recharging 12 volt batttery

[robbudo]

hi folks,

my halloween costume is coming along. a few more things (and probably posts in this forum), and i'll be all set. for those who have been serially answering my questions and havn't put the pieces together, i description of my costume is below.

so the question: i bought a 12V battery from radioshack (on sale for 15$), and it gives off 7Ah. I want a way to charge it without paying the 35$ they were charging for an adapter. i'm even willing to buy up to 30$ of stuff to build some kind of recharger, i think the knowledge gained would be worth it.


The whole picture: I'll be running 3 strands of red LED christmas lights off my 12V battery. Red LED lights, i found, use 1/2 the electricity as any other color LED lights. And LED lights use 8-10 times less electricity than standard christmas lights, YEAH for LED's. Anyhow, I also have a 3 channel color organ, so each of the three strands of lights responds to a different frequency of sound. It's pretty trippy, and should be awsome in a club. I found that if i wear the speaker (microphone) around my neck, the lights respond to my voice -- maybe i should have a sign under it that says "you turn me on". Lastly, as per this post, i'm running the lights off a battery and inverter. I calculated that i'll get 3 hours out of my fully charged battery, but that'll be fine. The lower i set the potentiometers, the lower the percentage the time the LEDs are actually 'on', thus increasing the overall amount of time i get to be a walking light show.

Finally, you might be asking, what i'm going to do with all these lights??? I'll be wearing all black, with cut-outs of red mylar paper in the shapes of flames, lined with my red LED lights. Red face paint. I'm going to go as fire.

If you've taken the time to read this post, and have helped me out with my questions along the way, i can only offer a simple 'thanks'.

cheers, rob

Added after 3 minutes:

I should have specified this is a rechargable "lead-acid" battery that i want to find a way to recharge myself.

rob

 

[echo47]

Here's a nice PowerSonic application note on the care and feeding of sealed lead-acid batteries:
**broken link removed**

Page 7 describes several charging methods and circuits, but it's not a how-to construction project. If you already have a bench power supply, then "Constant Voltage Charging" on page 9 is an easy way to do it. Basically, for a 12V battery, you connect a bench power supply set to 13.5 to 13.8 volts (if at room temperature), but limited to a few amps, and then wait for the charging current to eventually decrease to oh maybe 100mA. That'll do it! You can leave the power supply connected forever at that voltage to maintain full charge.

You can charge faster at around 14.7 volts, but once it's charged you must decrease to the 13.5 to 13.8 volt range, or you'll cause premature aging.

If you keep the battery charged, it will last a long time. Otherwise it discharges itself in a few months and begins to sulfate, and that will kill it in a couple/few years.

Be sure your charging power supply can safely survive reverse voltage. If somebody turns off the charger, you don't want the battery to shove current into the charger and burn it up. One easy protection is a series diode between the charger and battery. You'll need to slightly increase the power supply voltage to compensate for the diode voltage drop.

 

[robbudo]

nope, don't have a bench power supply.

r

 

[IanP]

If you are not after anything to fancy find an unregulated power supply (transformer+rectifying bridge or single diode) with voltage around 15V (rms, peak will be ≈20V), add a resistor which will just limit the current and charge your battery for required time (A * H) ..
See picture below ..
And after that time don't forget to turn the power OFF ..
Regards,
IanP

 

[sefton]

Use ultra hi bright LED's, they require much less current and will last longer. Why not wire them up in groups of 4 with 1 resistor for each group and then you wont need the inverter. Another idea - why not keep a spare charged battery in the car. I assume that you will be posting a photo of the finished product !!!

 

[robbudo]

wiring 120 or 180 lights in groups of 4 is a little daunting. i went with the inverter to keep things simple. i box of 60 LEDs take only 2.4 watts/hr running constantly. mine will be blinking so i'm hoping to have them on as little as a quarter of the time. i'll try to get a picture, taken of course, without a flash.

cheers, rob

 

[sefton]

wiring 120 or 180 lights in groups of 4 is a little daunting.

I see your point

i box of 60 LEDs take only 2.4 watts/hr running constantly

I assume you mean 2.4 watts (not watts/hour). If thats the case the LEDs should draw 7.2 watts in total. If they are on half the time that makes 3.6 watts. The electronics should draw less than 1.4 watts, which gives us a total of 5 watts. Assume the inverter is 60% efficient that makes 8.3 watts. At 12V this is about 0.7A. So with a 7 AH battery this should last about 10 hours.

i'm hoping to have them on as little as a quarter of the time

Well that depends on how many pretty women are at the party.