Simple DC Charging Circuit

Hi,

It looks to me, if you do not trigger the SCR's, all the voltage will be sitting over there and nothing elsewhere.

What is the purpose of the charger? What type of battery do you want to charge?

If you go for lead- acid batteries and forget the SCR's, or put them in a circuits that works, you will charge your battery, even without the capacitor. There is not a lot of use for it.

How your set up will react, depends on the current the battery will draw from the circuit - read: status of charge.

There is no intelligence in this schematic and unless you sit with it and watch your circuit charging, something will go wrong.

1. Your transformer, rectifier bridge, will burn out. OR if these are the strong parts:

2. your battery will be overcharged if not disconnected on time.

A overcharging battery starts boiling and runs dry.

Make sure also that you do not charge the battery with more current than advised.

Good luck. D

As reply on #2 and #3, I have to admit that I thought the SCR's were in the primary of the circuit, but that seems to be something else. I couldn't read the schematic very well. If you regulate your primary, the schematic could work as charger for a specific type of battery under permanent surveillance.

Getting pessimistic lately but if you can't tell the difference between an industrial diode and a SCR, your batts are in serious trouble. S.M.

I cannot read your block diagram so I do not know what battery chemistry or other charging you are trying to do. From what I can read of your schematic, I do not see any SCR in this schematic. What you have in your schematic is a basic full diode bridge rectifier that converts your 16.4 VAC transformer output to 16.4*√2≈23.2 VDC minus the diode forward voltage drop. The diode drop will be nominally 0.6V per diode so the voltage across the capacitor will be about 22V.

I have had a very similar problem when I try to speak Spanish. More than once I have said something I did not intend to.

Thanks so much for your patience and thanks for attempting to help a neophyte such as myself.

Also I did not know the pictures I posted would come out sooooo small.

Let me begin again.

I need a power supply to work in conjunction with a controller I already own. The two together are a charger

It's a Xantrex C-60 three stage charge controller capable of handling 12 - 55 volts DC with continuous duty cycle of 60 amps.

Here is what I hope is a better portrayal of the circuit I propose.

And here is a better flow chart.

So what do you think?

Will it work?

Is there a better way to do it?

OK now. For this current the capacitor must be in the 100000μF range or bigger, preferably at 50V. Your controller will protect battery, but if you use the transformer from your previous post, please stay near with an extinguisher handy for the first 10-15 minutes, because that's how long I BET the transformer will last, if your battery is enough discharged. (Of course if battery is fully charged this time, the show is postponed for next time) S.M.

I think you are wrong on a couple of counts here. First the C-60 charge controller is intended for use with a wide variety of rather unsteady sources, no way do you need 100,000uF,, (that's a great number for a 50A linear supply intended to supply relatively sensitive circuits) I think he could easily get a way with only 10,000uF. Secondly the C-60 charge controller is relatively intelligent, and will ramp the current up during the bulk charge portion and one can adjust the maximum allowable charge current to a level that the 50A transformer can deliver. Just how much the controller will deliver will also depend on the size of the battery being charged, if it's less than 100 amp hrs I very much doubt the controller would allow more than about 20A, due to excessive voltage at the battery.

Martin

Hi Martin. See this about what transformer we're talking about (and I'm familiar with) and this about what battery. And a max power tracking charger's voltage must not go under minimum working voltage plus it's logic depends on voltage curve to apply charging parameters. (this about capacitor value). In my posts you'll generally see I'm not making thinks up but my points are rarely understood. (LOL). S.M.

Ah, I see you are in possession of more info than I. I suspect that you are correct, with a battery bank that size the transformer will be fried in short order, unless the OP uses some sort of series resistor after the diode bridge to limit the max current, this would have the added benefit of reducing the size of the filter capacitor.

The Xantrex C60 charge controller is designed to work from solar panels, not what you are trying to do. I do not recommend you use it this way.

The C60 wakes up in the morning as the sun rises and when the solar array has enough power available the C60 connects it to the battery. As the battery voltage rises it will begin to regulate first giving a higher voltage for the absorption portion then dropping down to a lower voltage in the float portion and finally at night it will disconnect the solar panels to avoid reverse discharge.

I think you would be better off just buying a charger. IOTA makes great chargers which are all solid state and transformerless so they are small and light yet very powerful. I use an IOTA DLS12-75 charger for my battery bank which is charged by solar panels in the day and at night my charge controller connects the IOTA via a relay to keep the batteries full if they discharge too deeply.

Good Luck

John

John, the C60 is designed to work from a wide variety of sources, not just solar. In fact it can even be used with generator with remote start capability, (ie, start the generator to charge the battery bank when it becomes sufficiently discharged). It is quite flexible, one can change bulk charge rates, charge set points, absorption parameters, etc,, there is no reason he couldn't use this charge controller. Remember he's trying to save a bit of money and has this part on hand already.

Martin

I'm sorry, I was incorrect, on cannot adjust the bulk charge rate, just the set point for the bulk charge voltage. Now I believe we would have to know the size of the battery bank to determine if this controller could by used without burning the transformer.

Cheers, Martin

• Your circuit shows no SCR. You must not use any too.4 Diodes fine .
• You can remove the capacitor.Good for the System
• You should put some series resistor on the DC path to Battery-- may be variable - max say 2 OHMS
• The 2 Ohms can be inclusive of Winding resistance of TX secondary
  

KISS-- and all will be fine

You did not specify the battery chemistry, please do!

If LA, the charger (when switched on) will ruin the battery within 24 months, maybe even earlier because you have no apparent charge control.

If not LA, then the battery may explode in the fist 5 minutes.

Andy, unless I'm mistaken, his setup has this power supply feeding a Xantrex Charge controller, at least that's the way I read it.

Tom D.

OK, I didn't, but that does not mean I am right either....

I guess we wait up for some proper detailed infos.....he did not even mention type of battery either......DUUUUUHHHHHH!

Thanks for the heads up.

Thank you all for you comments!

I have received a good deal of useful information and you all have spared me from all the unnecessary excitement of a meltdown.

I know just enough to be dangerous.

SimpleMind made some reference to another thread. It's true I currently have two threads active. One trying to understand a transformer I have, supposedly out of a UPS and the other is this one. Trying to find what I had hoped would be a simple solution to a problem I have, which is living off a generator without a way to keep the batteries charged. Oh! And the other not so small issue is I haven't a lot of $$$ to throw at it.

SimpleMind thank you for pointing out that power supply circuit as proposed would have gone up in smoke!! I have done some more research and here is what I found.

The controller when bulk charging is wide open to sending all of the current available up to the limit of the controller, in this case, 60amps. The power supply will essentially see no controller while bulk charging. Herein lies the problem, most power supplies when connected to a battery will want to send a near infinite amount of current as the low internal resistance of a discharged battery will seem like a dead short to the power supply.

I don't understand though what keeps any battery charger's transformer from doing the same. I have peeked inside a number of battery chargers, in fact I have one here and it's certainly not any sophisticated piece of electronics.

So here is where I'm at.

I need to deliver ≈ 25 amps of charging current to the posts of my LA battery bank. This will of course decline as the battery's counter voltage increases.

The actual voltage at the battery post will vary dependant on the SOC and the controller in series with the power supply.

Voltage loss across everything up to the controller needs to be figured in so that the controller sees at least 15 volts. I have two possible transformers to use, one has 16.4 VAC and the other is 24 VAC.

I have a 120 volt 13 amp AC generator

The C-60 charge controller can work with 15.3 volts to 55 volts (delivering three stages of charging voltages) at 60 up to amps.

Here I do have a concern. The C-60 is a PWM controller (It pluses the charging current, truns it off and on) A DC regulator (C-60 controller) needs not to be over voltage or overshoot input voltage maximum when the charge controller goes into PWM mode the transformer may have some pretty good inductive kick.

So there is what I thought was a simple project. Is there any hope? Is anyone willing to take it on or am I looking at something that can't be feasibly done?

Once again thank you all for your selfless assistance!!

First, let me answer your question about why a transformer (not just a battery charging transformer) cannot provide unlimited current into a dead short. The resistances of the windings do limit the amount of current available to a small extent, but the major limitation is the size of the iron core itself. There is a maximum amount of energy that can be stored in the magnetic core per AC cycle. Calculating this total available energy per cycle is certainly possible but is usually not necessary to know for somebody to properly use a transformer. The maximum energy transform through a transformer should happen only during two brief conditions, charging a fully discharged capacitor and before protection circuitry opens the circuit. Some old battery chargers though were designed to use specifically this energy limitation aspect of a transformer. But this meant careful fabrication of the transformer and these transformers themselves lost a lot of power to heat.

Now to get back to your quest on assembling a usable external supply for your charger. My math on the value of the capacitor puts the capacitance instead at about 40,000 μF with a continuous draw of 50A and a ripple of only 10V to your charger per cycle of a full bridge. Seeing how robustly designed your charge controller seems to be though I agree that this may not be needed. So if you have an electrolytic capacitor lying around in the 100 ~ 1000 μF range (50V minimum) give it a try. If you must purchase a capacitor I would get at least a 10,000 μF capacitor to make it easier on your charger. I have two concerns though: (1) You should add some current limiting protection (fuse, magnetic or thermal breaker) on the output or input of this supply to prevent a mishap from growing to a disaster. (2) Ventilate this transformer, diode and capacitor chassis well enough that the capacitor does not get too warm. Heat shortens the lifetime of any capacitor.

Have fun, be safe and keep us informed.

So let me see if I understand what you're saying.

Transformers such as the one in my little 10 amp automotive battery charger were especially designed to be self limiting. This is primarily a function of the size of the iron core laminations.

I know there has been a great deal of discussion on this project and it is easy to get confused but output requirements have been down sized by half.

I only need 25 amps of charging current to the Lead Acid battery bank.

Using this formula:

I come up with 13000 µf

I looked very briefly at Digi-Key and found a 12000 µf with a working voltage of 25 volts for around $5. That is certainly workable. If I need to achieve the 13000 µf I could parallel another 1000 easily.

Yes a 25 amp bi-metal breaker on the output would be a simple an inexpensive solution but I have some concerns as to its actual performance. In practice I seldom discharge the battery bank below 50% so there is only a brief period of time when full charging current is required but even with a 50% discharged battery bank it will bulk charge briefly. I other words the battery would be fully open to the power supply. During that period of time the charging current will exceed the safe charging limits (of both the batteries and the transformer) if not controlled. A bi-metal breaker only has so many cycles in its lifetime and I afraid it would have a short life in this configuration.

Several individuals have made mention of placing a resistor in series with the secondary winding to limit current output. I don't understand how this works or what values I would be looking for. Care to comment of this.

Thanks!

fill in the missing values

http://www.the12volt.com/ohm/ohmslawcalculators.asp

I'm not sure where you got your equation or what numbers get plugged in where but I do not get the same number from your equation. I particularly do not understand where that constant (5) comes from. My formula is similar to yours:

C*dV/dt = i → C*ΔV/Δt= i → C = i Δt/ΔV Since your full diode bridge doubles the 60 Hz frequency Δt=1/(120 Hz). Your acceptable voltage drop into the Xantrex C60 charge controller is from 22V to 12V ΔV=22V-12V= 10V. Lastly your maximum continuous current is 25A = i so C≈ 20,800 μF. Remember though that this will be the maximum continuous current draw that will continue to provide 50 A from the capacitor while the bridge is OFF. I've not corrected for the actual shorter period of time that will happen because of the wave shape. So my calculated value for C is higher than needed but not by much. But likely the large voltage drop your proposed 13,000 μF capacitor will not produce an uncontrollable condition for the Xantrex C60.

As far as the proposed resistor, I suspect people proposing this resistor intend that it will be used instead of the Xantrex C60 controller to limit the peak current provided when a dead battery is connected.I anticipate that the controller's PWM charge controlling circuitry will handle this contingency.

Don't worry, this charger is self limiting at 85A(!) S.M.

That's what I thought, S.M. So if the OP intends on using the charger, no resistor is required.

Garthh

I'm good a building projects.

I understand the names and basic principles of electricty (electronics) but when it's all reduced to numbers I can get lost fast.

Thanks for the suggested web page. I placed it in my reference bookmarks.

However I'm not sure if I completely understand.

Does it mean if I have 15 volts at 25 amps

If I put .6 ohms of resistance in series with the transformer secondary winding it will be self regulating?

Confused:

"Lastly your maximum continuous current is 25A = i so C≈ 20,800 μF. Remember though that this will be the maximum continuous current draw that will continue to provide 50 A from the capacitor while the bridge is OFF."

If the maximum continuous current is 25A why must there be 50A from the capacitor while the bridge is OFF.

S.M. sometimes I have a hard time telling if you are serious or not. Remember you are the one the pointed out (extinguishers and all) the problem with an uncontrolled power supply feeding into the charge controller.

During the bulk charging cycle the C-60 controller will allow 60 amps continuously and then will shut down at 85 amps.

I have the data sheet and tech. manual but it is not legible as a .jpg within this forums format.

This is way too much charging current for my battery bank (25 amps) and most probably for the transformer in the power supply.

Isn't that what I read in previous posts? Maybe I read it and misunderstood. I just don't know.

Sorry but 50A was a brain fart on my part. The 50A number was residue left in my brain from the earlier higher current number. The capacitor will provide the desired 25A of current during the brief intervals that the diode bridge is not ON and the transformer is not providing current.

The linear unregulated power supply of transformer, diodes and capacitor is considered a voltage source and not a current source. In other words it will provide up to 25A of current to maintain the expected voltage of 17±5 volts. The current actually taken from your supply will be set by the controller during your charging process. Since your charger is used in solar and wind power management, the designers expected that there will be times that the battery cannot be charged from a lack of wind or sun. So I would not worry about scenarios where your supply voltage is less than ideal because a near dead battery has yet to come back to life.

Dear Mr tkc100

In the absence of battery amperehour AH rating i am not able to sugest anything.However i think the principle of battery charger and norml configuration of LA batttery may be understood so that u can choose/design your own circuit

Nominal LA battery cell voltage is 2 v which drops to appx 1.85v when fully discharged and raises to 2.2 volts when fully charged.Depending upon the requirement most batteries are assembed with either 3 cells to give 6v nominal or 6 cells to give 12 v nominal as can be seen in car batteries and truck batteries.

so the Dc output volt from charger is designed to give 2.2 times the number of cells

As far the charging current, 14% of AH rating is considerd such as for typical 60 AH car battery 8.4 A Dc out put is considered While starting the battery charging, the current is limited by the internal resistance of battery and back emf of battery depending upon its state of charge.If it is fully discharged it draws full load current and as the battery gets charged the nominal voltage increases and hence the back emf resulting in reduction of potential difference and correspondingly the charging current tappers down untill the battery is fully charged after which the battery is just floating.

As for the circuit is concerned a full wave single phase bridge as shown by u should be ok.Please note that the out put dc voltage will be about 90% of input ac.According to your Dc voltage required as calculated above( ie 2.2 times no of cells) you may choose the transformer secondary voltage .Regarding filter,it is not necessary to have filter for battery charging as ripple dc is sufficient.However as sugested earlier by a contributor a resistance may be insertd to avoid short circuting

There is not much point in having the capacitor in the circuit as indicated as the output will only see the battery voltage when it is connected, it serves no purpose.