Charging current is decreased when battery is connected

hi..
can any one explain how can I increase the charging current for 7.2Volts/2800mAH NiCd battery..??
I have tried many circuits like..

http://www.reuk.co.uk/print.php?article=LM317-High-Current-Voltage-Regulator.htm

NiCd Battery Charger - Fig3

**broken link removed**

etc...

They look good but in fact they are useless for me because when Battery is connected the charging current is reduced significantly to about 290 to 300mA... which is too low..

I'm using simple transformer supply (3A/12V) with full wave bridge rectifier+1000 µF Polar Capacitor...

when I charged the battery without any current/voltage regulator circuit i.e., direct from the rectifier's O/P(in parallel with the capacitor) the charging current was initial 1.3A and after about 30 minutes it was 0.8A..

Is there any method (circuit) that can draw even more current from that transformer supply....
I need 1.5A to 2A charging current at least( better be controlled as I have to use it with µController)..

Exerts out there, plz help me cause I'm already late..

With your 12V transformer and bridge rectifier you should get 17V or so of DC, if this only sends 1.3A into the batteries, then either the transformer set up is faulty (does it get hot?) or the batteries have a high internal resistance and will not accept charge very easily as you have found.
Frank

No the transformer never get hot...neither battery is faulty because I have three similar transformers and 2 similar batteries... I have tested both(transformer and battery).....

---------- Post added at 10:03 ---------- Previous post was at 09:57 ----------

How can we forcedly inject current into the battery with the same components mentioned above.....? I have listened that one of the method for doing this is by changing I/P impedance...! Is it true?

Actually, the standard charge rate for NiCd is C/10 so for a 2.8AH battery, 280mA is about right.

The first schematic you list is a constant voltage generator so do not use it - you risk serious damage to the battery.
The second one should produce a constant 1.25A until the battery is fully charged then it should reduce it.
The third should work, it is the same as the second without the charge indicator.

Connect the battery and then measure the voltage after the bridge rectifier, as mentioned earlier, it seems you do not have enough voltage ahead of the current limiter.

Brian.

all the data that you gave is normal.
the charging current will reduce as time passes for good battery.

what is your requirement ?
do you want more current in charging ?
then how will you control the heating of battery ?

You didn't report battery voltage during charge. The most plausible explanation is, that your battery is not suited for fast charge (or simply exhausted by previous deep discharge).

srizbf said:

all the data that you gave is normal.
the charging current will reduce as time passes for good battery.

what is your requirement ?
do you want more current in charging ?
then how will you control the heating of battery ?

Click to expand...

I made 1.25A constant current source using LM317, as mentioned in 2nd and 3rd link in my first post, when I connected Battery, the charging current was approax 270mA..
My requirement is that this current should remain constant(1.25A) even if the battery is connected.. or there is a change in battery's internal resistance..

Yes, more charging current (can be controlled) is better(2Amps max. is ok)..

I have read articles on Charging curve of NiCd/MiMH batteries and I have idea how to detect the peak plus I'll use a timer for safety.. this will prevent heating of the battery..

Thanks in advance..

if you have made the constant current charger using lm317 , first make sure that it is delivering the current of 1.25a with a test load.

are you using the pic controler shown in 2nd link ?

srizbf said:

if you have made the constant current charger using lm317 , first make sure that it is delivering the current of 1.25a with a test load.

are you using the pic controler shown in 2nd link ?

Click to expand...

In fact the short circuit current for that circuit is 1.25A and it works.. but as soon as battery is connected for being charged, it is dropped down to 270mA... When I short circuited the 1ohm,5watt resistance it became 300mA, looks it cannot provide current more then that...
it seems it will not appropriate for my application.... or can it be further modified to deliver more current with battery connected???

Yes, I'm using PIC16F676...

---------- Post added at 15:54 ---------- Previous post was at 15:50 ----------

FvM said:

You didn't report battery voltage during charge. The most plausible explanation is, that your battery is not suited for fast charge (or simply exhausted by previous deep discharge).

Click to expand...

No the battery is Ok... as I mentioned I have two similar batteries..

the lm317 not supplying more than 250mA is due to the max power limitation across lm317.

check your dc i/p voltage.
if you have very large voltage diff between i/p and o/p (within maxm allowed) , you may experience this problem.

measure i/p dc to lm317.
mesure o/p of lm317(i.e. battery voltage)

and make sure that you have the proper package of lm317.
an overheating of 317 is a possible indication of current limiting action.

No the battery is Ok... as I mentioned I have two similar batteries..

Click to expand...

I know you did, I was simply asking for the battery voltage during charge.