Controlproblem in a fuell cell.

[zodian]

Hi everybody.
Im danish, but writing you in english since my swedish is a little poor and to avoid confusion.

Im currently working on a projekt conserning fuell cells... My problem is that depending on the working load drawn from the fuell cell stack, the current is varying from about 3 A and 30 V to about 70 A and 20 V. I need to charge at 24 V ( two 12 V batteries) and my question is how to transform the varying current into a "stable" 24 V???

Hope you are able to help me... Im not very good at electronics... Answers in Swedish are also welcome.

Thanks. Zodian.

[victor_passe]

Look for a switching voltage regulator with a adjutable voltage that can take from 10V to 40V in its input.

Here are 2 suppliers:
http://www.maxim-ic.com/
http://www.linear.com/

[dar303]

A charging controller for home made wind or water-power could also work, try google!

[zodian]

For Victor_passe: Is that also what is called a "inverting regulator?"

For dar303: I have already tried to search the web for that solution. I (think) i found out that a charging regulator for a small wind turbine for use on boat or other places, could only receive 12, 24, 36 or 48 volts, but not vary between the currents...

[victor_passe]

The current that flows from the cells depends on the load.
The charger wont use more current then it needs at that voltage.

And I am not shure but i think that a switching regulator is a inverting regulator.
Or a inverting regulator is a regulator that makes.. say 12V to -12V

[zodian]

Ok... On the links you (victor_passe) gave me there is a load of switching regulators and i have quite a problem of finding out which one to use... Do i have to be worried about frying the regulator at 70 A?

[victor_passe]

If you can charge the lead cells with 70A then you need to pick a regulator that can handle 70A.
But if you only can charge your lead cells with 4A then the regulator only need to be capable to handle 4A.

Just because the feul cells can give 70A at 20V dosent meen that you need to use all the power.

If the charger is designed to charge with 2A the feul cells wont push 70A in to the charger.

[blueint]

1) "load drawn from the fuell cell stack, the current is varying from about 3 A and 30 V to about 70 A and 20 V."
2) "I need to charge at 24 V ( two 12 V batteries)"
3) "and my question is how to transform the varying current into a "stable" 24 V?"

Is your question how to convert the energy from the batteries to a fixed voltage for use with a load?
And how to use this output to charge batteries, or how to charge the batteries you mention?

It would be easier if you can clarify your actual setup.

[zodian]

For blueint: The fuell cell produces between 3 A at 30 V and 70 A at 30V depending on the current drawn from it. I need to charge 2 12 V batteries with this fuell cell... I have no system setup right now... that is what im hoping to build on the basis of the help im getting here...

[squiz3r]

So, the cells can give a maximum of 70A? If you have two 12-volt batteries, then it's not definite that you can charge them with that high current; it depends on the size of the batteries...

How does a fuel cell work, if you don’t take out more than, let’s say 4 amps, does the voltage rise then? If not, what happens to the excess power?

[blueint]

Have a look at wikipedia: Switched-mode power supply.
Especially look into: Sepic, Cuk, Flyback, Buck-Boost.

However IF your input voltage from the fuel cells are guaranteed to be above say 12V. It might be simple with a dual buck converter solution.

[zodian]

For blueint: It really seems like we are on the right track of finding the right solution... The problem is now that the fuell cell delivers 1 kW and both the buck-boost and the flyback can only handle up to 150 W. I do not know if this is the same case for the Sepic and the Cuk...
What difference does it make that the voltage output is inverted?

[blueint]

zodian, Inverted = The positive rail becomes negative. May complicate design.

[victor_passe]

Well, with what curret can you charge your batterys?

[bearing]

Lead acid batteries can be charged with high currents. I know Optima suggests charging their batteries at constant voltage (14.4V if I remember right) without a current limit. I also have read about a charging algorithm which used 50A constant current on 50Ah batteries.

A reasonable question to ask here is: Why do the voltage drop that much?
Galvanic cells in general have a pretty constant voltage (I think). The voltage drop may be due to high internal resistance. If that is the case then I think the cells will get hot at high currents and voltage drops.

If you are able to make your own buck regulator I think there is no limit to how much power it can handle. It's a matter of winding the right inductor (or inductors) and choosing beefy enough FET:s and drivers (in my unexperienced opinion).