Is this circuit compatible for charging my gel cell?

Discussion in 'General Electronics Discussion' started by Farukh Khan, Jul 20, 2015.

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1. Farukh Khan

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Jun 12, 2015
Hello Guys,

I got a 12V 7.2AH Gel Cell Battery. I want to make a charger circuit for my battery. I got a 24V 20A Power Supply for my whole system. So here I got a circuit diagram. But I am not sure if the circuit will charge my battery fast and safely using the power supply I have. I am attaching the circuit diagram in this post. If there is any fault in the diagram then share it with me. Please suggest me any changes that I need to make the circuit a safe and fast charger using my 24V power supply. As I think the circuit is for max 18V supply. So please suggest me how will I adapt it for my 24V supply.'

Thanks in advance......

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2. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
that circuit will do a relatively slow charge on your batteries. Nite that with a 24 volt or source the regulator will likely need a significant heat sink if you opt for a current anywhere near the rated maximum for the regulator.

3. AnalogKid

2,307
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Jun 10, 2015
P = E x I

Measure the battery voltage, and subtract that from 24. Let's assume 10 V for now. 24 - 10 = 14 V. That is the initial voltage across the LM317 regulator. Select a resistor from those on the drawing to set the charging current. For a 7 A h battery, keep it below 700 mA (capacity / 10, a safe max charging current for an unknown battery). A 1 ohm resistor sets the charging current at 0.65 A. 0.65 x 14 = 9.1 W. Again, gonna need a heatsink. Think golf-ball size or larger with lotsa fins.

ak

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Apr 24, 2015
5. Colin Mitchell

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Aug 31, 2014
24v supply is far too high for the circuit.
"A 1 ohm resistor sets the charging current at 0.65 A. 0.65 x 14 = 9.1 W. Again, gonna need a heatsink."
What rubbish - you need to read my basic electronics course.You know nothing about electronics.

AnalogKid likes this.
6. Alec_t

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Jul 7, 2015
Not rubbish, Colin. AK was referring to the current-sense resistor in the post #1 circuit.

7. Colin Mitchell

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Aug 31, 2014
"Not rubbish, Colin. AK was referring to the current-sense resistor in the post #1 circuit."

This make two of you that know nothing about electronics.
This is MY circuit and it is only designed for very low current to charge a 1.2AH battery.

Don't you think I know how it works?

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8. Farukh Khan

157
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Jun 12, 2015
AnalogKid your calculations seems right to me but as Colin Mitchell is the designer of the circuit I think he will know much better about it's working procedure than us. So, Colin please suggest me the changes in your diagram to be made for charging my 7.2AH Gel Cell Battery fast and most importantly safe.

Thanks in advance.....

9. Colin Mitchell

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Aug 31, 2014
As you can see by the circuit, the maximum current is 1300mA when you use a 2 amp plug pack.

10. AnalogKid

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Jun 10, 2015
In post #7 you said:
This is MY circuit and it is only designed for very low current to charge a 1.2AH battery.

1300 mA seems a bit high for a 1.2 A h battery.

ak

11. Farukh Khan

157
2
Jun 12, 2015
ak seems right to me...1300mA seems quite huge for charging a 1.2AH battery.

12. AnalogKid

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Jun 10, 2015
Yah. I've never seen C/0.9 as a recommended charging rate for *anything*.

ak

13. Alec_t

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Jul 7, 2015
I thought the battery was 7.2Ah, not 1.2Ah?

Last edited: Jul 23, 2015
14. Gryd3

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Jun 25, 2014
Regardless... let's get back on track here shall we?

@Colin Mitchell , You don't have to be such a stick in the mud.
The op want's to know if that circuit can be used. Obviously it's current configuration will work but will not charge the battery at a decent rate.

So the next question here, is what modifications can be made to this system to allow it to charge at a higher rate?

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15. AnalogKid

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Jun 10, 2015
Actually, if C = 7.2 A h, then 650 mA = C / 11, a good starting point for a battery with no manufacturer, part number, or age. A higher charging rate certainly is possible by changing the current shunt resistor as indicated in the schematic.

The discussion about a 1.2 A h cell refers to Colin's declaration in post #7 contrasting with the circuit notes in his own schematic in post #1.

ak

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16. Gryd3

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Jun 25, 2014
Good point on the lack of battery details.
Until the OP can get those details, it's unsafe to guess what current you will want for a 'fast charge'

17. Farukh Khan

157
2
Jun 12, 2015
The fact is not about charging the battery at fast rate. I want to modify colin's schematic so that my battery gets charged at a decent rate and the main thing I want is to keep the battery safe. So my point is to charge my gel cell at a quite decent rate (which I refer to charge it as fast as possible till the rate harms my battery life) . And the configuration of my battery is : Gel Cell 12V 7.2AH and chinese brand.

So guys please help me out with a new schematic or to modify colin's schematic or the gel cell charging procedure shown by Minder on reply #4

Thanks in advance...

18. Gryd3

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Jun 25, 2014
As AnalogKid pointed out... Unless we can get specifics on that battery, the best we can do is a generic charge rate..
If you do a little googeling, you will find that charge rates of Lead-Acid batteries, as well as a couple reports of Gell Cell being more vulnerable to damage from over voltage conditions.

Information here : http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
Give a charge time of well over 10 hours, so a charge rate of Capacity / 10 is well within the norm. If you want to take care of the battery, you may even want to go as low as Capacity / 12 to 16.
That said.. using a 1Ω resistor will provide a charge current within this range. There are no additional modifications needed, unless you can find a data sheet that specifically states you can charge this particular battery at a higher rate of charge.

19. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
the thing which will get hot is the regulator. It will be dissipating up to 10 W while charging and up to 15W if the output is sorted (all of these are for 650mA).

I would probably use a 4degC/W heat sink.

Note that the court charge voltage for lead acid battery is temperature dependant. If you're in an area where the temperature can vary widely, check out the range of optimum cell charge voltages for the range and use the lowest of the.

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20. AnalogKid

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Jun 10, 2015
The Maxim chip is smarter than the circuit in post #1, and Linear Tech makes a similar part that I have used with no problems. The nice thing about the smart chips is that they go into float or trickle mode automatically, so you don't have to set an alarm clock. But for simple and effective you can't do better than the circuit in post #1, no modifications, calculations in post #3.

ak

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