It says that the LM317 has a max. output of 1.5A, but my transformer will give 8.5A.

Ah well that’s more than the typical hobbyist power supply and requires a different solution. Same idea though - you need a regulator.

 

Ah well that’s more than the typical hobbyist power supply and requires a different solution. Same idea though - you need a regulator.

Wouldn't the addition of some "pass transistors" work? Like in the circuits shown here - http://www.bowdenshobbycircuits.info/page12.htm#317pass.gif

 

Your car battery may draw as much as 20 amps, depending on the state of charge. Trickle chargers that can produce only 2 amps are built so they don't try to give you more than that. If a transformer is not able to give you the amperage you need then all you'll get is what it's able to do. In your case, 8.5 amps. But I'd be cautious that your transformer doesn't overheat. A 20 amp load on an 8.5 amp rated transformer is going to produce a lot more heat unless it's specifically designed to do what you're asking of it. And yes, transformers can be built to limit current. It's done with shunt bars that redirect some of the magnetic energy back around so that it can't produce more than its rated value.

Years ago I worked servicing emergency lighting systems. We (the boss) bought a bunch of 32 volt transformers as replacements for failed transformers in systems that operate on 32 volts. The new transformers would typically last about 5 minutes when trying to charge batteries that were nearly depleted. The reason was they were not designed to limit their current. They'd go up in a spectacular light and energy show. (blow up)

If you're building a system with a limit of 8.5 amps then consider using a blast resister in your circuit that limits current to no more than 8.5 amps. If the battery wants 5 amps it will get it. If it wants 10 amps it will only get 8.5 amps. I'd also consider the RMS value of the transformer. If it's RMS value is 12 volts then multiply that by 1.414 to get an idea of the peak voltage. Someone before me already stated that. Also consider that the bridge rectifier will drop (as previously stated) 1.4 volts. So, 12 V RMS = 16.97 volts peak. Subtracting 1.4 volts for the rectifiers drops you to 15.57 volts. Assume it's 15.6 volts (rounded off). Divide 15.6 by the amperage rating you want and you get a blast resister of 1.83 ohms. Multiply that by 15.6 volts and you get 28.57 watts. A lightbulb in series with your charger may act as such a limiter. In fact, many of the smaller emergency lighting systems I've worked on used a bulb to limit charge current. When the light glowed brightly you knew the battery was taking a charge. As the battery got nearer and nearer to full voltage the light would dim, and in many cases go completely out. These smaller systems typically operated on 12 volt sealed lead acid batteries (SLA). We'd set their float voltage at 13.6 volts to keep them charged properly. In five years we'd often replace them because that was the typical life span of a SLA battery.

Use of a typical car light bulb like a brake light, often rated at 35 watts might do nicely. The NICE thing about using a light bulb is that when the current is high the bulb gets hot. That makes its resistance go up. That slows down the current, thus further protecting your transformer / rectifier circuit. As the current goes down the bulb cools and allows more current to flow. A nice sort of current regulation for charging a battery. And you can quickly get an idea if the battery is taking a strong charge or if it's just trickling, depending on how brightly the bulb glows.

But I don't think you need a capacitor in your circuit.

 

Wouldn't the addition of some "pass transistors" work? Like in the circuits shown here - http://www.bowdenshobbycircuits.info/page12.htm#317pass.gif

Sure, there are lots of ways to skin the cat. For an 8A charger, I'd look to a genuine lead-acid battery tending chip rather than a simple voltage regulator. Current limit while bulk charging, proper trickle for constant connection, that sort of thing. I thought the TS had a wall wart and that's why I brought up the LM317.

 

Aren't you all over-thinking this? The original poster just wants a simple power supply that he can use to charge a battery. Yes, there is a specific voltage/current profile that maximizes battery charge and life, but the OP just wants something simple.

OK, you have a transformer with the required primary and a 12 VAC 8.5 amp secondary. Connect that secondary to a diode bridge rated at 15 amps or greater, 50 V PIV or greater or equivalent individual diodes. Oh, and fuse the primary for safety and fuse the output lead at 10A in case you draw too much current on the output. And have spare fuse on hand.

If you want, you can connect that 2200 uF cap. If you want to use your new 15 volt power supply for other uses, then the cap would be helpful. The cap is not needed for battery charging.

Connect it to your battery, observing correct polarity. !! Important !! If you get the polarity wrong you might get some smoke, and will need to replace the output fuse. Turn it on.

If you have a voltmeter, observe the battery voltage, and when it get to 14.5 - 15 volts, or if the battery starts to bubble aggressively, turn off and disconnect the charger. The battery won't be 100% charged, but close. If you leave it on longer, you will be overcharging the battery and reducing its life.

Be careful with lead acid batteries. They do contain acid and can be chemically nasty that way. They also produce H2 gas when charging, and H2 is flammable/explosive, so no sparks near the battery - connect/disconnect the charger leads only with power off. Enjoy having made your won charger.

 

Aren't you all over-thinking this? The original poster just wants a simple power supply that he can use to charge a battery. Yes, there is a specific voltage/current profile that maximizes battery charge and life, but the OP just wants something simple.

He also made it clear that he was not familiar with how typical battery chargers work. This is a learning site, and the TS might choose differently once he understands the shortcomings of the simple approach.

We’re also not sure how big his batteries are. If it’s car batteries, the 8A supply isn’t such a big deal. If it’s motorcycle batteries or smaller, they’re at risk of serious overcharging.

 

We’re also not sure how big his batteries are. - - - If it’s motorcycle batteries or smaller, they’re at risk of serious overcharging.

Hadn't thought about that. Good point.

To the original poster; exactly what kind of batteries are we talking about?

 

Hadn't thought about that. Good point.

To the original poster; exactly what kind of batteries are we talking about?

Mostly the typical 12V "car" batteries

 

He also made it clear that he was not familiar with how typical battery chargers work. This is a learning site, and the TS might choose differently once he understands the shortcomings of the simple approach.

We’re also not sure how big his batteries are. If it’s car batteries, the 8A supply isn’t such a big deal. If it’s motorcycle batteries or smaller, they’re at risk of serious overcharging.

I actaully planned to charge smaller batteries, like one used on my lawn mower, but it seems like a bad idea since I pretty much don't know it's voltage nor amperage.

 

Aren't you all over-thinking this? The original poster just wants a simple power supply that he can use to charge a battery. Yes, there is a specific voltage/current profile that maximizes battery charge and life, but the OP just wants something simple.

OK, you have a transformer with the required primary and a 12 VAC 8.5 amp secondary. Connect that secondary to a diode bridge rated at 15 amps or greater, 50 V PIV or greater or equivalent individual diodes. Oh, and fuse the primary for safety and fuse the output lead at 10A in case you draw too much current on the output. And have spare fuse on hand.

If you want, you can connect that 2200 uF cap. If you want to use your new 15 volt power supply for other uses, then the cap would be helpful. The cap is not needed for battery charging.

Connect it to your battery, observing correct polarity. !! Important !! If you get the polarity wrong you might get some smoke, and will need to replace the output fuse. Turn it on.

If you have a voltmeter, observe the battery voltage, and when it get to 14.5 - 15 volts, or if the battery starts to bubble aggressively, turn off and disconnect the charger. The battery won't be 100% charged, but close. If you leave it on longer, you will be overcharging the battery and reducing its life.

Be careful with lead acid batteries. They do contain acid and can be chemically nasty that way. They also produce H2 gas when charging, and H2 is flammable/explosive, so no sparks near the battery - connect/disconnect the charger leads only with power off. Enjoy having made your won charger.

Thanks, clear and simple. But now when you mention it, my diodes used in the bridge are max. rated at 10A, would it be a issue since the supply will give a little less amps. (asking since you said "rated at 15 amps or greater" for the bridge)

 

EDIT: It's actually 7 amps at the output since I can't get the 8.5A one. The new one will be able to make:
1 x 18,6v or 2 x 9,3v
1 x 16,4v or 2 x 8,2v
1 x 14,4v or 2 x 7,2v