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Make your own Battery Tender/Charger

12oclocker

By 12oclocker
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12oclocker Mentor

12oclocker

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Here is my design for a 12v motorcycle battery tender/charger,

it will not overcharge, and it has a charging indicator LED to led you know the charging status.

very simple and low cost to throw together (around $15 to $20 in parts)

I will post up pics of mine through the build process soon...

here is my schematic...

my-12v-charger.jpg

(edited to show revised schematic...)

batt-charger.jpg

Edited by 12oclocker
show revised schematic
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12oclocker Mentor

12oclocker

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good idea, but can't you just buy one for like 25 bucks?

you could, but the cheap ones are lower current, and take longer to charge,

and not quite as fun as making it yourself. lol.

be careful not to get a cheap 'unregulated' charger, they can overcharge and harm the battery.

I had all these parts downstairs, so for me it cost $0.

Edited by 12oclocker
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12oclocker Mentor

12oclocker

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Excuse my ignorance, I've not dug into a circuit for a long time.

But isn't an LT317 limited to a 40V input to output voltage differential?

I'm actually using a LM317, but I think the LT may be drop substitute for my specs (its what my schematic program had in the part bin). I pulled all the pdf data sheets when I breadboarded it, and everything is in spec and within tolerance range.

You may be thinking of the absolulte max rating, the 317 has adjustable voltage regulation from 3v to 30v

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12oclocker Mentor

12oclocker

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So what's the estimated output amperage at the battery?

I'm asking rather than calculating... way too lazy...

it will give around 1.5amps if the battery is really really low, and it decreases the closer the battery gets to full charge, once the current decreases to less than 40mA, the opamp turns the charging led indicator off (indicating full charge), but the battery continues to be given float voltage to maintain charge.

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12oclocker Mentor

12oclocker

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Excellent, one last stupid question...

Are you really dropping stepdown voltage to 12v, or something a little higher?

the transformer is actually rated 12.6v at 3a, but when their is no current draw, the transformer secondary will read about 18 something volts, Im not utilizing all 3a, thus I can regulate the output at 13.2v and around 1.5a draw with still some extra to play with. Transformers are reactive to a load, I'm utilizing that phenomenon to get more voltage at a less current tradeoff than the actual transformer rating.

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ReconRat Grand Master

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Cool, you have just joined that elite group of bike riders, who build their own trinkle chargers. We used to do that back-in-the-day. Something about starving-student-with-more-time-than-money. Back then we didn't have the discrete devices to master control of the circuit. Ok, geez... we were lucky we had transistors...

Advanced circuit maybe has switchable hi-lo amperage rating? It never ends...

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12oclocker Mentor

12oclocker

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Cool, you have just joined that elite group of bike riders, who build their own trinkle chargers. We used to do that back-in-the-day. Something about starving-student-with-more-time-than-money. Back then we didn't have the discrete devices to master control of the circuit. Ok, geez... we were lucky we had transistors...

Advanced circuit maybe has switchable hi-lo amperage rating? It never ends...

lol

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12oclocker Mentor

12oclocker

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uhh i bought mine for 30..my time is worth more than 10/hr...

it's fun, thats why I do it.

My staircase at my house has touch adjust white LED's in the walls and sensors on the handrail, built into the wall when I made the staircase, so at night when walking up the stairs, I touch the railing, and the lights automatically come on.

One time I spent 2 years designing a transistor analyzer (in 11th and 12th grade) talk about dumping time into something. after i patented it I entered into some design competitions and did end up making a few thousand from it. back then I could not even afford an oscilloscope, i did the whole thing with measured values and calcuations, i did the board layout and etching by hand without any software, I did not even have a breadboard to test my design! I drew and drilled every trace by hand, 1038 something holes drilled by hand with precision drill bit set, and 3 months on the board design and drawing alone, 13 IC's bunched. in a 4" by 2.5" board, single sided, three 7 segment displays, and only 7 jumpers. got 3rd place in a world duracell design competition. and first in several others. I think I was 17 at the time.

Edited by 12oclocker
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that dude Grand Master

that dude

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it's fun, thats why I do it.

My staircase at my house has touch adjust white LED's in the walls and sensors on the handrail, built into the wall when I made the staircase, so at night when walking up the stairs, I touch the railing, and the lights automatically come on.

One time I spent 2 years designing a transistor analyzer (in 11th and 12th grade) talk about dumping time into something. after i patented it I entered into some design competitions and did end up making a few thousand from it. back then I could not even afford an oscilloscope, i did the whole thing with measured values and calcuations, i did the board layout and etching by hand without any software, I did not even have a breadboard to test my design! I drew and drilled every trace by hand, 1038 something holes drilled by hand with precision drill bit set, and 3 months on the board design and drawing alone, 13 IC's bunched. in a 4" by 2.5" board, single sided, three 7 segment displays, and only 7 jumpers. got 3rd place in a world duracell design competition. and first in several others. I think I was 17 at the time.

12 you lost me in all of that1 congrats- and id love to see the handrail idea..seems pretty cool..i guess if thats what you like to do in your free time, so be it!

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12oclocker Mentor

12oclocker

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12 you lost me in all of that1 congrats- and id love to see the handrail idea..seems pretty cool..i guess if thats what you like to do in your free time, so be it!

that motorcycles and programming are where its at, lol, I'll take pics of the staircase design tonight and post them up with the schematic, its pretty neat.

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12oclocker Mentor

12oclocker

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I've never been at your house in the middle of the night going upstairs to your bedroom so I've never seen it...haha. I'd like to see those pics and see how it works next time I come over man. :bow:

I never showed you that, lol, here it is...

at full brightness the whole setup only draws about 120mA, and that's directly from the 120VAC line, there is no transformer in this setup.

100_1433_sm.jpg

The silver thing is the touch sensor

100_1440_sm.jpg

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