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Such a thing as a variable voltage/current overcharge protection? Answered

I've had a couple of bad transformers in battery chargers in the last couple of months and I am thinking of building my own variable voltage/current battery charger with an ST Microelectronics L200.

Figure #23

But I would like to have overcharge protection so I don't have to monitor the batteries charging once I set the volts and current for that battery. Something that would just shut off or switch to a trickle charge when the battery reaches the voltages I want or set.

Also, another question - I am intending to use a salvaged transformer from an old chamberlain garage door opener for this project and need to know the best method for lowering the voltage from it to something the L200 can handle. It's a 1:3 and takes my 125v main to roughly 42v and the L200 is rated at a maximum of 40v input. I was figuring on just using a resistor, but I don't know how to calculate what I need since the current and voltage will be changing the load on a per battery basis.

Also I think the transformer might be a little small for this project, it only measures about 2"x2", while most of the chargers transformers were twice the size or more. So I'm wondering if it might not be able to handle the constant power or heat requirements for charging. *Added a pick of the transformer.

And since I've asked this many questions, I might as well add another. I was looking through the "how to get free electronic parts" pages and I didn't see any on solar cells. Does anyone know if any company does samples for cells the way they do for electronic parts?

I am capable of following simple to moderate schematics and can solder well. But when it comes to making new circuits and such, I tend to just hobble together something I've seen or used before that works. When it comes to programming and calculating circuit load and all that I become bogged down pretty fast. If I can see it and experiment with it till I get it right, I am better off and maybe learn something. So the simpler the better.


There are many specialised charging ICs around now which do the whole job for you. What battery technology are you trying to charge ?


A variety, NiMH, NiCAD, and SLA covers the spectrum. I'm trying to make a single charging station for the batteries that have lost/damaged chargers. Any help is much appreciated.

That is just shy of perfect, is there a version that will go up to 24v batteries? The ones listed only when to 17.6 volts. Thanks!

Unfortunately that's the rub. From the datasheet, the PCB would be relatively easy to put together. On the other hand, I have no host and wouldn't know how to program it if I did. This looks like it would be a great solution to my charging problems, and far cheaper then replacing or rebuilding the chargers I have too. Thanks for the info, you've give me some more refinement to my direction at the least.

That would be pretty easy to do if you have a micro-controller, since you can create subroutines for each of the chemistry/battery sizes you intended to work with. It would then be a matter of calling the appropriate subroutine to manage the associated charge operation.

In terms of a more analog solution, yes, could also be done, but it's a bit more convoluted, since each type requires a different charge profile. I really can't give you specific recommendations, since I have no idea of the batteries you intend to charge (nor will I. that takes footwork and footwork costs me time and therefore money.

You can place a couple of diodes in series with the load to drop the excess voltage. Make sure they're rated for the expected load current plus a bit for margin

If the transformer is rated for continuous operation above the highest charge current required, then you should be fine.