Constant Current Lead Acid Battery Discharger

[sa87]

I need to design a constant current Lead acid battery dis charger.
Specifications of the 12V Lead Acid Battery discharger
discharge current rating 0-8A
We need to get the constant current from the battery through out the discharge period.
I want to know about a suitable circuit diagram and parts for this design.
Can anybody help me.....

[Kelly_Bramble]

I'm not an electronics engineer, however I do know that Ni-Cd (Nickle Cadmium) Batteries discharge at a constant voltage/amperage until they are pretty much dead.

Have you considered changin the type of battery?

Also, a method to limit current is:

Constant current diode:

The simplest constant-current source or sink is formed from one component: a JFET with its gate attached to its source. Once the drain-source voltage reaches a certain minimum value, the JFET enters saturation where current is approximately constant. This configuration is known as a constant-current diode, as it behaves much like a dual to the constant voltage diode (zener diode) used in simple voltage sources.

Due to the large variability in saturation current of JFETs, it is common to also include a source resistor (shown in the image to the right) which allows the current to be tuned down to a desired value

Current_limiting_diode_schematic.png

[PinkertonD]

What voltage are you expecting as the output? A battery rated at 12v will only produce 12v for short period of time before it starts falling. It's a battery, not a magic Voltage holder. You need to start with a higher voltage than you need at the output. Then control that stable voltage to the current.

Do some research on "constant current output kit" and you will find thousands of designs and products, from DIY to final products.

[zeke]

You can use a power NPN transistor with a 3V zener diode from base to ground, a variable resistor from emitter to ground , a load resistor between +12V and the collector, and a resistor feed to the zener from + 12V .

The current is I=(Vz-Vbe)/Rv where

Vbe is the transistor base- emitter volt drop ( 0.62V) and Rv is the variable resistor.

[Kelly_Bramble]

See:

http://www.engineersedge.com/battery...view_10069.htm

[sa87]

thank you very much............

[sa87]

Value of the output voltage is not regarded. Only thing want to do is discharge the 12V lead acid battery in constant current. As the Terminal voltage of the battery is changing the output load also should be changed to keep the discharge current constant. Actually I need to know how to design a variable dc load which can dissipate constant current. Anyway thanx for your reply.

[zeke]

Value of the output voltage is not regarded. Only thing want to do is discharge the 12V lead acid battery in constant current. As the Terminal voltage of the battery is changing the output load also should be changed to keep the discharge current constant. Actually I need to know how to design a variable dc load which can dissipate constant current. Anyway thanx for your reply.

Then look at my reply; it doesn't show voltage dependence.

Or google for another circuit or go to a library and find a book of circuits if you don't have any basic understanding of electronics.

[sa87]

I need to design a battery test bench to perform capacity test for starting type lead acid batteries.
Full charged lead acid battery 12.6V and do the constant current discharge until 10.5±0.05v
According to the battery capacity discharging constant current should be able to change and these data should be able to take to computer. Timer should be included and also the voltage cutoff to stop the discharge at 10.5±0.05V. This should be automated one.
I was thinking about to do this using a switching dc load which keep the constant current through out the test when the battery voltage change. If somebody can give me a suitable switching load design to perform this it may be very helpful. Thanx

[zeke]

So you want us to design this system.

From your remarks,("I was thinking about to do this using a switching dc load which keep the constant current through out the test when the battery voltage change") you still don't understand the solution presented So..

I think you should hire an EE who understands this simple problem and can implement it.

You can get it done on the cheap by going to any university which has an engineering program and hire a 3rd year EE student.

[sa87]

So you want us to design this system.

From your remarks,("I was thinking about to do this using a switching dc load which keep the constant current through out the test when the battery voltage change") you still don't understand the solution presented So..

I think you should hire an EE who understands this simple problem and can implement it.

You can get it done on the cheap by going to any university which has an engineering program and hire a 3rd year EE student.

Actually I need a help to design it. Any way thank you very much....

[Kelly_Bramble]

I think the op needs a "Constant Current Regulator".

Many designs....

cc-regulator.gif
ireg.jpg