LM 317T Constant current charging problem

[Suraj143]

Hi all!

I’m using LM 317T constant current circuit (as in the data sheet) to charge my NEW 6V SLA battery.

The NEW battery I checked the voltage. It shows 6.2V.

1)When I use constant current circuit it shows 0 current when charging. Why is this?

The R1 I calculated 2.7 ohms. 1.25 / 450 = 2.7 ohms.
I used a 6V transformer to give the supply. The DC voltage showed 8.1V in the regulator.

Battery specifications

Capacity 6V / 4.5AH
Cycle use 7.25 – 7.45V

2)When I use constant current circuit will it charge all the time with the calculated current (450 mA) or will it decrease when the battery voltage increases?

Thanks

 

[k7elp60]

First off there is 1.25 volts drop across the constant current resistor and about 2 volts drop across the regulator itself so you don't have enough voltage a the battery terminals to charge the battery. It is best to have a little extra input voltage to the regulator.

 

[audioguru]

The battery needs 7.45V.
The 2.7 ohm resistor needs 1.25V.
The LM317 nees 2V.
So you need a DC source of 10.7V minimum when it has a 450mA load..
You have only 8.1V without a load.

 

[Suraj143]

Ok thanks. Then I will use a 12V transformer.

Will the battery overcharge if I use constant current with this 12V transformer?

Note: I need constant current, not constant voltage / current limited type.

 

[justDIY]

lead acid are flexible in their charging requirements - you should visit google and read about it, there is a lot of information

the easiest way is the constant voltage charge - which is printed right on the battery (7.25 to 7.45). build your lm317 to output 7.4 volts and thats all you need to do. when the battery is low, it will draw a large current as the regulator tries to pull the battery up to 7.4v. as the battery nears charge, the current will decrease. by 'cycle use,' they mean you cannot float-charge the battery at this voltage, as it will damage it eventually. consult your vendor or datasheet for a float charge voltage. it may be listed as 'standby use' or standby voltage

 

[Suraj143]

Thanks justDIY.
Then I will make a constant voltage type with LM317T.Need only two resistors.

Then the battery will draw its own needed current.

My battery says STAND BY VOLTAGE 6.8V – 6.9V what is this?
Do I have to adjust the charger to 6.8V?

What’s the transformer do I have to use?

Thanks.

 

[ericgibbs]

Thanks justDIY.
Then I will make a constant voltage type with LM317T.Need only two resistors.

Then the battery will draw its own needed current.

My battery says STAND BY VOLTAGE 6.8V – 6.9V what is this?
Do I have to adjust the charger to 6.8V?
As justDIY points out, the 7.4V is a cyclic charge rate [ when you want to charge the battery quickly, when charged, remove it from the charger and use it elsewhere]

The STANDBY VOLTAGE is the 'float charge voltage', thats when the battery is left connected to charger and used to power the 'system' when the mains power fails. When the mains is restored the battery charges up to the float voltage of 6.8V.

If you used a cyclic 7.4V charger permanently connected to the battery it would overcharge the battery and shorten its life or ruin it.

What’s the transformer do I have to use?

A 12Vac transformer would enable you use either cyclic or float, it depends upon how you plan to use the battery. Remember to use the correct VA rating for the transformer to suit the current charge rate.
Thanks.

Hope this helps.

 

[Suraj143]

Hi Eric I’m going to charge my batteries once in a while (once a week or two).
I will use cyclic voltage 7.4V.

I will make the constant voltage circuit (the two resistor circuit). Is it ok? Not the constant voltage/current limited one.

The constant current circuit I made but it’s charging current showed 0. I think the transformer in not enough voltage as earlier mentioned by others.

Now I’m going to use 12V / 500mA one.

 

[ericgibbs]

Hi Eric I’m going to charge my batteries once in a while (once a week or two).
I will use cyclic voltage 7.4V.

OK, but stop the charge when the battery is fully charged.

I will make the constant voltage circuit (the two resistor circuit). Is it ok? Not the constant voltage/current limited one.

I prefer the constant voltage chargers for SLA

The constant current circuit I made but it’s charging current showed 0. I think the transformer in not enough voltage as earlier mentioned by others.

Yes, as previous post have stated the 6Vac transformer you have is not suitable.

Now I’m going to use 12V / 500mA one.

Gayan was working on a SLA charger that would suit your requirement.

 

[justDIY]

I like these designs for a very simple current limited constant-voltage charger:

**broken link removed**

The current limit is really only needed if your power supply is very strong - the small transformer you mention 12v/500ma will already be limited in its current capacity.

Another thing to consider is input smoothing. If you indeed have a transformer and not a power supply, the input voltage will have ripple in it. large capacitance on the input to the regular will be required to keep it from dropping out as the AC voltage does its sine-wave thing. An argument could be made however, that this dropout is not a bad thing. The drop-outs will cause pulses in the output of the charger, which research shows is good for a lead acid battery.

 

[Hero999]

Are you sure you want constant current?

Most lead acid batteries are better charged of constant voltage.

I would float charge it to 7.0V, the internal current limiting in the LM317 should be sufficient to stop anything nasty happening.

 

[justDIY]

don't use 7.0 volts. use the voltages supplied by the manuf, printed right on the battery.

use 6.8 to 6.9 for continuous worry-free charging
or
use 7.25 to 7.45 for fast charging, which you will need to monitor to avoid damaging the battery.