Is this homemade battery charger circuit dangerous?

[Kanon Kubose]

The other day I found plans for a $3 battery charger that was just a diode and light bulb in series with the battery, using mains electricity. I posted a question about modifying it with a dimmer switch here: https://groups.google.com/forum/#!topic/sci.electronics.design/dW8otaIKNyE

Everyone there said I was a candidate for a darwin award and that I should come to sci.electronics.basics before I kill myself.

Taking their advice to heart, I have not built this darwin battery charger,but I do still have batteries to charge, namely a 7.2v nicd battery pack for a jigsaw. I found an 8.7vdc 360mA wall wart, and by putting a 12v car heater/fan in series with the battery, I got the amps low enough to trickle charge it. However, I suspect the 8.7v isn't enough, and also, as the chargegoes on, the current falls. After a few hours, I measured C/24, and overnight it was C/60. I've read that nicads need at least C/10 to fully charge (though another source says C/16 will work), so this wall wart seems to be insufficient. (Strangely, when the current reached C/24 and C/60, removing the heater didn't affect the amp rate at all. I wonder why?)

My new idea is to take a 24vac wall wart that I have and add a diode to remove reverse current, then put it in series with the battery and some current-limiting load. This is essentially the same $3 dangerous battery charger except using 24vac instead of 110 mains voltage. Is it still dangerous? People in the other group said even 25 mA is lethal if it crosses the heart, like would happen if you stupidly grabbed both leads of unshielded alligatorclips, and it doesn't matter that the current is limited. It's not limitedenough, especially when the lightbulb begins cold.

Other questions, assuming this circuit isn't dangerous:

1. Could I use 12v appliances or light bulbs as the current limiter or willthe 24v burn them out? Since it's attached to a 7.2v battery, the voltage will be lower, right?

2. Would a dimmer switch work to make it finely tuneable? That way I could monitor it and increase it as it falls, keeping the C/10 rate.

Thanks,

Kanon

 

[Mark Storkamp]

The other day I found plans for a $3 battery charger that was just a diode
and light bulb in series with the battery, using mains electricity. I posted
a question about modifying it with a dimmer switch here:
https://groups.google.com/forum/#!topic/sci.electronics.design/dW8otaIKNyE

Everyone there said I was a candidate for a darwin award and that I should
come to sci.electronics.basics before I kill myself.

Taking their advice to heart, I have not built this darwin battery charger,
but I do still have batteries to charge, namely a 7.2v nicd battery pack for
a jigsaw. I found an 8.7vdc 360mA wall wart, and by putting a 12v car
heater/fan in series with the battery, I got the amps low enough to trickle
charge it. However, I suspect the 8.7v isn't enough, and also, as the charge
goes on, the current falls. After a few hours, I measured C/24, and overnight
it was C/60. I've read that nicads need at least C/10 to fully charge (though
another source says C/16 will work), so this wall wart seems to be
insufficient. (Strangely, when the current reached C/24 and C/60, removing
the heater didn't affect the amp rate at all. I wonder why?)

My new idea is to take a 24vac wall wart that I have and add a diode to
remove reverse current, then put it in series with the battery and some
current-limiting load. This is essentially the same $3 dangerous battery
charger except using 24vac instead of 110 mains voltage. Is it still
dangerous? People in the other group said even 25 mA is lethal if it crosses
the heart, like would happen if you stupidly grabbed both leads of unshielded
alligator clips, and it doesn't matter that the current is limited. It's not
limited enough, especially when the lightbulb begins cold.

Other questions, assuming this circuit isn't dangerous:

1. Could I use 12v appliances or light bulbs as the current limiter or will
the 24v burn them out? Since it's attached to a 7.2v battery, the voltage
will be lower, right?

2. Would a dimmer switch work to make it finely tuneable? That way I could
monitor it and increase it as it falls, keeping the C/10 rate.

Thanks,

Kanon

We purchased a number of cheap cordless drills at work. The chargers
that come with them are nothing more than a wall wart, diode and
resistor. They work for a while, but eventually the batteries all died,
some of them simply would not take a charge, others got hot and melted
while in the charger.

Save yourself a lot of grief and just purchase a proper charger.

 

[Robert Roland]

The other day I found plans for a $3 battery charger that was just a diode and light bulb in series with the battery, using mains electricity. I posted a question about modifying it with a dimmer switch here: https://groups.google.com/forum/#!topic/sci.electronics.design/dW8otaIKNyE

Everyone there said I was a candidate for a darwin award and that I should come to sci.electronics.basics before I kill myself.

There is nothing wrong with the theory, but I agree with those that
say it is too dangerous.

When the battery is not connected, there will be full mains voltage on
the connector, since the light bulb has much lower resistance than
your body.

I've read that nicads need at least C/10 to fully charge

C/10 will charge it in about 14 hours. C/20 will charge it in about 28
hours, and so on.

(Strangely, when the current reached C/24 and C/60, removing the heater didn't affect the amp rate at all. I wonder why?)

A fully charged NiCd cell can easily reach 1.4V. In this case, there
simply is so little voltage difference between the battery and the
power supply, that, even if there is almost no resistance, only a very
small current flows.

My new idea is to take a 24vac wall wart that I have and add a diode to remove reverse current, then put it in series with the battery and some current-limiting load. This is essentially the same $3 dangerous battery charger except using 24vac instead of 110 mains voltage. Is it still dangerous?

24V is very safe. Most people won't even feel anything if they touch
the poles directly to the skin. On your tongue, it will sting a bit,
since your tongue is wet and has very thin skin.

People in the other group said even 25 mA is lethal if it crosses the heart, like would happen if you stupidly grabbed both leads of unshielded alligator clips, and it doesn't matter that the current is limited. It's not limited enough, especially when the lightbulb begins cold.

Ohm's law says current is determined by two factors, voltage and
resistance. More voltage gives more current, while less resistance
gives more current. Your body (the skin, mostly) has quite a high
resistance, so the 24V will not be able to drive a current that is
high enough to be dangerous.

1. Could I use 12v appliances or light bulbs as the current limiter or will the 24v burn them out? Since it's attached to a 7.2v battery, the voltage will be lower, right?

Appliances are not good. They will have a very unpredictable
resistance. A brushed motor, for example, may have a different
resistance depending on where in its commutation cycle it is parked.

Light bulbs, on the other hand, are excellent for this application.
They have a strong current regulating effect. If you increase the
voltage, the filament heats up and the resistance increases, so the
current will not increase as much as the voltage increase would have
caused in a linear resistor. Here's an illustration:
http://sub.allaboutcircuits.com/images/quiz/00091x02.png

In fact, I used to have a commercial battery charger that did use
light bulbs as the regulating element. 20-30 years ago, they were
quite common.

Since you are using a half wave rectifier, the 12V bulbs will probably
be OK. If you sprung for another three diodes and made a full bridge
rectifier, the 12V bulbs may be overstressed. When your battery is
fully discharged, it will probably be about 6V. That would leave 18V
for light bulb.

2. Would a dimmer switch work to make it finely tuneable? That way I could monitor it and increase it as it falls, keeping the C/10 rate.

Yes, a dimmer should work. However, if you can find the right bulb,
you will not need to adjust the current during a charge cycle. It will
remain plenty constant enough.

Note, however, that this type if charger is fully manual. It will keep
charging the battery until you disconnect it. In order to know how
long to charge, you must know the battery's initial charge state,
which in practice means that you must fully discharge the battery
before you can charge it.

Personally, I'd consider buying an automatic charger. It is
tremendously convenient:

- You can charge the battery without knowing its charge state in
advance.

- You do not need to remember to stop the charging at the correct
time.

Such a charger does not have to be expensive. Look for the term "delta
peak". That indicates the automatic charge state detection. Here's an
example:

http://www.ebay.com/itm/Orion-Advan...sGames_RadioControlled_JN&hash=item5657d9d8e4

You must, of course, pick one that is suitable for your specific
battery.

 

[ehsjr]

On 7/17/2013 2:40 PM, Kanon Kubose wrote:

I do still have batteries to charge, namely a 7.2v nicd battery pack
for a jigsaw.

I found an 8.7vdc 360mA wall wart, and by putting a 12v car heater/fan
inseries with the battery, I got the amps low enough to trickle charge
it. However, I suspect the 8.7v isn't enough, and also, as the charge
goes on, the current falls.

After a few hours, I measured C/24, and overnight it was C/60.
I've read that nicads need at least C/10 to fully charge (though
anothersource says C/16 will work), so this wall wart seems to be
insufficient.
(Strangely, when the current reached C/24 and C/60, removing the
heaterdidn't affect the amp rate at all. I wonder why?)

Because the closer to fully charged, the lower the current
the battery will draw, as you observed. The lower the current
drawn through your heater, the lower the affect the heater
will have on the circuit. The effect the heater has can be
computed by V = I * R where V is the voltage dropped in the
heater, I is the current drawn through it, and R is the
resistance of the heater.

Your circuit looks like this:

8.7 Vin----[Heater]---+
|
[NiCd]
|
Gnd --------------+

As the charge continues, the NiCd voltage rises, so the voltage
drop across the heater decreases. If the pack voltage reaches 8.7 volts,
there is no current drawn, and no voltage drop across the heater. It
would make no difference if the heater was in the circuit
or not.

Also, measuring small currents presents a challenge, even to
expensive multimeters. When used in series with the circuit
to measure current, they cause a voltage drop that affects
circuit performance. That can sometimes cause a significant
error in the measurement.

Ed