LM317 Issue??

ian field

Joined Oct 27, 2012
6,536
In that case, I'd probably take the easy way and just string enough 1N4001 diodes in series to drop the excess voltage.

With wall warts, the poor regulation of the old iron cored type is better for charging lead acid - a tight regulated SMPSU type will either give too much voltage or too little.
 

ian field

Joined Oct 27, 2012
6,536
Which circuit ,the lm317 charging circuit or lm317 voltage regulator ???
I am thinking charging circuit cause the lm317 is rate for so many amps and how could it charge at 3 amps then??
A good way to learn is to trace out the schematic for the circuit you have in front of you - it sounds like the sort of thing that would be simple enough to make that an easy task, then both you and we would know what we're dealing with.
 

Brownout

Joined Jan 10, 2012
2,390
Which circuit ,the lm317 charging circuit or lm317 voltage regulator ???
I am thinking charging circuit cause the lm317 is rate for so many amps and how could it charge at 3 amps then??
The circuit you made is NOT the problem. The problem is in your Jump Pack.
 

Lestraveled

Joined May 19, 2014
1,946
Your circuit is the problem. You are trying to charge a battery that wants a constant current but you are trying to charge it with a constant voltage source. The old wall wart did this nicely because the transformer saturated and became a crude constant current source. The normal LM317 regulator is a constant voltage source and your voltage is set too high (18 volts) causing an over current condition. A lot of people do not get that lead acid batteries want a constant current to charge and a constant voltage to float. That is why you buy multi stage battery chargers.

- My first suggestion is to get a wall wart with the same specs as the one that died and be done with it.
- If you want to build a LM317 charger circuit, I'll post a circuit that has current and voltage limiting, which I'll post shortly.

Edit: Dang, I was thinking about a different regulator. You can't do voltage and current regulation easily with a LM317. Constant current with a LM317 is very easy.
 
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Lestraveled

Joined May 19, 2014
1,946
Actually I edited too soon. There is a simple LM317 circuit that provides voltage and current regulation.
Look on page 9 of the attached data sheet. Figure 9 shows a circuit where a resistance is put in between the output and the voltage divider. This resistor incorporates current limiting. The current limit will be amps = 1.2/Rs. As the current drops to zero the max voltage will be controlled by the voltage set resistors.
 

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Brownout

Joined Jan 10, 2012
2,390
No, his circuit is NOT the problem. He connected his power supply directly to the charger board, and the same problem was there. He has a hosed jump kit. However, he can replace the existing charger with a home spun one, if the battery is good. But we don't know the condition of the battery.
 

Lestraveled

Joined May 19, 2014
1,946
Then your load requires more than 3 amps.
A lead acid battery capable of starting a car, during charging, will easily draw more than 3 amps, especially when connected to a voltage source set at 16 to 18 volts.

Brownout, I am sure you won't read it, but I am attaching the data sheet of my favorite lead acid charge controller. It clearly defines the industry standard of how to charge lead acid batteries. Perhaps Dr.Killjoy will read it and get something from it. The bulk charge for a lead acid battery is done in constant current mode. Meaning, the voltage is adjusted so the current stays the same. This accommodates the battery no matter if it is completely discharged or near full charge.

Another way of looking at a lead acid battery, when it is charging, is that it is a big zener diode. If the charger voltage is less than the "state of charge" (SOC) voltage of the battery then very little current will be drawn. Go slightly above the SOC voltage and the battery looks like a dead short, drawing maximum current. This is why a constant current mode charger is required. As the battery goes from discharged to full charge the charge current will stay the same. The voltage difference between dead and full charge on an automotive battery is only about 2.5 volts.

Dr.Killjoy
A simple experiment will show you a lot. Connect your battery directly to your bench power supply. You will notice that you will not draw any current until you get up to the battery voltage. (Your voltage meter will read the battery voltage until the power supply voltage gets above the battery voltage.) You will also notice that the current rises very quickly after that point. Next, adjust the voltage to get the max current out of you power supply. Notice anything??? The current keeps dropping. You would have to keep adjusting the voltage in order to keep the current the same. (If you have a shorted battery/system this will not happen.) If you set a fixed voltage and the current continually drops, you have a battery that is charging. That is what a constant current charger does for you. It adjusts the voltage to keep the charge current constant.
 

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Brownout

Joined Jan 10, 2012
2,390
A lead acid battery capable of starting a car, during charging, will easily draw more than 3 amps, especially when connected to a voltage source set at 16 to 18 volts.
Then why does the original charger work at 18vdc 500ma ??

I'm not arguing how a battery charges, but you haven't read the thread carefully enough to know that the TS is not trying to charge his battery with the circuit he made. The charger circuit is built into his unit, and he is only trying to power the charger, as the wall-wart originally did. He IS NOT connecting his battery to 18v, attempting to charge it. Please read the ENTIRE thread carefully before commenting further.
 
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snav

Joined Aug 1, 2011
115
NP. You info might be helpful to the OP should he decide to build his own. But first he must evaluate the condition of the battery.
The typical jump box has a 17Ah SLA with the cable clamps always hot, the cigarette lighter and battery test is across battery. The test through momentary switch and the lighter socket through a 20a thermal breaker. I recently rebuild two of these adding a battery disconnect switch for the cables and useing a Battery Tender charger that uses 0.75A charge current by connecting to the cigarette lighter socket.
The original charger in these two were internal wall wart types that were a transformer and single diode. The LM317 battery charger in application notes mentioned previously would be inexpensive and easily connected if the unit in question also has a cigarette lighter socket.

It's usually cheaper to buy a complete unit on sale than to replace the battery.
 
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