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LM317T need help

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Suraj143

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I want to charge my rechargeable batteries using LM317T.
Batteries 1.2V / 2500 mAh – Energizer.
I want to charge them at 0.1C that is 250 mA. I’m going to charge two cells in series.

I refered to the data sheet of LM317T.

**broken link removed**

1) If I adjust the R to give 250mA it will charge. But who knows the charging voltage there near the two batteries? Supply voltage is 6V.

2) If I use a 12V input voltage will it be harmful to the batteries. Because battery needs 1.6 x 2 = 3.2 V to charge minimum.

**broken link removed**

3) If I use this method I can calculate the correct voltage that is 3.2 V. But who knows the charging current?

4) What is that RS resistor?

5) Which circuit should I use to charge the two batteries? Do I need both constant current & voltage?

Thanks……
 
You'll be lucky if you can charge the battery to the full voltage from 6V using the first circuit.

lm217-dropout-gif.11469

The dropout voltage is about 1.75V @ 250mA, then you need to add 1.3V (the maximum output voltage), leaving only 2.95V for your cells. Although they wiil charge in the end, when the regulator goes into dropout they will charge very slowly to 3.2V.

Regarding Rs, did you look at the datasheet?
[latex]Z_O=R_S(1+ \frac {R2}{R1})[/latex]

Zo is just the output impedance of the charger; you set it so a value that will set the output current to a safe level.
 
hi suraj,
Get the LM317 datasheet, www.datasheetarchive.com

It has a number of applications, shows how to calculate the component values.

The GIF gives an example, recalculate as required.

The Rs resistor, is the current sense resistor.

The C10 is the 10 hour rate, also the charge to energy stored efficiency is about 66%, they require 15 hours at the C10 rate, DONT overcharge the cells.

Eric
 
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You don't need to limit the charging voltage. The battery limits its own voltage.

Go to Energizer's site and look at their Ni-MH battery applications manual. A cell's voltage is only 1.35V per cell when fully charged at only 1/10th its mAh rating.
 

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1/10th C is basically a trickle charge, looks like you have to charge at a minimum of 1C in order to see the obivous voltage depression that indicates the battery is full. I wonder if this is why the cheap charger I have sometimes badly overheats my 2500mah cells. It came with 1800mah cells and I think that's what it was designed for, maybe it can't see the voltage depression when the 2500mah pack cells are being charged.
 
audioguru said:
You don't need to limit the charging voltage. The battery limits its own voltage.
But if you fiddle the chager impedance and voltage so the continious charging current is safe then the battery won't overcharge.
 
Hi all thanks for the replies.
I adjusted erics diagram to suits my batteries.Pls see is the values correct or not?Output calculated 3.2V and 250 mA.
**broken link removed**

and another question
**broken link removed**

In this diagram sending only constant current.And the voltage is varying is that correct?
The first diagram its sending both constant voltage & current is that wright?(All the time its sending 3.2V)
 
The voltage will not exceed 3.2V. It starts at whatever the discharged state of the battery is. The current is approximately 0.75/R3, until the voltage limits (it may never limit with two NiMH cells, and if it does, they may be damaged). The current then goes to zero. R3 needs to be about 3 ohms.
In simulation, it oscillates at high frequency. You can eliminate this by adding 100nF from base to collector of the transistor.
This still does not solve the problem of accurately determining when the charge is complete, as has been noted by others.
NOTE: I'm not endorsing this circuit, I'm just explaining how it works.
 
1.6V per cell is too high and the cells will overcharge.
Look at Energizer's graph I posted. At a charging current of 1/10th the cell's capacity it is fully charged at 1.4V and will never reach 1.6V.
Your circuit will keep pumping 250mA overcharging current into the battery forever. Energizer says its life is shortened by overcharging.
 
1) Hi shall I adjust the output voltage to 2.8V is it ok? 3.2V is high as audioguru said.
What’s the correct voltage need to charge two 1.2V cells. If I have another brand batteries how I know in which voltage should I charge.

2) Hi Ron H how this come? Current = 0.75/R3?

3) Is the current goes to zero when I’m charging?

4) If I charge at 0.1C rate it will not harmful to batteries too much isn’t it? (Even if I leave the charger)
 
Suraj143 said:
1) Hi shall I adjust the output voltage to 2.8V is it ok?
Adjust so there is 2.8V across the two cells in series when they are fully charged.

What’s the correct voltage need to charge two 1.2V cells. If I have another brand batteries how I know in which voltage should I charge.
Look on the datasheet for the cells that you use.

4) If I charge at 0.1C rate it will not harmful to batteries too much isn’t it? (Even if I leave the charger)
Energizer says to reduce the trickle charge current to 1/40th of the cell's rating or its life will be shortened.
 
Hi audioguru I downloaded the datasheet in my other GP batteries it says charge voltage must not reach over 1.5V per cell.
Here is a graph but it doesn’t have a 0.1C curve. Can I charge at 1.5V with 250mA?
**broken link removed**

Also I don’t need to discharge NIMH it because less cause to memory effect is that right?

Here is the datasheet.
**broken link removed**
 
Suraj143 said:
1) Hi shall I adjust the output voltage to 2.8V is it ok? 3.2V is high as audioguru said.
What’s the correct voltage need to charge two 1.2V cells. If I have another brand batteries how I know in which voltage should I charge.
2) Hi Ron H how this come? Current = 0.75/R3?
Because the current limits due to the 2N2222 starting to turn on, and it is on when the base-emitter voltage (basically the voltage across R3) is about 0.75V. Almost all the charging current flows through R3. The transistor actually senses the voltage across R3, and reduces the voltage from the regulator to maintain a voltage across the battery that ensures it is being charged at 250mA. As the battery voltage increases, the transistor slowly turns off, maintaining approximately 250mA.

3) Is the current goes to zero when I’m charging?
It goes to zero when the voltage across the battery reaches 1.25*(R1+R2)/R1.
4) If I charge at 0.1C rate it will not harmful to batteries too much isn’t it? (Even if I leave the charger)
 
It's also a good idea to connect resistors across the batteries to ballance the cell voltages.
 
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