Battery charging with SMPS question.

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.


[Klaus, Tim, Petrus and others, thank you for answers to my previous posts]
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Not 100% stiff.

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.

Reply to

You're missing several ingredients. Modern automatic battery chargers follow a protocol of voltage and current. There are chips available that can help with this. The quickest way to wreck a rechargable battery, including lead-acid batteries, is to overcharge them.

Sniff around on the web. Information on the best way to charge a given battery chemistry is not hard to find.

Reply to
Don Foreman

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

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.

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

I don't think so.

what difference would that make, and why?

you or I.

Bye. Jasen

Reply to
Jasen Betts

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