Battery charging with SMPS question.

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siliconmike - 15 Dec 2005 10:53 GMT

I'm trying to build a basic car battery charger.

The charger will use a 14.4V, 4A SMPS to charge the battery.

The SMPS output will be directly connected to the battery terminals
[other suggestions?]. The SMPS will be a flyback convertor and I have
two options in its design:

1. either to take feedback from the 14.4V output using opto-isolator
technique.
2. or to have a separate feedback winding

Consider this situation:
----------------------

The battery EMF is a stiff source (right?) say at 12 Volts.

Now if I am using option 1, the SMPS controller will never be satisfied
from the feedback, because the output is forced to be 12 Volts by the
stiff battery EMF, whereas the SMPS controller is constantly wanting
the output to be 14.4 volts.

So the duty cycle and the output current will peak. It would be an
over-load situation.

So I concluded that option 2 is the way to go.

Am I right here? Or missing an ingredient?

If you have tips about design please do not hesitate to mention.

-Mike

[Klaus, Tim, Petrus and others, thank you for answers to my previous
posts]

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CWatters - 15 Dec 2005 15:06 GMT

> I'm trying to build a basic car battery charger.
>
> The battery EMF is a stiff source (right?) say at 12 Volts.

Not 100% stiff.

> Am I right here? Or missing an ingredient?

Yes, current limiting. What you need to build is a constant voltage source
with current limiting. When a really flat battery is connected the charger
will initially operate in current limited mode. Then quite quickly the
battery voltage starts to climb and the charger switches back into constant
voltage mode.

Current limiting is also useful to protect against damage caused by shorting
out the charger battery clips. Make sure the heat sink is big enough because
in current limiting mode the power dissipation in the charger might be high.

If you make the output voltage adjustable the charger can also operate as a
permanant float charger.

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Jasen Betts - 16 Dec 2005 10:01 GMT

["Followup-To:" header set to sci.electronics.misc.]

> I'm trying to build a basic car battery charger.
>
[quoted text clipped - 12 lines]
>
> The battery EMF is a stiff source (right?) say at 12 Volts.

no. the EMF will vary as the battery charges upto aaround 13V somewhere.

> Now if I am using option 1, the SMPS controller will never be satisfied
> from the feedback, because the output is forced to be 12 Volts by the
> stiff battery EMF, whereas the SMPS controller is constantly wanting
> the output to be 14.4 volts.

no. the battery voltage will float above the no-load voltage while being
charged and will maintain that voltage for a small time dur to internal
capacitance etc... it should maintainf it long enough to get sufficient
signal to control the powersupply.

> So the duty cycle and the output current will peak.

when the battey is depleted it will accept a rapid chaarge. as the voltage
rises the SMPS shoudl throttle back.

> It would be an over-load situation.

I don't think so.

> So I concluded that option 2 is the way to go.

what difference would that make, and why?

> Am I right here? Or missing an ingredient?

you or I.

Bye.
Jasen

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Electronics Forum / Electronics / December 2005

Battery charging with SMPS question.