Recharging circuit for 4V rechargeable battery

Hello,
I want to make a recharging circuit for a 4V lead acid rechargeable battery from the home ac 220V power supply. I have a circuit for this but I am confused whether a step down transformer is actually required. The following circuit shows no step down transformer.


But from another site the circuit shows a step down transformer as shown below.



So is step down transformer mandatory?

thanks for replies

okbro said:

Hello,
I want to make a recharging circuit for a 4V lead acid rechargeable battery from the home ac 220V power supply. I have a circuit for this but I am confused whether a step down transformer is actually required. The following circuit shows no step down transformer.
View attachment 137056

But from another site the circuit shows a step down transformer as shown below.

View attachment 137057

So is step down transformer mandatory?

thanks for replies

Click to expand...

Only if you want it to be safe.

can we simply apply regulated 3V or 5V power supply across the battery to charge it?

The transformerless supply in the mosquito killer will only charge the battery at around 62 mA, and will not isolate from the mains. Larger currents would need a larger capacitor.

There is no way of regulating the charge in that circuit, so the battery will be overcharged eventually.

To regulate a transformerless supply, all you can do is dump the excess power as heat.

With a transformer, even with no regulator, more current will be taken when the battery is more discharged. If you want to regulate the charge, you can use a switch or regulator to take less current from the transformer.

And, of course, while you are trying to work out why your first prototype doesn't work, as long as you have good insulation around the mains input to the transformer, the output is safe and won't give you a shock. Unless the transformer is huge, if you short it out, it will take a long time to get hot enough to do much damage.

okbro said:

can we simply apply regulated 3V or 5V power supply across the battery to charge it?

Click to expand...

The correct float charge voltage for a two cell lead acid battery is 4.6V
It can be supplied with that forever.

You may get around that odd voltage by using a 5V adapter in series with a schottky diode, plus a low value resistor to limit the charge current. Try a one ohm 5W, to allow for the battery being connected when near dead.

Use a 3A schottky diode.

Make sure the 5V power unit can supply enough current, and keep an eye on the voltage across the battery.

What 4V lead-acid battery do you have?

crutschow said:

What 4V lead-acid battery do you have?

Click to expand...

battery is like this one:

Is it possible to use the PC PSU transformer as shown in the picture below and is the voltages shown at the pins correct?


I think the CT is the ground, the 12V means +12 and -12V, similarly for the two other 3V and 5V.

That's a switched-mode PSU transformer, which needs a high frequency drive - possibly anything from 20KHz to 250KHz, depending on the design.

The outputs are AC at the drive frequency.

You need a normal 50/60Hz transformer plus rectifier and smoothing capacitor etc., then a voltage regulator.

rjenkinsgb said:

That's a switched-mode PSU transformer, which needs a high frequency drive - possibly anything from 20KHz to 250KHz, depending on the design.

The outputs are AC at the drive frequency.

You need a normal 50/60Hz transformer plus rectifier and smoothing capacitor etc., then a voltage regulator.

Click to expand...

hi,

isn't the input and output frequency the same for a transformer?

also what capacitor value should I use if I want for 5V? how is the capacitor value selected for example if I want 12V or any other output voltage value?

thanks

okbro said:

isn't the input and output frequency the same for a transformer?

Click to expand...

Yes - as long as it is working within it's acceptable frequency range.

A high frequency transformer such as that will appear to be a dead short at mains frequencies! You would get nothing out except smoke from the transformer/house wiring/fuses!

They are driven by a transistor circuit in the PSU which is supplied with DC from a rectifier - not mains AC frequency. That's why they are called "Switched Mode" PSUs, as a transistor switch drives the transformer.
High frequency means the transformer can be a fraction the size of a 50/60Hz one for the same power through.

You must use a proper mains transformer.

I suggest you use a 5Vdc wallwort for your charging supply.
It's relatively cheap and safe for your application.

Do you want to keep the charger continually connected, or will you leave the charger attached only until it is charged.
If removed after it is charged, you likely will only need to add a small resistor in series with the 5V supply to limit the charging current to a safe level.

crutschow said:

I suggest you use a 5Vdc wallwort for your charging supply.
It's relatively cheap and safe for your application.

Do you want to keep the charger continually connected, or will you leave the charger attached only until it is charged.
If removed after it is charged, you likely will only need to add a small resistor in series with the 5V supply to limit the charging current to a safe level.

Click to expand...

hi,
I have already build the charger circuit with variable dc adaptor and tested and it is working. I am looking to add charge indicator circuit which detects battery level and disconnects.

I am also working on using transformer but I have been searching for the capacitor value after the bridge rectifier.

thanks

okbro said:

I am looking to add charge indicator circuit which detects battery level and disconnects.

Click to expand...

As long as the voltage is set precisely, there is no need to disconnect - a lead-acid battery can be left on float charge forever. The charger needs to be current limited though, to prevent it charging too fast.

The charge happens in two stages; first the voltage increases until it reaches the set value, then the current gradually falls off until it gets down to the trickle charge level - probably somewhere between 20 - 50mA for that battery.


When the battery voltage reaches maximum, it's only somewhere between a third and half full charge - you must monitor the current as well, to work out the state of charge.

(If you look at any old, non-microprocessor type battery charger they have an ammeter not a voltmeter, as you know the battery is charged when the current gets to a low value).
eg.


If you try to copy some online supposed battery charge controls that use voltage detection only, it will not work properly at all. Never assume online or youtube posted circuits are correct - there are more incorrect than there are working!


The battery you have is only 1AH capacity, so the maximum charge current is probably around a 250mA.
Too much current will wreck the battery very quickly.

The capacitor value depends on the transformer. The voltage should be at least double the transformer secondary voltage; higher is Ok.

The capacitor value depends on the load current. I'd probably use 4700uF as a rough guess value.

okbro said:

battery is like this one:
View attachment 137104

Click to expand...

Is it like that one or it is that one.
Does your battery have numbers on it like Voltage and Ah ratings?

Mike.