Wall Wart


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tom84std
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Wall Wart

Postby tom84std » Mon Feb 11, 2013 10:28 am



I took a 14volt DC 200mA wall transformer and put a .5 OHM 3 Watt resistor in series with the output and attached alligator clips. All soldered and heatshrunk. I put it on my bike's battery which had been sitting for several weeks untouched. I monitored the voltage and current for several hours with it plugged in. The battery was a little low at first but over a period of about an hour it was brought to around 12.5 volts. I left it connected overnight and in the morning there was no part of the transformer's case, the resistor or the battery which was noticably warm to the touch but the battery was a full 12.6 volts. The total cost was about $3 and I'm confident that with the added resistor I could just leave it attached for extended periods without harm to the transformer or the battery.
Poor man's Battery Tender?




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littlebeaver
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Re: Wall Wart

Postby littlebeaver » Mon Feb 11, 2013 3:33 pm

so what you are saying is that you made a trickle charger right...Now dismantle it and show us what you had to do because not everyone is a electrial wizard, especially not me..You can tell us all you want but take some photo's and give us a step my step..PLEASE :D thanks man..Beave... I would love to see how its done..I'm positive others would too... :D I think :shock: :shock:

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virgilmobile
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Re: Wall Wart

Postby virgilmobile » Mon Feb 11, 2013 4:12 pm

Sounds OK to me...I haven't done the math yet but a few hundred miliamp at that voltage should be plenty enough to keep the battery up.Good Idea about the load resistor to keep the power supply from running max and burning up too.

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tom84std
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Re: Wall Wart

Postby tom84std » Mon Feb 11, 2013 9:43 pm

If you go to Goodwill, Salvation Army or any other thrift store they usually have a bunch of these wall transformers. I've found that many of these stores usually have a box full of them in the back which couldn't be matched to a product but they keep them around and would give them away or sell them for less than $3. You can usually find a 12-14VDC one somewhere. The tricky part is finding the right resistor. I have a surplus electronic supply house near me where I bought my resistors for .25 each. I know most people don't have such a place nearby so that's going to be the difficult part.
I've been using a similar one for years without the resistor. The battery was dead in my Miata a few days ago so I hooked it to that one and left it for about three days. It fully charged the battery but the transformer got hot enough in the process to melt the case a little. That's the reason I decided to add the resistor. The resistor would have increased the charge time but would have protected the circuit from the overload. My logic is that a dead battery has a low resistance to a battery charger therefore increasing current to full output of the charger. Adding a resistance, in this case .47 OHM will drop about a volt at full output current, reducing the load considerably. With the resistor in series with the power, full load is really never seen by the supply because as the battery charges, it's own internal resistance increases, adding to the external resistor. This lowering in current reduces the voltage drop accross the resistor, raising the charging voltage slowly in a curve similar to a capacitor's curve allowing the battery to rise to full 12.6V very slowly. By the time the battery reaches it's full charge, it's voltage is full theoretical voltage and it's charging current it almost nil.
I'm going to build a couple more of these in the next few days, I'd be happy to do a pictorial.

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littlebeaver
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Re: Wall Wart

Postby littlebeaver » Mon Feb 11, 2013 10:14 pm

Well alrighty then... :shock: :shock: :shock: I cannot wait to see the photo's and read you're how too...awesome man... :shock:

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redial
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Re: Wall Wart

Postby redial » Mon Feb 11, 2013 10:21 pm

12.6v doesnt sound enough. A normal 'wet' cell should produce 2.2v when fully charged, so a '12v' battery with 6 cells should come out to 13.2v when fully charged. It is also not a good idea to leave a battery on charge for so long, unless you have a "smart" charging system, which adjusts to nothing as the voltage and resistance in the battery increases.

I dont mind if someone contradicts this, but that is my understanding.
Len in Kapunda

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tom84std
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Re: Wall Wart

Postby tom84std » Mon Feb 11, 2013 11:14 pm

I don't know Redial, but it's been my experience over the years that the average automotive battery should be somewhere around 12.6 volts fully charged at rest. When I read your reply it brought up that question so I checked a couple of the batteries in my vehicles simply to satisfy my own curiosity. I'm using a Fluke 77 multimeter I've owned since 1990. It's pretty old and has never been calibrated so it could be off a little. I've used it in the field all those years but I've never had any problem with it so I trust it to a degree. On the BSA I keep in my kitchen at about 70* the battery is measuring 12.5V. On my Wing kept under the carport at around 49* tonight I'm measuring about 12.7. I also keep my 2005 KIA under the carport at the same temp and it's measuring 12.6V. My '99 Miata is also kept outside and is measuring about 12.57V. Don't confuse alternator output with battery voltage. I've had a fairly accurate analog voltage meter attached to my Wing's speedometer for about ten years since I had to replace my fried alternator stator in about 2002. With it running and everything normal it reads a steady 13-14V. On my BSA with a Tympanium rectifier, regulator unit it averages 13-14 volts engine running. They both read a little over 12V with the engine off.




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