Charging nicad batt pack with variable voltage current limited bench power supply

[Wes]

I was out hanging drywall in my garage when my Black and Decker Firestorm 14.4v charger or
drill failed.

From what I've read, the chargers are a bit flaky but then maybe the drill died. I had a
brain storm, I have a Tenma variable power supply that I can set to limit current. That
seems like a battery charger to me.

So I set it to 14.4v open volts, 2.0A current. Atm I'm putting 12 volts into it and after
a few minutes, I could get the drill to work so the pack is charging and the drill works.

So what strategy do I use to charge this? I figure I have to go above 14.4v, I've read
that the 1.2v cells need to be taken to 1.45v for full charging. Since I'm not checking
temperature, I'm worried about just setting 17.4 volts and walking away.

Thanks in advance.

Wes

 

[Baron]

I was out hanging drywall in my garage when my Black and Decker
Firestorm 14.4v charger or drill failed.

From what I've read, the chargers are a bit flaky but then maybe the
drill died. I had a
brain storm, I have a Tenma variable power supply that I can set to
limit current. That seems like a battery charger to me.

So I set it to 14.4v open volts, 2.0A current. Atm I'm putting 12
volts into it and after a few minutes, I could get the drill to work
so the pack is charging and the drill works.

So what strategy do I use to charge this? I figure I have to go above
14.4v, I've read
that the 1.2v cells need to be taken to 1.45v for full charging.
Since I'm not checking temperature, I'm worried about just setting
17.4 volts and walking away.

Thanks in advance.

Wes

When I do that with a battery pack I set the current to about 1/20th of
the rated drain current. You should see the current drop off on the
meter as the batteries charge. If I need the battery quickly then I
turn up the wick. 10 minutes at 1 amp usually gives me enough to
finish what I need to do. The pack gets warm at this current but not
enough to cause undue concern.

 

[Wes]

When I do that with a battery pack I set the current to about 1/20th of
the rated drain current. You should see the current drop off on the
meter as the batteries charge. If I need the battery quickly then I
turn up the wick. 10 minutes at 1 amp usually gives me enough to
finish what I need to do. The pack gets warm at this current but not
enough to cause undue concern.

I started out at 2 amps, never got warm. I have no idea of what the rated drain current
is. How far above nominal voltage do you charge at?

Thanks,

Wes

 

[[email protected]]

I was out hanging drywall in my garage when my Black and Decker Firestorm 14.4v charger or
drill failed.

From what I've read, the chargers are a bit flaky but then maybe the drill died. I had a
brain storm, I have a Tenma variable power supply that I can set to limit current. That
seems like a battery charger to me.

So I set it to 14.4v open volts, 2.0A current. Atm I'm putting 12 volts into it and after
a few minutes, I could get the drill to work so the pack is charging and the drill works.

So what strategy do I use to charge this? I figure I have to go above 14.4v, I've read
that the 1.2v cells need to be taken to 1.45v for full charging. Since I'm not checking
temperature, I'm worried about just setting 17.4 volts and walking away.

Thanks in advance.

Wes

14 volt pack = 12 x 1.2 volt batteries = 14.4 volts.

end of charge voltage is 1.4 volts/cell
12 * 1.4 = 16.8 volts

Set the power supp;y output to 16.8 volts.
Set the current limit to 200 ma (0.2 Amp).
Charge the battery overnight, then use it to determine the level of
charge. Adjust the current up in 100 ma (0.1 Amp) steps until you get
a full charge in 10 hours. If the battery pack has an amp hour
rating, divide that by 10 to get the proper charge current.

This is trickle charge, not fast charge, because you don't have a way
to monitor the pack's internal temperature and turn off the power
supply when the battery reaches maximum charge. Overheating a NiCd
during charge can cause high internal pressure, which causes a vent to
open (releasing the pressure and usually some electrolyte) and
rendering that cell (and thus the entire pack) unusable.

John

 

[Baron]

I started out at 2 amps, never got warm. I have no idea of what the
rated drain current
is. How far above nominal voltage do you charge at?

Thanks,

Wes

John put it much better than I would have done.

But to answer your question, I don't bother monitoring the voltage at
the outset ! The current is by far the most telling parameter. If you
set a reasonable charge current you should find that the voltage will
be about a 1.0v to 1.5v above the pack voltage.

 

[ehsjr]

I was out hanging drywall in my garage when my Black and Decker Firestorm 14.4v charger or
drill failed.

From what I've read, the chargers are a bit flaky but then maybe the drill died. I had a
brain storm, I have a Tenma variable power supply that I can set to limit current. That
seems like a battery charger to me.

So I set it to 14.4v open volts, 2.0A current. Atm I'm putting 12 volts into it and after
a few minutes, I could get the drill to work so the pack is charging and the drill works.

So what strategy do I use to charge this? I figure I have to go above 14.4v, I've read
that the 1.2v cells need to be taken to 1.45v for full charging. Since I'm not checking
temperature, I'm worried about just setting 17.4 volts and walking away.

Thanks in advance.

Wes

This is a _generally_ safe strategy for charging a drill
battery pack, Set your supply to 20 volts. Then use this:

1N4001
+---------|<---------+
| |
+20---+---Vin[LM317]Vout---+-----+
Adj | |
| [10R] [47R]
| | |
+----------+-----+---------Batt +

Gnd -------------------------------------->Batt -

That will deliver ~150mA to the pack - the rate recommended
for an ovenight (~14 hour) charge, based on the battery
Mike Terrell mentioned.

Far better would be *knowing* the mAh capacity of your pack.
You can then design a circuit that is closer to guaranteed
safe. Furthermore, you can add a TL431 on the output to steal
current when the full charge voltage (1.43 volts per cell) is
reached, thus reducing the current going into the pack. That
way, if you forget to remove the pack from the charger, it
won't cook at the higher current.

Ed

 

[Wes]

That will deliver ~150mA to the pack - the rate recommended
for an ovenight (~14 hour) charge, based on the battery
Mike Terrell mentioned.

Far better would be *knowing* the mAh capacity of your pack.
You can then design a circuit that is closer to guaranteed
safe. Furthermore, you can add a TL431 on the output to steal
current when the full charge voltage (1.43 volts per cell) is
reached, thus reducing the current going into the pack. That
way, if you forget to remove the pack from the charger, it
won't cook at the higher current.

The pack is 1400 ma/hr if I can trust what I found on the net.

Thanks for the diagram!

Wes

 

[ehsjr]

The pack is 1400 ma/hr if I can trust what I found on the net.

Thanks for the diagram!

Wes

Ok, in that case, change the 47 ohm resistor to 100 ohms.
That will set the charge current to ~137 mA, very close
to the recomended C/10 (140 mA) rate. If you want to add
the TL431 circuit:


+20-------Vin[LM317]Vout---+-----+
Adj | |
| [10R] [100R]
| | | 1N4001
+----+-----+-----+------>|--->Batt +
| | |
| [220R] [2.2K]
| | |
| +---[LED]---+
P |
5K o---/_\ TL431
t |
| |
Gnd -----------+----+----------------------->Batt -

To adjust: monitor the pack as it charges. When it
reaches 17.16 volts, adjust the 5K pot until the LED
just turns on. The TL431 circuit will draw a little
over 3 mA when the LED is off with the battery voltage
below the 17.16 volt set point. When the voltage
reaches the set point, the circuit will draw about
87 mA. The LM317 continues to deliver ~ 137 mA, but
the battery will "see" only about 50 mA after the set
point is reached, charging at roughly the C/30 rate.

Ed