Is this homemade battery charger circuit dangerous?

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
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

Re: Is this homemade battery charger circuit dangerous?

Quoted text here. Click to load it

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.

Re: Is this homemade battery charger circuit dangerous?
On Wed, 17 Jul 2013 11:40:44 -0700 (PDT), Kanon Kubose

Quoted text here. Click to load it


You could do it if you were very careful.  

Quoted text here. Click to load it

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


Quoted text here. Click to load it

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
We've slightly trimmed the long signature. Click to see the full one.
Re: Is this homemade battery charger circuit dangerous?


Quoted text here. Click to load it

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.  


Re: Is this homemade battery charger circuit dangerous?
On Thu, 18 Jul 2013 22:16:13 +0100, "Ian Field"

Quoted text here. Click to load it

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
We've slightly trimmed the long signature. Click to see the full one.
Re: Is this homemade battery charger circuit dangerous?


Quoted text here. Click to load it

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


Re: Is this homemade battery charger circuit dangerous?
wrote:

Quoted text here. Click to load it

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    

We've slightly trimmed the long signature. Click to see the full one.
Re: Is this homemade battery charger circuit dangerous?


Quoted text here. Click to load it

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.  


Re: Is this homemade battery charger circuit dangerous?
wrote:

Quoted text here. Click to load it

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  
Quoted text here. Click to load it

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

Quoted text here. Click to load it

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.


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

We've slightly trimmed the long signature. Click to see the full one.
Re: Is this homemade battery charger circuit dangerous?


Quoted text here. Click to load it

Probably quite common if you're careless.

Quoted text here. Click to load it

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.  


Re: Is this homemade battery charger circuit dangerous?
On Fri, 19 Jul 2013 17:38:27 +0100, "Ian Field"

Quoted text here. Click to load it

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

--  
JF

Re: Is this homemade battery charger circuit dangerous?

John Larkin wrote:
Quoted text here. Click to load it


   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.

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

Re: Is this homemade battery charger circuit dangerous?


Quoted text here. Click to load it

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.  


Re: Is this homemade battery charger circuit dangerous?
On Sat, 20 Jul 2013 01:01:38 -0400, "Michael A. Terrell"

Quoted text here. Click to load it

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


Quoted text here. Click to load it

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

Quoted text here. Click to load it

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

Quoted text here. Click to load it

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




--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

We've slightly trimmed the long signature. Click to see the full one.
Re: Is this homemade battery charger circuit dangerous?


Quoted text here. Click to load it

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.  


Re: Is this homemade battery charger circuit dangerous?

John Larkin wrote:
Quoted text here. Click to load it


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


Quoted text here. Click to load it


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


Quoted text here. Click to load it


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


Quoted text here. Click to load it


   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.

Re: Is this homemade battery charger circuit dangerous?
On Tue, 23 Jul 2013 04:54:43 -0400, "Michael A. Terrell"

Quoted text here. Click to load it

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.


Quoted text here. Click to load it


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
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

We've slightly trimmed the long signature. Click to see the full one.
Re: Is this homemade battery charger circuit dangerous?

John Larkin wrote:
Quoted text here. Click to load it


   Have you seen a doctor about your paranoia?  The 'parts stores'
around here will test a car battery under load & for free.  I see more
people with Harbor Freight's chargers than the ones sold at 'parts
stores'.


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

Re: Is this homemade battery charger circuit dangerous?


Quoted text here. Click to load it

I had an old starter motor bolted to a lump of wood for checking batteries -  
would've been a more realistic test if it was bolted to an engine, but such  
as it was.  


Re: Is this homemade battery charger circuit dangerous?
On Wed, 24 Jul 2013 16:28:43 +0100, "Ian Field"

Quoted text here. Click to load it

---
So what you were really testing was the lump of wood's resistance to a
transient moment arm instead of the starter motor's ability to deliver
power to a load?

Good thing you weren't around when Rolls Royce's Merlin was on the
drafting table.

--  
JF

Site Timeline