Question about charging a 12v NiCd battery with lm317

[protocol217]

Hello i have a question about charging a 12v NiCd battery pack with a simple lm317 charger like this one:
**broken link removed**
What i want to know is if i leave this lm317 charger connected to the battery without a power adapter (ac-dc 15v-24v adapter) will it discharge the battery on a longer time ?

I want to build in this lm317 circuit inside the battery housing and from out side only connect the power adapter when needed charging.
And i have another question what is the maximal voltage that i can use for charging a 10 cell 12v nicd pack, is 24v good or too high ?

 

[audioguru]

The simple constant current charger keeps the same current even when the battery is fully charged. That overcharging is wrong if the current is high.
If the power adapter is disconnected then since the LM317 circuit is not connected to the negative terminal of the battery then the battery will not discharge.

The manuals of an old Ni-Cad cell or a new Ni-MH cell from Energizer shows that its voltage is 1.4V to 1.55V when fully charged. Then 10 cells will be 14V to 15.5V. If the input voltage of the LM317 is higher than 17.5V then the extra voltage times the current makes the LM317 hot. Here is the voltage of a Ni-Cad or Ni-MH cell when charged at different currents. See where the voltage has a peak then drops? That is what a battery charger IC detects as a full charge so that it stops the charging.

 

[JimB]

if i leave this lm317 charger connected to the battery without a power adapter (ac-dc 15v-24v adapter) will it discharge the battery on a longer time ?

If the incoming supply is completely disconnected, there is no path for the current to flow back from the battery pack through the LM317.

what is the maximal voltage that i can use for charging a 10 cell 12v nicd pack, is 24v good or too high ?

The absolute maximum input/output voltage for the LM317 is 40 volts, so in your case 24v charging 12v gives a deltaV of 12v, so you are OK there.

I want to build in this lm317 circuit inside the battery housing

Be aware of the power dissipation of the LM317, it will get hot, a heatsink would be a good idea.
The power dissipation will be the voltage across the LM317 times the charging current.

JimB

 

[protocol217]

thanks a lot, it logical when the lm317 is not connected to the negative side of the battery there is no discharge, i did not se that at the moment.
if i charge with 24v 250ma will i need a big heatsink or not, with fan or not?

 

[audioguru]

If the battery is dead at 10V, the LM317 has an input of 24V and the charging current is 250mA then simple arithmetic shows the heating is (24V - 10V) x 250mA= 3.5W. The LM317 datasheet shows its TO-220 case has a thermal resistance from its chip to the ambient without a heatsink of 50 degrees C per Watt so more than 2W is too hot. The thermal resistance from its chip to its case is 4 degrees per Watt and the mounting to a heatsink is about 0.5 degrees C per Watt so for its chip to be safe at 120 degrees C in ambient air that is 30 degrees C then you need a heatsink rated at (120 - 30)/3.5W= 26 degrees C per Watt.

 

[Colin]

Charging current depends on the capacity of the battery (cells) (cell) and if you keep the current to about 100mA you will not have to worry about removing the adapter. If you work out the current for a 20 to 24 hour charge, you will always remember to remove the charger and the low current will not damage the cells.

 

[protocol217]

My battery is 2Ah, bud i want to charge them with 250ma so it charges little faster, and if i use a AC transformer with rectifier, i should not put a capacitor across the DC side of the rectifier am i right? if yes what is the minimum AC voltage that AC transformer should put out for charging 12v, 14.4, and 18v nicd batterys?

 

[audioguru]

Aren't you going to use the current regulator with the LM317? Rectifying a transformer output and connecting directly to a battery is NOT a charger circuit. Nothing will limit the current except the transformer might be overloaded.
Ni-Cad batteries develop shorted cells that might cause the transformer to smoke and burn if there is nothing to limit the current.

The datasheet for the LM317 shows an input capacitor that must be used and an output capacitor that should be used.

In a previous post, I showed the voltage of a charging cell at different currents. Determine how many cells in series you want to charge, refer to the graph for the current you are charging at and add the 2V or 3V extra shown on the datasheet of the LM317.

 

[Colin]

If you are going to use a transformer and rectifier, you will not know what current is flowing.
Put a one ohm 0.25watt resistor in series and touch it with your fingers. If you can hold it, the current flow will be 250mA or less.

 

[protocol217]

Yes i am going to use the lm317 charger circuit, but i am puting it on the DC output of the rectifier, i was asking what minimum AC voltage should the transformer give out. and should i put a capacitor across the DC side of the rectifier ? this is the same that i disribed before only i am not using the original ac-dc adapter i am making it my self.

 

[Colin]

You need to know the floating voltage of your battery when charging, plus 2v for the constant current section and about 3v across the regulator.
Now, the head voltage depends on the impedance of the transformer.
All you have to do is sign-up for a 3 year course in electrical/electronic engineering and you can work out this value.