Hello
I hope that this is the correct forum for these questions.
A friend asked me for a battery charger for his car battery recently. I gave him the (very old) one that I had.
However, it seems that it no longer worked. As it was very old, I decided to build a new one and I found this circuit (A)
online.
I acquired all the parts and then (stupidly, as I now had the parts) started to question the circuit.
A reader also says it doesn't work (B).
I asked the following questions in the comments section of the website, but received a strange reply (C).

1. Is this circuit OK in general?

2. Changes I made are indicated on the attached schematic (D) with red blocks and are consistent with the datasheets.
2.1 Inclusion of a current sensing resistor
2.2 Inclusion of input short-circuit and output capacitor discharge protection diodes.
2.3 Addition of 2200uF and 0.1 uF caps on the input, a 10uF cap on the Adjust pin, and a 1uF cap on the output.

3. Can I swop the leds? I wish to have the Red led for charging and the Green led for fully charged.
3.1 Where is the current limiter for the fully charged led?
3.2 Should I change the current limiting resistor for the charging led?

4. Is the 11V 1W Zener diode correct? Shouldn’t it be at least 13V?

5. The author says: "battery charger SHOULD have over voltage protection, short circuit protection and reversed polarity protection"
5.1 Does the following constitute the over-voltage protection? Or should I still do something else?
"Now the base of the transistor gets the sufficient current to turn on so that the output current from the LM317 voltage regulator
is grounded through the transistor Q1. As a result Red LED indicates the full of charge"

6. Rs resistor.
6.1 Is the current sensing resistor a “normal” resistor or is it a special type?
6.2 Will this resistor automatically set the current correctly?
6.3 I couldn’t find a 2 Ohm resistor, only a 2.2 Ohm. Is this ok?

Thank you
Max

 

Hello
I hope that this is the correct forum for these questions.
A friend asked me for a battery charger for his car battery recently. I gave him the (very old) one that I had.
However, it seems that it no longer worked. As it was very old, I decided to build a new one and I found this circuit (A)
online.
I acquired all the parts and then (stupidly, as I now had the parts) started to question the circuit.
A reader also says it doesn't work (B).
I asked the following questions in the comments section of the website, but received a strange reply (C).

1. Is this circuit OK in general?

2. Changes I made are indicated on the attached schematic (D) with red blocks and are consistent with the datasheets.
2.1 Inclusion of a current sensing resistor
2.2 Inclusion of input short-circuit and output capacitor discharge protection diodes.
2.3 Addition of 2200uF and 0.1 uF caps on the input, a 10uF cap on the Adjust pin, and a 1uF cap on the output.

3. Can I swop the leds? I wish to have the Red led for charging and the Green led for fully charged.
3.1 Where is the current limiter for the fully charged led?
3.2 Should I change the current limiting resistor for the charging led?

4. Is the 11V 1W Zener diode correct? Shouldn’t it be at least 13V?

5. The author says: "battery charger SHOULD have over voltage protection, short circuit protection and reversed polarity protection"
5.1 Does the following constitute the over-voltage protection? Or should I still do something else?
"Now the base of the transistor gets the sufficient current to turn on so that the output current from the LM317 voltage regulator
is grounded through the transistor Q1. As a result Red LED indicates the full of charge"

6. Rs resistor.
6.1 Is the current sensing resistor a “normal” resistor or is it a special type?
6.2 Will this resistor automatically set the current correctly?
6.3 I couldn’t find a 2 Ohm resistor, only a 2.2 Ohm. Is this ok?

Thank you
Max

 

Sorry, I struggled a bit to upload the files....

 

The LM317 can supply up to 1.5A and a common car battery would be 40Ah, so this circuit will take 40Ah / 1.5A ≈ 26 hours which would be alright providing you are not late for work, or need to go to the shops to get some lunch.

 

AlbertHall thanks for the reply. Hadn't realised the current limitations...
Any ideas on where to look for a good reliable schematic with auto cut-off (over-charge protection), charging and fully charged led indications and
a overnight charging time (8 hours).
I've been searching but can't seem to find a schematic with all of the above features.
Max

 

Oh and an adjustable current setting....

 

If you want to charge car batteries it would be much easier and cheaper to buy one.
http://www.ebay.co.uk/itm/8AMP-6V-1...882115?hash=item33a6bf39c3:g:RukAAOSw9N1Vu4YG

 

Thanks AlbertHall.
I live in South Africa and the shipping costs from the UK to here are astronomical (often much more than the item itself), not to mention the exchange rate.
Sometimes I think that living in the UK or the States must be great for all the electronic stores.
Also, as I mentioned, I have a complete charger, which no longer works, but all the peripherals are there, the case, fuse holder, switch, transformer, rectifier, crocodile clips, etc. All I need is a circuit to install.
Of course there is the added benefit of having built it oneself...
Thanks though for your helpful comments. I will probably keep searching.
Regards
Max

 

What's wrong with the one you've got?
If you've got a multimeter it should be fairly easy to find out what's wrong with it and repair it (providing the transformer is OK - that would be difficult to replace). We can help you with that too, if you want to go that way.

 

You only need a 15v transformer, bridge rectifier for a car battery charger, thats the best design, lead acids limit their own current as it charges up.

 

I think you will have problems using a standard transformer. I agree with this quote from another thread on here:
"The desired behavior is very dependent on the exact turns ratio of the tranny. Think about the Sears buzz-box unregulated type of battery charger. They purposely build a lot of leakage inductance into the transformer to help with the current-limiting. They also set up the turns ratio so that as the battery voltage climbs toward the end of charge, the current drops off as the battery tops up.

If you substitute a higher quality transformer, then there is less current limiting because of the better flux coupling. If you substitute a transformer with a slightly higher turns ratio, then there will be higher initial current, and less tapering as the battery tops up."

 

Thanks AlbertHall.
I live in South Africa and the shipping costs from the UK to here are astronomical (often much more than the item itself), not to mention the exchange rate.
Sometimes I think that living in the UK or the States must be great for all the electronic stores.
Also, as I mentioned, I have a complete charger, which no longer works, but all the peripherals are there, the case, fuse holder, switch, transformer, rectifier, crocodile clips, etc. All I need is a circuit to install.
Of course there is the added benefit of having built it oneself...
Thanks though for your helpful comments. I will probably keep searching.
Regards
Max

What was the maximum current on the bad one?
That will tell us how much to plan for.

 

What's wrong with the one you've got?
If you've got a multimeter it should be fairly easy to find out what's wrong with it and repair it (providing the transformer is OK - that would be difficult to replace). We can help you with that too, if you want to go that way.

Well, as I know the charger is (was) so old (the 2N3055 is rusted), I didn't look for the fault.
So I thought it best to scrap the circuit and build a new more modern smart charger.

You only need a 15v transformer, bridge rectifier for a car battery charger, thats the best design, lead acids limit their own current as it charges up.

I'd like to build a smart charger with auto cut-off etc.

What was the maximum current on the bad one?
That will tell us how much to plan for.

Ronv, I'm not sure to be honest. This was a commercial charger made by a company called Electro Accoustics, of which I can find no information on the internet regarding power supplies.

Which actually brings me to another question: batteries have various Ah ratings and a charger should charge at 1/10 of the battery's rating.
How is this achieved in practice to charge batteries with differing Ah ratings if one has a charger which has a pre-set Ah charging rate?

Also is it possible to use a Darlington pair to increase a LM317's current rating as shown in the attached link?


http://www.reuk.co.uk/wordpress/electric-circuit/lm317-high-current-voltage-regulator/

 

Well, as I know the charger is (was) so old (the 2N3055 is rusted), I didn't look for the fault.
So I thought it best to scrap the circuit and build a new more modern smart charger.


I'd like to build a smart charger with auto cut-off etc.


Ronv, I'm not sure to be honest. This was a commercial charger made by a company called Electro Accoustics, of which I can find no information on the internet regarding power supplies.

Which actually brings me to another question: batteries have various Ah ratings and a charger should charge at 1/10 of the battery's rating.
How is this achieved in practice to charge batteries with differing Ah ratings if one has a charger which has a pre-set Ah charging rate?

Also is it possible to use a Darlington pair to increase a LM317's current rating as shown in the attached link?


http://www.reuk.co.uk/wordpress/electric-circuit/lm317-high-current-voltage-regulator/

You might think about a larger regulator like the LM338.
http://www.ti.com/lit/ds/symlink/lm101a-n.pdf
You will need to change the 0.2 ohm resistor to raise the current. You could also switch in/out some resistance to limit the maximum current for various size batteries.
The switch from charge to float in your circuit is not real well defined. We could maybe come up with a better way or you can try it first to see how close it comes to the switch from charge to float.
I don't think you want the 100 ohm resistor.
Let me know if you are going to build it and I'll see if I can simulate it.

 

You might think about a larger regulator like the LM338.
http://www.ti.com/lit/ds/symlink/lm101a-n.pdf
You will need to change the 0.2 ohm resistor to raise the current. You could also switch in/out some resistance to limit the maximum current for various size batteries.
The switch from charge to float in your circuit is not real well defined. We could maybe come up with a better way or you can try it first to see how close it comes to the switch from charge to float.
I don't think you want the 100 ohm resistor.
Let me know if you are going to build it and I'll see if I can simulate it.

Thanks ronv. Let me go back to the drawing board and redesign the circuit with the LM338 (or maybe 2 of them in parallel as my own car battery has a 95Ah rating. I'll post it here later. I don't think I need the float charge.
Incidentally, how accurately must the current be set compared to the Ah rating of the battery? I know that it should be 1/10 of the Ah rating but what are the tolerances, if any.
Please forgive me if I appear to be reinventing the wheel...
Max

 

The 1/10C is a good safe current for all rechargeable batteries. As I'm sure you're aware many batteries can be charged in 1 hour and that is a current greater than 1C. Car batteries are less critical than most.

 

Thanks ronv. Let me go back to the drawing board and redesign the circuit with the LM338 (or maybe 2 of them in parallel as my own car battery has a 95Ah rating. I'll post it here later. I don't think I need the float charge.
Incidentally, how accurately must the current be set compared to the Ah rating of the battery? I know that it should be 1/10 of the Ah rating but what are the tolerances, if any.
Please forgive me if I appear to be reinventing the wheel...
Max

No problems.
The regulators are difficult to parallel. If the original had a single 2N3055 or something similar it is unlikely it put out more than 3 or 4 amps. The limit after the regulators is probably the transformer, so I wouldn't try to get much more out of it.

 

The 1/10C is a good safe current for all rechargeable batteries. As I'm sure you're aware many batteries can be charged in 1 hour and that is a current greater than 1C. Car batteries are less critical than most.

Thanks AlbertHall for that information. I'll stay within that limitation.

No problems.
The regulators are difficult to parallel. If the original had a single 2N3055 or something similar it is unlikely it put out more than 3 or 4 amps. The limit after the regulators is probably the transformer, so I wouldn't try to get much more out of it.

ronv, I had a look at the 2N3055 datasheets and they say that the Collector current is 15A continuous. Can I assume then that the transformer (which has no markings whatsoever) is able to generate this amount of current? Or at least 10A? I'm just trying to reduce charging time from 20+ hours to maybe 10+ hours.
Also, I have attached 2 schematics with parallel LM338s

http://www.eleccircuit.com/1-20v-10a-adjustable-dc-power-supply-by-lm338/

http://www.eleccircuit.com/high-power-supply-regulater-0-30v-20a-by-lm338/

 

I had a look at the 2N3055 datasheets and they say that the Collector current is 15A continuous. Can I assume then that the transformer (which has no markings whatsoever) is able to generate this amount of current? Or at least 10A?

No, you can't make that sort of assumption.

 

Thanks AlbertHall for that information. I'll stay within that limitation.



ronv, I had a look at the 2N3055 datasheets and they say that the Collector current is 15A continuous. Can I assume then that the transformer (which has no markings whatsoever) is able to generate this amount of current? Or at least 10A? I'm just trying to reduce charging time from 20+ hours to maybe 10+ hours.
Also, I have attached 2 schematics with parallel LM338s

http://www.eleccircuit.com/1-20v-10a-adjustable-dc-power-supply-by-lm338/

http://www.eleccircuit.com/high-power-supply-regulater-0-30v-20a-by-lm338/

The problem is not so much the maximum current, but the maximum power in the transistor. For a regulator there is usually a fair amount of voltage across the transistor. Let's say 4 volts so if the current is 10 amps that's 40 watts in the transistor. Well you might say the 3055 is good for 115 watts and it is. But only if you can keep the case at 25C. So even at 40 watts the heat sink gets quite large. I should have ask you how big the heat sink is. Or sometimes they mount them on the case and use that.
I think the first circuit may be ok. The second is not so good. It will work, but the regulators won't share equally. Either way you somehow have to get rid of the heat.
Maybe we can "guesstimate" your transformer. Any idea how much it weighs or how big it is? Is there a fan?

3C per watt.
I guess you could just build it and measure how hot it gets.