12 volts sealed battery charging circuit designing with constant voltage & current

[rajaram04]

12 volts sealed battery charging circuit designing with constant voltage & current

Hello

Given standby use : 13.5v - 13.8v
for a 12v sealed battery is there

A single or many chains of LED etc ll act as load further for hours

So how to make a charging circuit which can deliver charge with constant voltage & constant current supply ???

 

[KlausST]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

Hi,

measure the voltage and current directely at the battery and you are safe.

Klaus

 

[Audioguru]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

All the details of your many personal messages to me about this project are missing in this thread.
Before, you said occasional use so I selected the standby voltage. Now you say the load will be for hours which is not standby so the charging voltage for frequent use should be used.
Before, I discussed using an LM317 as a current regulator followed by another LM317 as a voltage regulator. You said you had a certain transformer.

We did not discuss charging the battery and powering the load at the same time.

 

[rajaram04]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

Hi,

measure the voltage and current directely at the battery and you are safe.

Klaus

no actually i hv to deal with provided powerline for battery . .
means the power source for charging must have a constant voltage & current source

- - - Updated - - -

All the details of your many personal messages to me about this project are missing in this thread.
Before, you said occasional use so I selected the standby voltage. Now you say the load will be for hours which is not standby so the charging voltage for frequent use should be used.
Before, I discussed using an LM317 as a current regulator followed by another LM317 as a voltage regulator. You said you had a certain transformer.

We did not discuss charging the battery and powering the load at the same time.

No its just for say 6 hrs or 8 hrs n so on & rest is on standby so forget the load part as its a part in absence of power supply

just i wrote the load details so as to start the thread as per advice

We need only concentrate on charger part as we discussed for 4v battery . .

please proceed sir

 

[KlausST]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

Hi,

no actually i hv to deal with provided powerline for battery . .
means the power source for charging must have a constant voltage & current source

If you measure voltage and current then you have all information for a constant I constant U charging.
Just keep charging current constant as long as voltage is below 13.7V then switch over to constant voltage charging.

Klaus

 

[rajaram04]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

o i see , so is there any design which we could implant in our system for auto switching as you said ?

 

[Audioguru]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

I looked in Google and found many lead-acid battery charger circuits the same. They use an LM317 voltage regulator IC and a transistor that senses the current and it limits the current.
But I cannot remember the recommended charging current of your battery so I cannot calculate the current sensing resistor value.

 

[rajaram04]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

ok let me reqrite the details laballed over the battery

its 12v 7.5Ah

constant voltage charge
voltage regulation

cycle use : 14.4v - 15v

standby use : 13.5v - 13.8v

maximum charging current : 2.25v



Too sir as the diagram talks about a 15v transformer , what if i apply a 18v in place ?

 

[Audioguru]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

If you use an 18V transformer then the LM317 will get hotter and will need a larger heatsink.

We cannot calculate the charging current without knowing the maximum allowed continuous current from your 18V transformer.

 

[rajaram04]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

If you use an 18V transformer then the LM317 will get hotter and will need a larger heatsink.

We cannot calculate the charging current without knowing the maximum allowed continuous current from your 18V transformer.

& what if we convert 18v to 15v via LM317 before applying to our main circuit ?
what disturbances ll happen there ?

& my transformer is 18v 500mA

 

[Audioguru]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

The little 18V/0.5A transformer has a maximum output of only 9VA. Its peak voltage is 25.5V so the maximum current you can charge with is 9VA/25.5V= 353mA.
If your 7.5Ah battery is dead then it will take 7.5/0.353= 21.2 hours to be fully charged. I cannot remember its standby maximum charging current.

When loaded, the 25.5V peak is reduced to 23.5V by the bridge rectifier then ripple reduces it further to 23V.
With such a low current the 13.7V LM317 will heat with only (23V - about 12V) x 353mA= 3.9W which is not much. A fairly small heatsink can be used. An additional 15V regulator is not needed.

 

[rajaram04]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

The little 18V/0.5A transformer has a maximum output of only 9VA. Its peak voltage is 25.5V so the maximum current you can charge with is 9VA/25.5V= 353mA.
If your 7.5Ah battery is dead then it will take 7.5/0.353= 21.2 hours to be fully charged. I cannot remember its standby maximum charging current.

When loaded, the 25.5V peak is reduced to 23.5V by the bridge rectifier then ripple reduces it further to 23V.
With such a low current the 13.7V LM317 will heat with only (23V - about 12V) x 353mA= 3.9W which is not much. A fairly small heatsink can be used. An additional 15V regulator is not needed.

wowww if it is 353mA then its ok for experiment i guess . .

& ya i ve that heat sink for the purpose

so . . . finally should i apply this 18v directly to the main circuit ?

may i proceed sir ?

 

[Audioguru]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

What is the maximum standby current rating of your battery? If it is less than the charger current then the battery must be disconnected from the charger when it is fully charged and you will need a circuit to detect that it is fully charged.

Are you using an LM317 in a TO220 package?
If you use thermal paste and if your heatsink is rated for cooling 3.9W then an input of 18VAC to a bridge rectifier and large main filter capacitor is fine.

Did you calculate the value of the main filter capacitor?
Did you calculate the value and power rating of the current sensing resistor?

 

[rajaram04]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

What is the maximum standby current rating of your battery? If it is less than the charger current then the battery must be disconnected from the charger when it is fully charged and you will need a circuit to detect that it is fully charged.

Are you using an LM317 in a TO220 package?
If you use thermal paste and if your heatsink is rated for cooling 3.9W then an input of 18VAC to a bridge rectifier and large main filter capacitor is fine.

Did you calculate the value of the main filter capacitor?
Did you calculate the value and power rating of the current sensing resistor?

standby current rate of battery is not laballed so please tell any method to find it out ?

Though i ve encountered so many cincuits but in my case how could i design the detector circuit to know about the battery is fully charged ?

yeah i am using only TO220 packacge

I hv heat sink paste but what should be the area of heat sink plate so as to rate it for 3.9W ???? No idea,please help

Its bridge rectifier i am applying & filter capacitor value is 1000uF

Whats the procedure to calculate value and power rating of the current sensing resistor ?

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sample indicator diagram for the very simple indicaton of low charge full charge & over charge . . here a 13v zener is mentioned
I ve got it here on this site only

but now hv to make the over charge indication for 13.7v

 

[Audioguru]

Re: 12 volts sealed battery charging circuit designing with constant voltage & curren

standby current rate of battery is not laballed so please tell any method to find it out ?
Though i ve encountered so many cincuits but in my case how could i design the detector circuit to know about the battery is fully charged ?

You can learn about batteries in Google or at www.batteryuniversity.com . It says the recommended continuous trickle charging current and how to measure a full charge (reduction in charging current?) for a few types of batteries.

yeah i am using only TO220 package
I hv heat sink paste but what should be the area of heat sink plate so as to rate it for 3.9W ????
We use an inexpensive manufactured heatsink with fins that has a W rating.

Its bridge rectifier i am applying & filter capacitor value is 1000uF

That is fine for 350mA or less.

Whats the procedure to calculate value and power rating of the current sensing resistor ?

You look at the datasheet for the transistor. It shows a graph of its typical Base-Emitter Voltage Vs Collector Current. You need the voltage when its collector current is low at about 1mA, at 25 degrees C.
Then you use Ohm's Law to calculate the resistor value and its power rating.
The Maximum power from your little transformer is 353mA when it will be very hot inside. Use 300mA to calculate the current sensing resistor to be safe.