Charging A Lead Acid Battery

Yes, it does, to get the 540 mA you specified, and the "about 400 mA" (depending on what you mean by "about" 400 mA) you specified.

But you did not specify any current rating.

You are trying to argue a point not being debated. There are many chargers that are *better*. No debate on that.

My objection to your post was that what you claimed was not correct. You *cannot* specify that the current will be some particular number without regulation. That regulation adds complexity to your circuit.

By the way, I assume your question "Why is your simple charger so complicated?" was facetious, and you were just busting balls. 6 simple parts does not make a complicated charger! Your 2 part charger is even simpler - no debate - and it may be better.

Ed

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Mind you, you can have problems with a simple float charger too. If you rely on the regulator's current and thermal limit to hold the current under a certain level it will get HOT. I also had a National part that the info in the datasheet didn't match real life and we blew out a few wall warts before I caught it.

Jim

I'm amazed at this thread. The "very simple charger" I diagrammed *stinks* if _used_ as a float charger, which is what you were de facto discussing when you talked about forgetting and leaving the batteries on it too long. If used that way, it *might* prevent damage to the batteries by the cumulative voltage drop which you and Mark have discussed, but it is the *wrong* tool for that job, and the wrong usage of the tool.

I am glad to see that you are taking the discussion to a float charger. I'm responding below because you mentioned relying on the chip's thermasl & current limits and that some wall warts blew.

It would be poor practice to design a float charger that relied on the regulator's current and thermal limit to hold the current under a certain level, assuming by "the regulator's" you mean the IC chip. Those things - the current and thermal limit - only indirectly hold the current under a certain level. They are design maximums for the chip, not for whatever load the chip is feeding.

The circuit design needs to keep the current under the maximum rating of the chip under worst case conditions. The designer specifies a heat sink and/or a design that keeps the chip temperature below the maximum spec. He/she needs to ensure that any other limitations (eg Vin-Vout rating) for the chip are adhered to.

A float charger may not need current limiting for normal conditions, but it does for worst case: a shorted battery. That's where a float charger without current limiting fails. Under normal conditions, the battery will limit the current drawn as the battery voltage increases, and additional limiting may not be required. Still, you need to consider the whole circuit. You mentioned that you had some blown wall warts. If the batteries require more than the wall wart can deliver, that may be a specification rather than circuit problem. (ie use a bigger wall wart) If they blew because the circuit relied on the chip to shut down when it got too hot, that's a design issue. You indicated an error in the National datasheet was the cause - do you still have the details? It could be helpful to know which part and what spec was wrong.

Ed

When the datasheet and app note show the part being used in this very way, I would assume the manufacturer has rated the part as such. Wouldn't you?

The IC had a built in current limit and, like I mentioned above, National had several design examples that used over current protection as an integral part of the design. The problem was that the limit was wrong on the datasheet and in the app note. 1 - 1.2A was suppose to be min to max range, it turned out to be 1.9 - 2.2A or some such.

The part was a National LM2941CT and the wall warts blew because the current limit of the IC was wrong on the datasheet. I didn't want to go to a bigger wall wart, we didn't want to charge at a greater rate anyway. I'm sure (hope) they have corrected the datasheets by now, but the funny thing was one of the applications engineers sent me a "fix" for this problem that entailed a small sense resistor and a transistor to implement a half assed current regulator and they had that wrong too. It was set up to pull the feedback pin lower as the current increased, that would increase the voltage out as the current demand increased causing the system to slam hard against the + rail. I think they were relying on the internal current limiting to do its' thing at the level the datasheet had listed erroneously. I had already fixed the problem myself. The point was I didn't want to add complexity to the design, that was why I chose the part in the first place, but I ended up adding parts anyway.

As a side note, they have changed the datasheet, but there is still one hold over. In the application notes they still show a "1A Coil Driver" or some such, that is suppose to limit the current to 1A using the internal current regulation of the part. Well, it won't do it.

Jim

That would depend on the datasheet, but in general, yes, I would probably trust it.

Thanks, that's good information to know. Also as a side note, I've read comments by others who are skeptical, or suspicious or uncertain (searching for the right term, not sure what it is) of LDO's in general because they've been burned by them, too.

Ed