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Hi Leo,
Welcome to our forum. ;D
The author of the project has many similar complaints about this project on his website.
It just doesn't work! (Another one!)

Since it is no longer made (for 10 years), did you use a substitute for the opamp?
I think the LM301 opamp has its input common-mode voltage range exceeded in the circuit. It might work with a TL071 or TL081 opamp that have the positive supply within their input common-mode voltage range. Try it. ;D 


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Hi Sasi,
Thanks, you fixed it!  ;D
The other people also complained that its voltage was too high at about 16.5V, no wonder the opamp never sensed that the battery was fully charged.

The project's text and National's schematic doesn't say what should happen when the battery is fully charged but I think it is supposed to stop charging and light the LED brightly.

With your trimpot adjusting the open-circuit voltage to 14.5V, the opamp's (+) non-inverting input will be slightly less. When the battery voltage reaches its 13.8V full charge and its current is still 0.3A or less, then the voltage across the 0.22 ohm resistor wil be enough to switch the opamp into its full charge state.

Does a car battery draw a charging current of only 0.3A or less when its voltage reaches 13.8V? I thought the current would be more.

BTW, the LM301 operates perfectly with its inputs up to  the positive supply voltage, most opamps don't.

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Hi Guys,
I'm still looking for information on the web about how much current a car battery draws when fully charged, to see if it is low enough to allow our battery charger project to automatically shut-off.
I found out that car batteries are charged at a higher voltage today than they used to be.
Older batteries had lead-antimony for both plates and were charged to 13.8V at room temperature. But they frequently had to have water added.
Newer "maintenance-free" batteries are charged to about 14.5V because one plate is changed to lead-calcium and they don't need as much water to be added.

A good article about charging car batteries is here and has a link to a FAQ section:


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My 12V 150AH battery fully charged state (13.8V) drawn 23ma current when I applied 13.8V.

I think that since it was drawing such a low current for such a powerful battery, it wasn't fully charged because the charger voltage was too low.

But when I increase the charging voltage to 14.5, first it had drawn 2.3A than slowly decreased to 200ma.

I think that 200mA is a reasonable indication that such a big battery is fully charged. 200mA through its 0.22 ohm current sense resistor will drop the 14.5V charger set voltage to 14.46V across the battery.
When you load the battery its voltage will quickly drop to its operating voltage of 13.8V.

We can't trust the battery because it isn't marked with the manufacturer's recommended charger voltage. ;D
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Hi Sasi,
I like your charger. ;D
It is good (gud?) that you included a float-current adjustment. Your charger must be pumping many Amps into a battery that is completely discharged.

I was thinking about using a similar charger for quick-charging a Ni-MH battery. If it is set to 1.4V to 1.5V per cell like my timer-controlled (I am its timer) charger delivers, I don't think the battery will explode. What do you think?


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I made in the past a car ttery charger with thyristors.  The cct is simple and works fine.
It was modified to work with 24V (two batteries in series) but can be modified back for 12V operation very easyly.
Just make R1=1 to 1K2 and zener diode D1 should change to 6v.
For 12V operation use the red coloured values.



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Guest Alun

What did you power this circuit from?

There's nothing to limit the current, Q1 will just turn on and as much current will flow as the battery's internal resistance and that of the power supply will allow to flow.

The LED will glow when the voltage on the battery reaches a level detrurmined by the dividing ratio of the potentiometer with R4 =l to 1.3+0.7 or about 2V.

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I think Q1 will never turn off and a high trickle current will be maintained.
I have never seen an SCR that stops conducting when its gate voltage is removed like a transistor.

The circuit is basically only the fuse in series with a diode (the conducting SCR) that is charging the battery. Plus a latching battery voltage indicator. If the pot can pass the LED current, the 2nd SCR isn't needed. ;D

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Perhaps he could use a Gate Controlled Switch instead. Those can turn off, unlike a SCR.

An ordinary transistor turns on and off pretty well too. ;D
Instead, he used a 500V(!) SCR.

I checked, its minimum holding current is only 7mA. So you would need to disconnect the battery to turn it off.

He says it works fine. My car battery has unlimited charging current and continuous overcharging in the car and it works fine too! ;D
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Hi Born,
Are you joking?
If a 4000W inverter is 80% efficient, it would draw 5000W from the 12V battery at full output. That's a current of 417A! If a car battery doesn't explode or boil, it would last about 22 minutes. Connecting wires and for the primary winding of the transformer about the size of your thumb! 84 output power transistors and maybe 8 more as driver transistors!
I think it would be very expensive and use a bus-load of paralleled car batteries.

Since it would need a custom-made transformer and multiple batteries anyway, connect the batteries in series for 48V to reduce their current to about 104A, use only (!) 20 output transistors and only 4 driver transistors. ;D

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Hello again,
About the cct I posted yesterday.  You made a lot of questions and I am sorry I had not said an important detail.
A  transformer of 13-14V a.c. is used.  Then a full wave bridge rectifier is connected to the cct.
So, we have d.c. cycles (positive) at he upper part of the cct and the negative is the lower part, where the batteries negative terminal is conected.
Due to the fact that we have unfiltered d.c. supply, i.e. full wave rectification without electrolytic caps to smooth and keep it at high levels, the Thyristor will turn off every time the voltage reaches the zero level (100 times per second).

Now, Q1 will conduct unless Q2 is on.  Q2 is on as soon as the voltage across the battery is high enough and could pass through the zener. (So, we have voltage regulation)

The LED lights when there is no charging, Q2 conducts.

The battery is 12V, not 24 as indicated in the diagram.

The cct works fine for many years in many commercial applications. It is a part of a battery charger in standby generators (24v operation). It keeps the battery charged and does not dry it out. 

Transformer current capabilities, Q1's size and headsink define the supplied current ability of the cct.


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Guest Alun

Shouldn't there be at least something to limit the charging current though the battery?

Even if it's just a small resistor in series with Q1, it still needs current limiting.

You may have got away with out it due to the intrenal resistance of the transformer and the battery.

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Certainly current limittation is only achived by the transformer capabilities.
A flat battery would absorb high currents at the begining of its charging until the voltage reaches  a level near the set voltage by the pot.  At this level, conduction is not for the full cycle but for a smaller part than before, hence less heating. 
Anyway, this is not an actual current limitting technic and the charger is intended mainly for keeping the battery at a charged level. 

If someone wants current limittation then a series resistor could be incorporated but the heat losses increase.

A 1ohm series resistor will limit the current of a flat battery (supposed a 10V across it) to about 3-5 Amps depending on transformer voltage and charger settings.  The heat on the resistor would be about 25Watts which means that a resistor of about 50Watt rating must be used (due to high temperatures arising at the resistor)

In this case, the cost of the resistor is almost equal to the cost of the charger  :)

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Hi Audioguru,

Fuse?? Why use them they just break all the time! No, but seriously no blown fuses, the short bursts of current combined with the resistance in the battery leads (a few meters of 1 sq/mm) can be the answer. And if you have ever had a completely flat battery you might have experienced the opposite, no reading at all on the A-meter to begin with? After that slowly climbing over a period of time until the chemical process is back to “normal”.

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Hi Ante,
Yeah, I've had a lead-acid battery that was sitting for some time completely discharged, and it took a long time for it to begin charging normally. I think it became "sulphated".
I have also left my car's lights on after a morning drive to work in the dark and getting there at dawn when it is bright. Many hours later the lights were very dim and the battery didn't have enough charge remaining to crank the engine. After wating for about half an hour with the lights off, it started almost normally. After a short drive with the alternator probably pumping many amps into the battery, it acted like normal.
There isn't a fuse. The current is limited (?) by the alternator's windings, rectifiers and wiring. ;D

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