12v Lead Acid Charger, LM317 Question

[duffy]

You're right, I'm sorry. You could replace Rs in the original circuit with a 2.2 ohm, 1w resistor to limit the maximum current to 500 ma so you could read it with your meter - but this is all getting very silly. You don't need a current regulator or a hobbled circuit, you need an $8 multimeter.

 

[joecool85]

You're right, I'm sorry. You could replace Rs in the original circuit with a 2.2 ohm, 1w resistor to limit the maximum current to 500 ma so you could read it with your meter - but this is all getting very silly. You don't need a current regulator or a hobbled circuit, you need an $8 multimeter.

What was the equation you used to get to 2.2 ohms? I really appreciate your help. Part of the reason I want to use the meter I have now is that it fits perfectly into an old external zip drive enclosure and you can see it in the "window" on the drive case. If I can get this all working, it'll be a real slick little charger thats perfect for charging my motorcycle battery. Heck, at 500ma it'll even give me a rate of C/14 - C/24 depending on which battery I'm charging - which is exactly where I like it.

 

[duffy]

What was the equation you used to get to 2.2 ohms?

The one you posted on page one. Use 2.2Ω for Rs, and you get a source impedance of around 24Ω, with a completely flat battery (or shorted output)that will give you a maximum of about half an amp.

 

[duffy]

You understand that won't give you a constant 500 ma, that's just the maximum. It will quickly taper off.

But I just came up with a better idea. This will give you constant current with limited voltage. Replace R2 with a 1N5242B zener. Make Rs 2.6 ohms, or use this pot to make the current adjustable -
Digi-Key - CT2150-ND (CTS Corporation Electrocomponents - 026TB32R5AOB1A1)

When the output hits 1.25V + zener voltage the LM317 will shut off. It may be a half-volt lower than 13.75V, but I can tell you how to adjust it after you get it built.

So the schematic would look like this:

 

[joecool85]

That zener idea sounds pretty good, would I be able to get 14.8v out of it though? From all my searching it seems thats going to be the best voltage to charge my specific batteries at.

Also, I liked your idea of using the voltmeter function to check the current by reading the voltage over the resistor. Wouldn't it be possible to use a transistor or op amp to raise the voltage from 0.3v to say...5v or something? I have a handful of 2n3904 laying around as well as an lm386 I could use.

 

[duffy]

>using the voltmeter function to check the current by reading the voltage over the resistor. Wouldn't it be possible to use a transistor or op amp to raise the voltage from 0.3v

It's not .3v anymore, now it's 1.3v. Use ohm's law.

>would I be able to get 14.8v out of it though?

You can get a higher voltage out of it by putting a higher voltage zener in it. 14.8v is higher than what I would use.

 

[joecool85]

Wait, why is it 1.3v? Due to the zener in the circuit? If so, I was disregarding that as I don't have any zeners on hand right now.

Couldn't I put a 3-4w bulb before the lm317 in parallel with the circuit to use up a few ma, therefore lowering my overall maximum charging current? I was considering adding a light anyway. I was going to use a LED, but a regular bulb would work, and if it was dual purpose and helped the circuit do what I want it to, even better.

From what I was reading, it is important to pass the gassing voltage of a maintenance type wet cell to get a full charge. Gassing voltage at 50F for mine is 14.8v which is the coldest my basement gets where the battery would be charging.


**edit**
Also thanks again for all the help. I love messing with electronics but I always have a bunch of questions. And some people get frustrated because even though I have a lot of questions I can still be very set on going a certain direction sometimes.

 

[duffy]

>>I can still be very set on going a certain direction sometimes.

Believe me, I understand this. I have made many crazy contortions to circuitry because I wanted it to fit in a certain project box or work with a certain display or whatever.

>>Wait, why is it 1.3v? Due to the zener in the circuit?

Not the zener, just ohm's law, like I said. Rs is now 2.6Ω. At 500ma, that's 1.3V.

>>Couldn't I put a 3-4w bulb before the lm317 in parallel with the circuit to use up a few ma, therefore lowering my overall maximum charging current?

Yes, but if you run the current through Rs, it will read the same on the meter. If you bypass Rs, it will goof up the regulator. If you want a pilot light, just put it ahead of the regulator so it runs off the 18V supply, that way it won't affect anything.

>>it is important to pass the gassing voltage of a maintenance type wet cell to get a full charge.

Yes, I've hear that gas-station lore about "She ain't charged till you see the bubbles in the battery". While it's true that it will start to bubble when it's as charged as it can possibly get, I would cheerfully sacrifice that extra one or two percent of potential capacity just so it doesn't gas all the time and destroy the battery.

 

[joecool85]

Is it possible to us an L7809CV and L7805CV in series to get 14v? It would be driven from one transformer, not two seperate ones. I ask because I just gutted an old soundcard for ICs and I got both of those chips. Since I don't have an LM317T yet (was going to buy one from RadioShack for $2.29), I was wondering if these might work.

 

[duffy]

Uhh... not in series... you might be able to "stack" them so the ground reference of the 7805 is the output of the 7809, put a load resistor on the 7809... I really wouldn't suggest doing it this way, I don't know how they would behave. Spend a couple bucks, get the LM317, get a digital multimeter.

 

[joecool85]

Uhh... not in series... you might be able to "stack" them so the ground reference of the 7805 is the output of the 7809, put a load resistor on the 7809... I really wouldn't suggest doing it this way, I don't know how they would behave. Spend a couple bucks, get the LM317, get a digital multimeter.

Yeah, well it was worth asking. And "stacking" them as you mentioned is what I meant. I need to get a new DMM anyway since my old one died. I already had another idea for the 7809 (9v guitar pedal board power supply chip for my brother), not sure what I'll do with the 7805. I have another one of those as well. I don't really have anything that needs 5v though.