Need help for building a 24v nicad battery charger

[protocol217]

I had two 14.4 volt nicd battery's and i took them apart and made a 24 v battery all in series from 20 1,2 volt 1200 mah cells.
And i need a charger for it to charge all 20 cells at once 24 volts.

the old charger that i had for 14.4 volts was simple small it had two led's and 3 resistors, ( one was big with 5-6 watts and was hot when charging ) and it had an 18 volt 400 mamps rated adapter.

so is it possible to build a charger to charge 24 v and what adapter do i need, and can someone give me a circuit diagram so i can build one.

i saw 24 volts chargers on ebay like sites but they had 3 pins not 2 i need just two.

 

[tvtech]

Hi protocol

Basics first: Please read this link: **broken link removed**

Regards,
tvtech

 

[protocol217]

thanks for the reply, i already know nicd battery basics i need a charger for 24 v ( 20 cells in series ) these battery are from a cordless drill. i intend to use them for an audio amp ( from pc speakers that work with 24 volts) now i need a charger.

 

[tvtech]

You can always build one..

Attached file below.

 

[protocol217]

i am more a beginner in electronics, sure i can build one if i have a circuit diagram and every resistor and capacitor... calculated values for charging 24 volts nicd battery's . but i don't have that and i don't really know how to do those calculations so i asked here for already done circuit diagram for charging 20 nicd cells connected in series = 24 volts.

 

[tvtech]

Hi protocol

I now know you need a completed design to suit your needs....I am not the one that can help you....many Members here can though.

Me, I will try and point you in the right direction

I work with Electronics daily....and ETO is where I relax.

All the best,
tvtech

 

[ChrisP58]

Basic battery charging only needs two pins, so if the ebay chargers you have looked at have 3 pins, one of them may not be used.

But, a better way to charge NiCd cells uses a thermister in the pack to detect the pack temperature. The thermister needs another connection, for a total of 3 pins.

I would suggest looking more closely at the ebay chargers to see which of these possibilities apply. And if the charger needs a thermister on the 3rd pin, it is very easy to add one to your battery pack.

 

[protocol217]

I don have, thermister, and i dont need one i want to build it with only two pins, i need a circuit diagram so i can get started building one.

 

[protocol217]

Basic battery charging only needs two pins, so if the ebay chargers you have looked at have 3 pins, one of them may not be used.

But, a better way to charge NiCd cells uses a thermister in the pack to detect the pack temperature. The thermister needs another connection, for a total of 3 pins.

I would suggest looking more closely at the ebay chargers to see which of these possibilities apply. And if the charger needs a thermister on the 3rd pin, it is very easy to add one to your battery pack.

so you say that if i would buy one from ebay that has three pins i can charge my nicd battery pack on two pins and leave the third one empty or is the thermister on battery pack necessary for that kind of charger ?

 

[ChrisP58]

so you say that if i would buy one from ebay that has three pins i can charge my nicd battery pack on two pins and leave the third one empty or is the thermister on battery pack necessary for that kind of charger ?

You need to look at the details of the charger in question.

 

[protocol217]

i found this on instructables: https://www.instructables.com/id/Homemade-Cordless-Drill-Battery-Charger/?ALLSTEPS

**broken link removed**

can i build a charger like that ? it is simple, but can someone do the math for me here please.

i need: these resistor values and what input adapter should i use ( what voltage, what amperage rating, and AC or DC adapter )

i have a similar charger for 14,4v it uses 18v dc adapter.

 

[chemelec]

First off, You will need a Transformer with about 24 Volts AC Output.
This Circuit Should also have a Filter Capacitor (About 470uF) between the Out of the 1N4004 and Battery Negative.
When Rectified, and Filtered, it will give you about 33 Volts DC.
A Full Charge on your 24 Volt battery is about 28 Volts.

But Either way, that is NOT a Very Good Design.

Your Charge Current Should NOT Exceed 120 mA.
So R1 could be a 48 Ohm Resistor.
This should give you about 100 mA charge as the battery gets up to full.

R2, Possibly a 150 Ohm Resistor.

 

[protocol217]

First off, You will need a Transformer with about 24 Volts AC Output.
This Circuit Should also have a Filter Capacitor (About 470uF) between the Out of the 1N4004 and Battery Negative.
When Rectified, and Filtered, it will give you about 33 Volts DC.
A Full Charge on your 24 Volt battery is about 28 Volts.

But Either way, that is NOT a Very Good Design.

Your Charge Current Should NOT Exceed 120 mA.
So R1 could be a 48 Ohm Resistor.
This should give you about 100 mA charge as the battery gets up to full.

R2, Possibly a 150 Ohm Resistor.

thanks for a very useful reply chemelec, i have i few questions:

OK, i have 24 v AC transformer, 2 amps rated can that one work or do i need a smaller one or the output amperage of the transformer does not mater ?
and R1 = 48 ohm but how many watt should the resistor be ?
and the Led diode will stop glowing when the battery is full, am i right ?
and you say that this is NOT a Very Good Design, what are the disadvantages of this charger ?

when this questions get answered i will start building my charger, i hope it will charge my battery's.

 

[chemelec]

The Amperage of the transformer is Not Important, as long as it Exceeds the Required current.
And it Does.

I would recommend a 5 watt resistor for R1.
R2 can be just a 1/4 watt.
Yes the LED should either go Dimmer or Off.

A BETTER CIRCUIT would be CONSTANT CURRENT Controlled.

The Problem with this circuit is If Any Part of the Battery is Bad, It will Deliver EXCESSIVE Current.
Possibly catching the battery or other parts on FIRE.

A 1/8 or 1/4 Amp FAST BLOW FUSE between the charger and the Battery Would be a GOOD THING.

BUT it is NOT a CIRCUIT that I would use.

 

[protocol217]

thanks for the explanation, i already have 3 chargers like this, for 12 v, for 14.4v and for 18v, they have 2 led 's one for power and one that goes out when the battery is charged.
the 12v one has a 3 pin transistor ( S9012 H128 ) and one led plus the resistors. and they work well so far, so i am going to build this charger with the values that you gave me and when charging will leave it on the safe place in case it catch on fire ( because the battery's are old. )

one more question what is the minimal current that the transformer should deliver ? and when you said 1/8 or 1/4 Amp FAST BLOW FUSE that is 125 Mamp fuse and 250 Mamp fuse am i right, if so which one should i put 125 one or 250 one ?

and is this how it should be rectified and filtered and is the fuse on the right place?:

**broken link removed**

and another question/problem:
i cant find 48 ohm resistor only 47 ohm ( 4 watt ), when i measure the resistance of that 47 ohm resistor i get a reading 48,8 ohm - 49 should i add in series one 1 ohm resistor by color code ( on the multimeter measurement 3 ohms) or not ?

i have 2 more smaller resistors 47 ohm by color code and 49 ohm on the multimeter which i plan to add in parallel to increase the wattage over 5 watts. so should i add a 1 ohm resistor or not, or is it already to much ohms ?

here are the photos of my resistors from an old TV:
**broken link removed**
**broken link removed**

update: i just found i transformer rated: 24V 0,8 A on the voltmeter it gives 25,5 V AC i will use that one because it is smaller.

 

[chemelec]

The Fuse is Just to Protect against Excessive Current, So EITHER of those fuses is OK.

Just the 47 Ohm, 4 Watt resistor should be OK.

And that 24 Volt, 0.8 Amp transformer is Also OK.

Yes Put in the Capacitor as you Show.
But if using the Full Wave Bridge, You can REMOVE the 1N4004 Diode.

 

[protocol217]

great, thanks i am starting to put it all together, and i will put the two smaller resistors from the first picture in parallel with the big ceramic one to increase the wattage over 5 watts.


i am curios in which case will it deliver more current to battery's?
with 48 ohms or 47 ohm resistor, i think 47 but i am not sure ?

 

[chemelec]

DON'T Add the Smaller Resistors.
The 4 WATT one is OK by itself.

Yes, The LOWER the Resistor value, the Greater the Current flow.

The Actual Current will be determined by the Resistance, As well as the VOLTAGE DIFFERENCE between the Supply Voltage and the Battery Voltage.

So if the Battery is really Low, the Current will me HIGHER, Compared to if the Battery is Not so low.
And I can't Predict What this Lower Voltage might be!

This is Why a "CONSTANT CURRENT SUPPLY" Would be "MUCH BETTER".

For this type of Charger, Ideally you want a MAXIMUM Current of 1/10 of battery Capacity.
So for 1200 M/AH Batteries, a Charge Current of 120 mA or Less.

 

[protocol217]

give me a circuit diagram of some constant current chargers for my case if you have.
and if it helps i have already few LM317 regulators.

 

[chemelec]

Here is the Circuit.
But DO NOT Short out the Output.

Input to Output Voltage Can NOT Exceed 40 Volts