[SOLVED] Question about circuit diagram of battery charger

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m.mokhtar

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Hello Every Body,

In the circuit diagram of 12 V battery charger in page 13 of the datasheet of LM338,
I calculated the output voltage of the charger, it will try to reach 16.4 V,
and in this case the LM338 will enter the current limit mode.

After the battery current reaches 380 MA the output voltage will switch to 14 V
provided that at the anode of 1N457 is at 1 V with respect ground
(the voltage drop on the diode and the output of the LM301A).

The 16.4 V, and the 14 V are not acceptable for the lead acid battery, and they must be

(14.6 to 15 V), and (13.6 to 13.8 V) respectively.

I need your advice, and It will be highly appreciated.

View attachment New Picture (4).bmp

Best Regards,
M.Mokhtar
 
Last edited:

Let Vout = 14.4 (less gassing)
Using my REFDES , R2 = R1*Vout/1.25 -R1 = (15+230)*14.4/1.25 - (15+230) = 2.6k

Then with your R4//R1 = my R2

Let Vout=13.6V, R2=2.4k so 1/{2.6k} + 1/{R4} = 1/{2.4k} , R4=31k
thus your R4 goes from 15k to 31k

This is a very old design and design has a problem with output saturation and diode drop with a wide range 1~3V, so replace Op Amp with Open Collector Comparator and delete diode..

THEN Add diodes for reverse portection on LM338 with power diodes.
 
Hi SunnySkyguy

Many thanks, I appreciate your efforts and helping me; but your answer makes me to ask the following questions:
1- you said : output saturation and diode drop with a wide range 1~3V.
For few MA do not you agree with me it is no more than 1 V ?.

2-you said : Let Vout = 14.4 (less gassing)
If the voltage is 15 V. will the battery gas before it is fully charged ?.

3- You said : it is old design, if you have something new , kindly post it to me.

4- Does the circuit you posted can be connected permanently?. I think so ; but I wanted to be sure through you.

My specifications is a charger permanently connected to a battery. The battery is 12V/10 AH, and it is written on it : cycle use is (14.6 to 15 V), and stand by use is (13.6 to 13.8 V) .

As you know when the charging current decreases to a predetermined value that indicates the battery is fully charged and the voltage is switched to stand by use voltage (13.6 to 13.8 V), how to determine that value of that current?. As far as I know that switching current is 0.1 of the initial charging current which is not known.

Your response will be highly appreciated.

Kind regards,
M.Moktar
 

 
Hi SunnySkyguy
I am very grateful for your quick response.

1- you said : output saturation and diode drop with a wide range 1~3V.
For few MA do not you agree with me it is no more than 1 V ?.

Not according to IC specs

I have re-read the data sheet of Lm301A for National Semiconductor and SGS they did not mention any thing about low output voltage value and as you know the voltage drop for the diode at few MA are 0.5 - 0.6 V, how come the 1~3V ?.

For :3- You said : it is old design, if you have something new , kindly post it to me.

Read my last answer again . It told you what to change.


I Understood what you said and I agree with you cent per cent but I thought there is a completely new technique.

As for : 4- Does the circuit you posted can be connected permanently?. I think so ; but I wanted to be sure through you.

YES then CHANGE the other parts for precise current limit instead of diode and the output of the LM301A.. Old parts could be used but output saturation tolerance was too high to make it accurate.

I meant the circuit you posted yourself and not mine.

Kind regards,
M.Mokhtar
 
Last edited:


LM301 Output Voltage Swing VS = ±15V RL = 10 kΩ ±12min ±14nom < ( from specs)
Above specs indicate 1~3V drop from supply rails 15-12=3

The "newer" technique (not that new is to use an open collector drive to replace an Op Amp and diode combination with a large drop from supply rail that will affect current limit R value.

I hope that is clear. If you already are using the LM301, it can be made to work by select on test result for R values with CUrrent limit.
 
Many thanks for helping me.

Kind regards,
M.Mokhtar
 

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