Battery charger design

[alphacat]

Hello.

I'd like to have your comments on the following design.
I need the batteries to power my unit which consumes 3.3V @ 80mA.
I wanted to ask a few questions please.

**broken link removed**

1. Did I design it correctly?
I'd set the ?VAC and the ?:? ratio after understanding numerous things with your help.

2. Should I use a capacitor post the bridge-rectifier?

3.
I thought of using 3 or 4 1.5V AAA batteries and a 3.3V regulator to power my unit.
Any better way?
Would 3 be ok, or since the current drawing is pretty high, I'd need 4?

4. How long will batteries last at such rated current?
Shall I decrease the current drawing?

5. Say that the batteries in series consume X voltage totally, what would be Vrectifer(out) according to x?

Thank you very much.

 

[Diver300]

1. You don't need any converter between the mains and a transformer. A transformer like this BLOCK|AVB1.5/2/6|TRANSFORMER, 1.5VA, 2 X 6V | Farnell United Kingdom will convert 230 V mains to 6V.

2. A smoothing capacitor is always a good idea.

3. If you are using Ni-MH batteries, 3 should be enough with a low dropout regulator

4. Ni-MH AAA size are about 700mAh so you should get about 9 hours at 80 mA

5. You would need the rectifier output to be more than about 5 V.

It is a very good idea to regulate the charging with a proper charge control circuit.

 

[Externet]

If you make a regulated voltage supply from 220VAC to 4.0 VDC , you could use a single Li-ion cell by inserting a 1N4001 diode ( -|>|- ). The Li-ion cell connects between (+) and (-)

+4.0V---------------(+)---------------|>|-------------- +3.3V

gnd------------------(-)---------------------------------- gnd

Or; If you use a larger voltage (12V?) battery and a 3.3 fixed voltage regulator IC, it will last longer time.

 

[audioguru]

Nobody makes a 3.3V rechargeable battery.
He forgot to say if the battery is lead-acid, Ni-Cad, Ni-MH or lithium and they all charge differently.

 

[Hero999]

If you make a regulated voltage supply from 220VAC to 4.0 VDC , you could use a single Li-ion cell by inserting a 1N4001 diode ( -|>|- ). The Li-ion cell connects between (+) and (-)

+4.0V---------------(+)---------------|>|-------------- +3.3V

gnd------------------(-)---------------------------------- gnd

Or; If you use a larger voltage (12V?) battery and a 3.3 fixed voltage regulator IC, it will last longer time.

Sorry but that's a really bad idea and is very dangerous: it doesn't limit the current and there's nothing to stop the battery charging.

To charge a lithium ion, the current needs to be limited to 0.5C, then when the voltage reaches about 4.1V, it should be float charged until the current drops to 1/10th o the original value, then it needs to be disconnected from the voltage source otherwise it might explode.

 

[alphacat]

Hello and thanks for the help.

1. You don't need any converter between the mains and a transformer. A transformer like this BLOCK|AVB1.5/2/6|TRANSFORMER, 1.5VA, 2 X 6V | Farnell United Kingdom will convert 230 V mains to 6V.

2. A smoothing capacitor is always a good idea.

Is this design good?
**broken link removed**

The problems with that design are:
1. the mains voltage could vary from 220V to 240V in different countries, wouldnt it hurt the battery charging?

2. You need two different chargers for 120Vac mains and 230Vac mains.

3. Could you use only one primary coil to convert the 230Vac straight to 4Vac?

3. If you are using Ni-MH batteries, 3 should be enough with a low dropout regulator

4. Ni-MH AAA size are about 700mAh so you should get about 9 hours at 80 mA

5. You would need the rectifier output to be more than about 5 V.

If you use 3 1.5V batteries, why would you want more than 4.5Vdc at the rectifier's output?

It is a very good idea to regulate the charging with a proper charge control circuit.

Could you please refer me to such circuit?
Is it connected at the charger's side or at the battery's side?

 

[MrAl]

Hi there,


Battery charging isnt as simple as it seems at first. The first thing you have to know is what kind of battery chemistry you are dealing with. This is extremely important as the charge circuit must be designed according to the type of battery (NiMH, NiCd, Li-ion, Lead Acid, etc.). In fact, if more than one type is to be charged with the same charger then you may have to design two or more algorithms into the charger. That's just the way it must be.

In general though the ac is stepped down and then a dc switching converter is used in some fashion, or even a linear in some cases.

So do begin with, you should state the battery chemistry(s) you will be working with and we can go from there.

 

[Diver300]

Hello and thanks for the help.


Is this design good?
**broken link removed**

The problems with that design are:
1. the mains voltage could vary from 220V to 240V in different countries, wouldnt it hurt the battery charging?

2. You need two different chargers for 120Vac mains and 230Vac mains.

3. Could you use only one primary coil to convert the 230Vac straight to 4Vac?

You can buy transformers with two 120 V primary windings. You connect them in series for 240 V and parallel for 120 V.

Also, you can get transformers at 4.5 or 5 V.

If you have a good charging control device, it won't matter if the supply is a little more than optimum. It also won't matter it the mains is 220 or 240 V.

In fact, it is almost impossible to get the output voltage from a transformer to be exactly what you expect, which is another reason not to bother trying, but have some regulator or charge control device after the transformer.

 

[alphacat]

@MrAl
Thanks for the advice.
I'll be reading on the different types of batteries.

@Diver
Could you please refer me to battery charging control schematics?
Thank you.

 

[Diver300]

https://www.electro-tech-online.com/custompdfs/2010/02/46686.pdf is a typical battery charger controller with some example circuits.

 

[audioguru]

Go to www.batteryuniversity.com and read about charging and discharging the different kinds of batteries.
Then go to a semiconductor manufacturer like Maxim-IC to see their smart battery charger ICs.

 

[alphacat]

Thanks alot guys.
I'll do my readings.