15A Lead Acide Battery Charger!

[Shahriar]

Hi
Someone has asked me to build a battery charger. The battry is a 24V / 150Ah from those kinds which is used in big vehicles. I think they Are called Lead acide battery!!! Am i Right?

the battery is to be charged with a max Current of 15A! I decided to charge it with a circuit like the one I have attached, because of High current for my circuit, I decided to use TWO ATX (PC power supply) 12V rail. {They are 16A Nominal}, but the problem is how do I limit the current at 15A.

I have an Idea, Tell me if it works:
There is a pin in ATX Ps which is for Stand by, And PC pull it down to Start the ATX. I want to use a .05 Ohm resistor at the output and when Over-Current Happen, Pull this Pin Up? Can I have a current Control In this Way?

Of course, I have to increase 12V rail of Two ATXs a little bit higher to creat 27V (13.5 + 13.5 Two ATXs in series) to Charge Batteries.

ThanX
Shahriar

Battery_charger.pdf

 

[indulis]

Not really. If the battery you were charging was trying to draw more than 15A, the charger would just bounce on and off. You don't want to limit the current to  15A, but regulate it to no more than 15A. Take a look at some of the apps notes from National on 5A adjustable regulators. You should find a schematic for a battery charger. Use 3 of the regulators and re-calculate the values for 24V operation.

 

[ante1]

Hi Shahriar,

You have to decide first which type of charger you like it to be. A common charger does not limit the current; the resistance of the battery sets the current and the current drops as the charge proceeds. Besides this kind there are a few other types which work in different ways. 
BTW, 27Volts is not enough to fully charge a 24Volts set of batterys.

 

[Shahriar]

Hi

I attached that file to show that my device is going to work like the cicuit I have attached, the only difference is High current for my device.

The circuit I have attached is a Current limited power supply. when The Battery wants to draw more that i.e. 1A the Output voltage drops to decrease current at 1A, when battery get charge the current decreases gradually down to Zero, thats when the battery has been charged to 13.8V, So it never get Over charged!

My circuit is same just differs in Current (15A) and voltage (27V).

Dear indulis
I didn't undersatnd the Meaning of ""  regulate it to no more than 15A ""
Can you explain a little more!!!

Dear Ante
I hope I had answered your questions!  ??? 
How much voltage is enough to charge a 24V Battery?

ThanX
Shahriar

 

[indulis]

What you just said is pretty much what I meant.

You need circuit that can operate in voltage mode and current mode, basically voltage foldback. Under certain conditions you want the output voltage to control the output current, and under other conditions you want the output current to be able to control the output voltage. At the same time you need to make sure the output current will never be more than 15A. Those

 

[Shahriar]

Dear indulis

I have attached  a file in my first message, (at the top of the Page). Its name is battery charger.PDF and has been downloaded 3 times.

In which condition the output Current Will control th output voltage? 
Like a LM317 Over-Current Protection, It is always, the output voltage that controls the current to be Under i.e. 1A!!!

ThanX
Shahriar

 

[indulis]

None of those circuits can do 15A.

If you look at the 1.2A & 2.5A chargers, there is a current sense resitor that turns on the NPN transistor connected to the "adj" pin. As the current goes up, the voltage across the resistor will increase until the NPN starts to turn on and pull down the adj pin. In this condition, the ouput current is controlling the output voltage. The LM317 itself has a internal current limit of 1.5 to 3.4 amps when  Vin - Vout is 15V or less. Not very exact!!!

 

[audioguru2]

Old-fashioned "add water" lead-acid batteries were fully charged at 13.8V.
New "maintennance-free" sealed car batteries are different and are fully charged at 14.4V.
Measure the voltage of the battery that is charging by the alternator on a new car.
I don't think anyone makes "13.8V" batteries anymore.

 

[Shahriar]

Dear indulis

I know that LM317 can not Handle 15A. I send that schematic to show how my circuit is going to charge that battery.

INSTEAD OF that LM317, I want to use a PC ATX Power supply. Just to limit the current I want to pull Up the stand By pin PC Power supply. The Only thing I want to know is That: "" Is it possible to limit current at 15A By pulling up the Stand By Pin Of an ATX Power Supply, Or that ATX will turn on and Off repeatedly??? ""

Sorry to say this, But I am writing the same sentences as I have written in my previous Post....  ??? ??? ???


ThanX
Shahriar

 

[ante1]

Hi Shahriar,

I think you are trying to make a unnecessarily complicated charger here, why do you need 15A, and why use ATX PSUs?
You need to reach 2.39Volts/cell (28.7V) before the 24V battery is fully charged and remain here for some time (based on the battery data and the current used) then you can switch to trickle charge (27.5V).

 

[Shahriar]

Dear Alun

It is a 150Ah Battery, Can you tell me how  much current do I need to charge it, and what kind of Power supply I need to Charge this battery?

ThanX in advance
Shahriar

 

[Shahriar]

Oh sorry, I meant Ante, I mis spelled it with Alun

 

[bwriter]

Just thought it worth mentioning that Lead-Acid rechargable batteries like a raw recharging supply. In other words, rectified but unsmoothed. The battery itself will have a smoothing effect on the supplied voltage but that is irrelevant. Look at the battery recharging circuitry in a vehicle and you will find no smoothing components are used. I'll see if I can dig up some more info on this for you as it's a long time since I researched this. I do remember though, that a raw DC supply was more efficient at reversing the chemical process involved.

 

[indulis]

No, you can't current limit the ATX power supply by pulling on the standby pin, as was already stated.

Pure DC is BEST for charging, it is too expensive to build/sell 20 or 30A pure DC chargers!!

 

[ante1]

Hi indulis,

I disagree; pulsed DC has a much better capability to solve and prevent the buildup of sulphate in a lead acid battery than “pure” DC!

 

[indulis]

Greetings Ante,

From the www:

Charging the lead-acid battery

The charge algorithm for lead-acid batteries is similar to lithium-ion but differs from nickel-based chemistries in that voltage rather than current limiting is used. The charge time of a sealed lead-acid battery is 12-16 hours (up to 36 hours for larger capacity batteries). With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less. Lead-acid cannot be fully charged as quickly as nickel or lithium-based systems.

It takes about 5 times as long to recharge a lead-acid battery to the same level as it does to discharge. On nickel-based batteries, this ration is 1:1, and roughly 1:2 on lithium-ion.

A multi-stage charger first applies a constant current charge, raising the cell voltage to a preset voltage (Stage 1). Stage 1 takes about 5 hours and the battery is charged to 70%. During the topping charge in Stage 2 that follows, the charge current is gradually reduced as the cell is being saturated. The topping charge takes another 5 hours and is essential for the well being of the battery. If omitted, the battery would eventually lose the ability to accept a full charge. Full charge is attained after the voltage has reached the threshold and the current has dropped to 3% of the rated current or has leveled off. The final Stage 3 is the float charge, which compensates for the self-discharge.

Much has been said about pulse charging lead-acid batteries. Some experts believe there is a benefit in reduced cell corrosion but manufacturers and service technicians are not in full agreement on the effectiveness. There are also disagreements on the "equalizing charge". An equalizing charge raises the battery voltage for several hours above that specified by the manufacturer. Although beneficial in reversing sulfation, the side effects are elevated temperature, gassing and loss of electrolyte if the service is not administered correctly. A periodic discharge of about 10% is said to benefit the battery but little conclusive evidence is available.



In doing some research for questions posted on a few golf cart forums, I discovered there are also many misconceptions about lead acid batteries

 

[ante1]

Hi indulis,

I agree with most of it, but:

” Much has been said about pulse charging lead-acid batteries. Some experts believe there is a benefit in reduced cell corrosion but manufacturers and service technicians are not in full agreement on the effectiveness.”

I hope you don’t ask your car salesman if he thinks it’s time for you to buy a new car!

I mean; why would a battery manufacturer tell you to treat your battery in some way that make it live longer? I think that would be bad business for him!

And:

“There are also disagreements on the "equalizing charge". An equalizing charge raises the battery voltage for several hours above that specified by the manufacturer. Although beneficial in reversing sulfation, the side effects are elevated temperature, gassing and loss of electrolyte if the service is not administered correctly. A periodic discharge of about 10% is said to benefit the battery but little conclusive evidence is available.”

Yes, longer periods of over voltage and/or high temperatures are damaging to the battery this is correct! That’s one reason to pulse charge a battery, a short pulse with high current and voltage followed by a longer rest will remove sulphate and still not raise the temperature.

Gassing and loss of water starts at 2.39 V / cell with a continuous charge but it is possible to go as high as 2.7 V (during pulse) when pulse charging without water loss or heat issues.

The standard rating on a lead acid is Ah/5h that means the maximum discharge current for the capacity is over a 5-hour period.

If anyone thinks it’s OK to use tap water in a battery: Buy maintenance free! ;D

 

[indulis]

Since the "battery experts" themselves can't agree as to what is best, how can we determine which camp is "right"? I suppose one could argue that if the "pure DC" camp is correct, and since the vast majority of battery chargers are of the peak-pulse variety as opposed to the multi-stage DC type, the battery manufacturers have ensured themselves a long line of customers!! One of our sister divisions here at C&D Technologies makes batteries....... I'll have to see if I can get their postion/opinion.

 

[harrychoo]

I have been study this type of charger quite a while.

Now i wish to charge 12V 7Ah SLA battery.

To charge the battery, u need a constant voltage and current source. First, u need to charge the SLA battery for 2.4V/cell. Charging current should be around 0.1C. The charging current will drop throughout the charging process.. When it reach like 10mA-40mA, u need to stop the charging process or change it to float charge which is 2.35V/cell, else it will be overcharged.

I had built the charger using LM317 and some pot. However, now i wish to do it in switching type charger.

I try to use some switching regulator like HA17384/385 or UC3844 current mode PWM to do the switching and with some opto-coupler and TL431 to do the feedback. All this i wish to do it with flyback topology.

But now i still in study stage. I had study quite some of the application notes but still I not too understand how to produce constant current source in switching type. I manage to do cosntant voltage but not constant current.

Any idea?