Forums

Is this homemade battery charger circuit dangerous?

Started by Kanon Kubose July 17, 2013
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 poste=
d a question about modifying it with a dimmer switch here: https://groups.g=
oogle.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 f=
or a jigsaw. I found an 8.7vdc 360mA wall wart, and by putting a 12v car he=
ater/fan in series with the battery, I got the amps low enough to trickle c=
harge 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 overni=
ght 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 i=
nsufficient. (Strangely, when the current reached C/24 and C/60, removing t=
he 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 rem=
ove reverse current, then put it in series with the battery and some curren=
t-limiting load. This is essentially the same $3 dangerous battery charger =
except using 24vac instead of 110 mains voltage. Is it still dangerous? Peo=
ple in the other group said even 25 mA is lethal if it crosses the heart, l=
ike 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
In article <b1949b2f-0b8c-4a35-9fe6-f3c6bd3fc2e7@googlegroups.com>,
 Kanon Kubose <kanon11@gmail.com> wrote:

> 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.
On Wed, 17 Jul 2013 11:40:44 -0700 (PDT), Kanon Kubose
<kanon11@gmail.com> wrote:

>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.
You could do it if you were very careful.
> >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.
24VAC is safe. In the US, 48 volts AC is generally considered safe.
> >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. >
My car was dead one day, some light left on. The battery was zero volts. I bought a battery charger at Kragen, and it wouldn't charge the battery. These modern switchmode chargers will not push current into zero volts. Personally, I think it's a scam to sell more batteries. "It won't take a charge" the guy said "you need a new battery." So I got my money back on the stupid charger. I found an old DSL modem supply in the junk bin, 18 volts AC. In series with a diode, it put a couple of amps into the battery. It was getting hot so I figured it would trip a thermal overload, so I started testing various appliances as series resistors. A belt sander got it down to about half an amp. Left overnight like that, the car started. I couldn't jump it because it was head-in to my garage, and the path to the street is short and steep uphill. I guess I could have run a long extension cord out to a car on the street and paralleled the batteries. That wouldn't crank directly, but it would have recharged the dead one. Now I keep a DC bench supply at home. It's handy for all sorts of things. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
On Wed, 17 Jul 2013 11:40:44 -0700 (PDT), Kanon Kubose
<kanon11@gmail.com> wrote:

>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-Advantage-IQ801-1A-Delta-Peak-Charger-ORI30198-/370841082084?pt=UK_ToysGames_RadioControlled_JN&hash=item5657d9d8e4 You must, of course, pick one that is suitable for your specific battery. -- RoRo
On 7/17/2013 2:40 PM, Kanon Kubose wrote:

<snip>

> 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
> > Kanon >

"John Larkin" <jlarkin@highlandtechnology.com> wrote in message 
news:ce7eu8t36r1jjt34cg3cu30c90oqluub15@4ax.com...
> On Wed, 17 Jul 2013 11:40:44 -0700 (PDT), Kanon Kubose > <kanon11@gmail.com> wrote: > >>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. > > > You could do it if you were very careful. > >> >>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. > > 24VAC is safe. In the US, 48 volts AC is generally considered safe. > > >> >>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. >> > > My car was dead one day, some light left on. The battery was zero > volts. I bought a battery charger at Kragen, and it wouldn't charge > the battery. These modern switchmode chargers will not push current > into zero volts. Personally, I think it's a scam to sell more > batteries. "It won't take a charge" the guy said "you need a new > battery." So I got my money back on the stupid charger.
Most modern battery chargers have polarity detect - if the battery is totally dead flat, there's no polarity *TO* detect and the charger won't operate. I keep an old fashioned charger with iron cored transformer to get things started on a *DEAD* dead battery. Once the battery can sustain a few volts on the terminals, the automatic charger can detect polarity and work as the designer intended.

"Robert Roland" <fake@ddress.no> wrote in message 
news:jjpfu816pal6ut8eu64qngp3d8d1vbkcaa@4ax.com...
> On Wed, 17 Jul 2013 11:40:44 -0700 (PDT), Kanon Kubose > <kanon11@gmail.com> wrote: > >>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.
Seems I missed the original thread - but it sounds like the old "bridge rectifier spliced into a lamp circuit" trick. The first and most obvious trick is to splice the bridge into the neutral lead so in normal use the rectifier and load isn't on the live side. There's various ways of limiting the voltage, like a hefty zener or a crowbar thyristor. Not the safest way to charge batteries - but the dangers can be managed. Its handy for emergency desulphating lead acid batteries - but if they've got that bad they'll never be 100% again.
On Thu, 18 Jul 2013 22:16:13 +0100, "Ian Field"
<gangprobing.alien@ntlworld.com> wrote:

> > >"John Larkin" <jlarkin@highlandtechnology.com> wrote in message >news:ce7eu8t36r1jjt34cg3cu30c90oqluub15@4ax.com... >> On Wed, 17 Jul 2013 11:40:44 -0700 (PDT), Kanon Kubose >> <kanon11@gmail.com> wrote: >> >>>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. >> >> >> You could do it if you were very careful. >> >>> >>>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. >> >> 24VAC is safe. In the US, 48 volts AC is generally considered safe. >> >> >>> >>>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. >>> >> >> My car was dead one day, some light left on. The battery was zero >> volts. I bought a battery charger at Kragen, and it wouldn't charge >> the battery. These modern switchmode chargers will not push current >> into zero volts. Personally, I think it's a scam to sell more >> batteries. "It won't take a charge" the guy said "you need a new >> battery." So I got my money back on the stupid charger. > >Most modern battery chargers have polarity detect - if the battery is >totally dead flat, there's no polarity *TO* detect and the charger won't >operate. > >I keep an old fashioned charger with iron cored transformer to get things >started on a *DEAD* dead battery. > >Once the battery can sustain a few volts on the terminals, the automatic >charger can detect polarity and work as the designer intended.
The designer was an idiot, or designed it specifically so Kragen can sell more batteries. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation

"John Larkin" <jlarkin@highlandtechnology.com> wrote in message 
news:nkqgu8p4tb92g77c6amapr895lddnfi1pj@4ax.com...
> On Thu, 18 Jul 2013 22:16:13 +0100, "Ian Field" > <gangprobing.alien@ntlworld.com> wrote: > >> >> >>"John Larkin" <jlarkin@highlandtechnology.com> wrote in message >>news:ce7eu8t36r1jjt34cg3cu30c90oqluub15@4ax.com... >>> On Wed, 17 Jul 2013 11:40:44 -0700 (PDT), Kanon Kubose >>> <kanon11@gmail.com> wrote: >>> >>>>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. >>> >>> >>> You could do it if you were very careful. >>> >>>> >>>>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. >>> >>> 24VAC is safe. In the US, 48 volts AC is generally considered safe. >>> >>> >>>> >>>>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. >>>> >>> >>> My car was dead one day, some light left on. The battery was zero >>> volts. I bought a battery charger at Kragen, and it wouldn't charge >>> the battery. These modern switchmode chargers will not push current >>> into zero volts. Personally, I think it's a scam to sell more >>> batteries. "It won't take a charge" the guy said "you need a new >>> battery." So I got my money back on the stupid charger. >> >>Most modern battery chargers have polarity detect - if the battery is >>totally dead flat, there's no polarity *TO* detect and the charger won't >>operate. >> >>I keep an old fashioned charger with iron cored transformer to get things >>started on a *DEAD* dead battery. >> >>Once the battery can sustain a few volts on the terminals, the automatic >>charger can detect polarity and work as the designer intended. > > The designer was an idiot, or designed it specifically so Kragen can > sell more batteries.
You might find it was designed that way to stop people killing the rectifier sticking the battery on wrong way round.
On Fri, 19 Jul 2013 16:02:48 +0100, "Ian Field" <gangprobing.alien@ntlworld.com>
wrote:

> > >"John Larkin" <jlarkin@highlandtechnology.com> wrote in message >news:nkqgu8p4tb92g77c6amapr895lddnfi1pj@4ax.com... >> On Thu, 18 Jul 2013 22:16:13 +0100, "Ian Field" >> <gangprobing.alien@ntlworld.com> wrote: >> >>> >>> >>>"John Larkin" <jlarkin@highlandtechnology.com> wrote in message >>>news:ce7eu8t36r1jjt34cg3cu30c90oqluub15@4ax.com... >>>> On Wed, 17 Jul 2013 11:40:44 -0700 (PDT), Kanon Kubose >>>> <kanon11@gmail.com> wrote: >>>> >>>>>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. >>>> >>>> >>>> You could do it if you were very careful. >>>> >>>>> >>>>>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. >>>> >>>> 24VAC is safe. In the US, 48 volts AC is generally considered safe. >>>> >>>> >>>>> >>>>>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. >>>>> >>>> >>>> My car was dead one day, some light left on. The battery was zero >>>> volts. I bought a battery charger at Kragen, and it wouldn't charge >>>> the battery. These modern switchmode chargers will not push current >>>> into zero volts. Personally, I think it's a scam to sell more >>>> batteries. "It won't take a charge" the guy said "you need a new >>>> battery." So I got my money back on the stupid charger. >>> >>>Most modern battery chargers have polarity detect - if the battery is >>>totally dead flat, there's no polarity *TO* detect and the charger won't >>>operate. >>> >>>I keep an old fashioned charger with iron cored transformer to get things >>>started on a *DEAD* dead battery. >>> >>>Once the battery can sustain a few volts on the terminals, the automatic >>>charger can detect polarity and work as the designer intended. >> >> The designer was an idiot, or designed it specifically so Kragen can >> sell more batteries. > >You might find it was designed that way to stop people killing the rectifier >sticking the battery on wrong way round.
I think it was deliberate to sell batteries. It would be trivial to shut off the charger if it saw, say, -0.6 volts at its terminals. A car battery charger that won't charge a dead battery is idiotic at best. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators