the motor-run cap on the AC side. It creates the AC impedance that limits the current. You can also use an inductor to do the same. But why limit it to 15A, when every Walgreens has a 70A J1772 EVSE? Go to plugshare.com and see how many of these stations are popping up. They're multiplying like rabbits. As soon as someone creates an "improved Freddy" charger, fast charging will be the norm.

I am sure there are better but more expensive or complicated solutions for a home made charger, but one thing is certain about this charger: it is a constant current charger and the current is determined and limited by the capacity of the capacitor. The larger the capacity, the larger the current. If you short A en B, the current will still be only 2,5A for a 40 µF capacitor, voltage will be close to zero. If you put a 50 Ohm resistor the voltage between A en B will adjust to reach the 2,5 A current. So, the ampsare limited by the capacitor and not by the other components Amberwolf mentioned. I promise it works. But when you open A and B,de voltage will reach the AC-voltage level, trying to reach the 2,5 Amp current.
You have to look at it as follows: each cycle the capacitor is fully charged twice. The capacity puts a limit on the charge the capacitor can handle. Next, the capacitor is decharged. Because of this mechanism the voltage will only reach the level corresponding to the fixed current.
The price of such a capacitor would be 10$ to 15$ as shown at the ebay links in my first message. Increasing the capacity even further would indead increase the charge current.

Check out Modular EV Power: http://modularevpower.com/
for J1772 to 240Vac adapters. So you take a high current charger and one of these adapters for fast charging your ride off J1772 just about anywhere you want to go. It would be nice if that high current charger could be a cheap modified "Freddy" charger.

This charger circuit is not constant current. As the load resistance is decreased the current will increase. If the load is shorted the bridge and series capacitor will blow.

oh details, details.



this got me thinking of my first science project when i was about 14, back in 1960. back then they wanted everybody to become a scientist so we could beat the russians to the moon. even before gingrich decided to go to the moon.

anyway, ramjets were big in all the popular science magazines so i decided i would build a ramjet for a science project, using the gas bunsen burner in the lab for the fuel source. i found a length of downspout to make the "cylinder" of my ramjet, and put the bunsen burner underneath with the tube pointing to the ceiling of the lab. no pretesting so the ignition source i rigged up from a 12V doorbell transformer with the 120V wall source applied to the 12V windings and the high side, the 120V but now really 1200V, was inside this cylinder with a big spark gap to achieve ignition so my ramjet would work.

so i went in early and was setting it up on the bench, turned on the gas and tried for along time to get the ramjet to ignite, but the igniter would never make a spark. so it never got lit.

i still remember how dejected i felt that i had failed to get my project to work, the ramjet cylinder never ever lit in spite of my pushing tons of natural gas from the bunsen burner through the cylinder into the room for about 15 minutes before i gave up.

this is back when we had floor to ceiling windows, overlooking the schoolyard where everybody was coming to school that day.

i coulda been famous. failure sometimes has been a blessing.

Samson said:

This charger circuit is not constant current. As the load resistance is decreased the current will increase. If the load is shorted the bridge and series capacitor will blow.

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I have to deny this. The only thing that happens is that the voltage between A en B drop to zero.

I think that the real popular name for tehse kind of charger was BAD BOY CHARGER

Doc

I have had good results desulphating lead batteries, with this circuit, using 24 micro farad cap at 120 volts

Us with CATION!! connect D.C side first, than A.C.

Wow, I just built one of these with a 3 dollar 8a bridge rectifier from radioshack, a 70-82 mfd cap I pulled from an old dead air compressor and for the output caps, a pair of 330uf caps out of an old psu, they are amazing. Its small, compact, silent and stays stone cold throughout the charge process, plugged it into a 1s konion group, 4.6a charge, plugged it into my 24s race bike pack, 4.6a charge. Build a 160v + headroom adjustable cutoff for it and this could be the next big thing for charging.

Farfle, I am glad that you tried it. You proved that it works. Indead, as Torqueon saidlug in the DC first and make sure the circuit does not open during operation. If that happens the voltage rises to the AC-voltage level which could be dangerous for BMS or other parts in the system or for yourself.
Can you help me with the adjustable cutoff and post the schematic for building it. I am not sure how to make it.
Thanks,

Farfle said:

Build a 160v + headroom adjustable cutoff for it and this could be the next big thing for charging.

Click to expand...

Until the AC cap fails shorted....

Then it'll be the next big thing for your fire insurance.


I did some research and these chargers do work, but if they fail there is little to stop them from damaging things. So that cutoff circuit is definitely desirable. Caps don't usually fail shorted, but sometimes they do. So...be careful.

amberwolf said:

Farfle said:

Build a 160v + headroom adjustable cutoff for it and this could be the next big thing for charging.

Click to expand...

Until the AC cap fails shorted....

Then it'll be the next big thing for your fire insurance.


I did some research and these chargers do work, but if they fail there is little to stop them from damaging things. So that cutoff circuit is definitely desirable. Caps don't usually fail shorted, but sometimes they do. So...be careful.

Click to expand...

Could a simple 20A fuse prevent disasters? Thats what these things are ment for.
I am wondering what would happen if the BMS cuts out the circuit. Then there will be 220V (for Europe) or 120V for US on the BMS. That could hurt, isn't it?

A fuse sure would be a good idea on one of these.

This setup works, and is stupid cheap, small, light, and effective. However, its an extremely dangerous way to charge batteries, especially ones that tend to go up in fireballs when overcharged.

It's not something you should 'set and forget'.

It's not something you should 'set and forget'.

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If you do, you better run.

I would also like a schematic of one finished. it can't be just 3 parts or is it?

Dan

DAND214 said:

It's not something you should 'set and forget'.

Click to expand...

If you do, you better run.

I would also like a schematic of one finished. it can't be just 3 parts or is it?

Dan

Click to expand...

Ill get some pics up on Monday, and it's more like 7 parts, you have to include the input and output wires, and the input and output fuses.

Just a photo of the charger, the fuse is not implemented yet. Here I am charging a scrap gel battery. The voltage adjusts to 14,6V. When charging 4 full batteries in series, the voltage increases to 60V. The power factor is only 0,08 for this 14,6 volt, for the 60 volt is increases to 0,25.
All components are, mainly for convenicence, mounted to a heat sink from a broken monitor, but actually only the rectifier heats up and needs cooling. This set-up provides about 2,8 Amp. The current can be increased by using a larger capacitor or putting aditional capacitors in parallel.



and zooming in

It's important to use a motor run capacitor that has a continuous rating.
I made a similar circuit for impedance testing cells. The current really does stay nearly constant.

A ground fault circuit interrupter plug from an old hair dryer would be a great addition.
Mulitiple AC capacitors and a switch could make the current selectable depending on the capacity of the source.
A solid state relay (or mechanical for that matter) could be used to turn off the power when the output voltage exceeds a preset limit. This would take a small circuit, but not too complex.

I ran this circuit through an LTSpice simulation. Using 110 VAC input and a 100 uf series capacitor and I show the RMS current through the battery for various battery voltages to be as follows.

Battery Battery
Voltage RMS Amps

• 
  70 3.46
  60 3.64
  50 3.95
  40 3.99
  30 4.14
  20 4.28

Not constant current but much better than I would have thought and good enough over the normal charge voltage range.

Another caution though. Make sure that the controller and BMS are disconnected when you charge. If the battery is bad (high impedance) or becomes disconnected from the rest of the circuit the peak voltage would destroy the controller caps and Fets. The peak voltage from a 220 VAC supply is 310V !