Is this homemade battery charger circuit dangerous?

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:

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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?

  1. 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

Reply to
Kanon Kubose
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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.

Reply to
Mark Storkamp

You could do it if you were very careful.

24VAC is safe. In the US, 48 volts AC is generally considered safe.

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 

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Reply to
John Larkin

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.

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

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.

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.

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.

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:

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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.

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:

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

--
RoRo
Reply to
Robert Roland

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

Reply to
ehsjr

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.

Reply to
Ian Field

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.

Reply to
Ian Field

The designer was an idiot, or designed it specifically so Kragen can sell more batteries.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

You might find it was designed that way to stop people killing the rectifier sticking the battery on wrong way round.

Reply to
Ian Field

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.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

A bit inconvenient perhaps - and very rarely at that, its not a good idea to discharge a lead acid battery so low it won't activate the polarity detect, that inconvenience is a good reminder of that. If a battery has got that dead in storage - it probably *IS* scrap.

You don't need a fully fledged old style charger, pretty much any DC output wall-wart can jump start the polarity detect.

Reply to
Ian Field

A light left on, even a door not fully closed, can drain a battery to zero. That's not all that rare.

its not a good idea to

That's what the battery sellers say. After I recharged my zero-volt VW battery, it worked fine.

Fine if you're an EE and have the means to do that, like I did. Ordinary mortals buy a garbage charger and then a new battery that they don't need.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

Probably quite common if you're careless.

It wouldn't have been if you'd left it in that state even as much as only a week - they sulphate PDQ when discharged dead flat.

Reply to
Ian Field

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Sounds like hearsay to me, and where's the quantitative evidence that 
supports your assumption?
Reply to
John Fields

Sure it is. Who wants to make sure someone doesn't try to charge a battery with a shorted cell and start a fire?

Who cares about lawsuits, or killing their customers?

Other than the roll around garage chargers, I haven't seen anything other than a float charger that will try to charger a dead battery.

My charger was built from a motorhome battery eliminator and a Variac. I can set the initial charge rate from a few mA, to 30 amps and see how the battery is charging. I built it over 40 years ago. I have used it to start a car with a 'dead' battery more than once.

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Anyone wanting to run for any political office in the US should have to 
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Reply to
Michael A. Terrell

I have a couple of optimates that have desulphate pulse modes, but they won't start without at least 2V on the terminals to detect correct polarity, so if the battery is dead flat it needs an external DC source to get it started.

Generally battery recovery is satisfactory unless its stood any length of time dead flat.

Maybe they should include a start button, perhaps current limited at the desulphate pulse voltage.

Reply to
Ian Field

What's going to catch fire? The battery? Some idiotic Chinese electronic charger that doesn't current limit?

Why kill them when it's more profitable to rip them off?

Old fashioned cheap transformer-rectifier chargers worked fine on a dead battery. The new electronic chargers are too smart to do that.

I keep a Lascar bench supply around now. It current limits.

--
John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

The bulk charge phase is usually constant voltage, there may not be much in the way of current limiting until it gets to the float charge phase.

Reply to
Ian Field

A car battery with a shorted cell can boil over and rupture. Not that you give a damn.

Sigh. They have to be idiot proof for the uneducated masses. No one who builds battery chargers gives a damn about your problem.

Complain to their company lawyers, who don't want more lawsuits.

Good for you. I haven't had a completely dead battery since 1997, when a #8 AWG wire shorted to the frame and pulled the battery to zero volts. With the hot summers in Florida batteries die without warning. I pulled into an electronics supplier in Destin one day. When I went to leave, the car wouldn't even click the solenoid. Like several other failed batteries, a cell had opened.

--
Anyone wanting to run for any political office in the US should have to 
have a DD214, and a honorable discharge.
Reply to
Michael A. Terrell

If it shorts while it's fully charged, yes, but that's very rare. Car batteries are amazingly rugged. But that's not the situation we're talking about here.

If it's already shorted and stable that way, charging current will apply zero power to a shorted cell, so it won't "boil and rupture." I^2 * R = 0 when R = 0. Dinky chargers don't have enough power to boil a battery anyhow.

Beside, the cheap electronic chargers would still pump current into a battery with one zero-volt cell. They just won't put current into a battery with all zero volt cells.

They design them that way to sell to parts stores, who sell the chargers and then sell batteries to replace the ones that "won't take a charge."

--
John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

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