APC Battery to run LED strip

I have an APC battery backup that I got a new battery for and ended up not using it. Could I use that battery (12.4V when using a multi-meter - APCRBC110) to power a section of 12V LED strip in a ceramic Christmas tree? Would I need to use any resistors or anything? I was thinking of an on/off switch on the positive side and just run it direct to the LED strip. I would probably run it a couple hours a day maybe. Is there anything I need to worry about with that set up? I am thinking I could get through the Christmas season maybe with 1 or 2 charges.

Technical Details
Part Number ULSAD011201
Item Weight 6.4 ounces
Package Dimensions 4.2 x 4.2 x 2.8 inches
Item model number ULSAD011201
Color Daylight White
Style Modern
Material Plastic
Shape Linear
Power Source Corded-Electric
Voltage 12 volts
Wattage 12 watts
Item Package Quantity 1
Type of Bulb LED
Special Features Not Dimmable
Batteries Included? No
Batteries Required? No

At 12W that means it draws 1 A.
What is the Ah rating of the backup battery ?

From this link, it says 8Ah.

It looks plug and play to me. The led strip would have the resistors because it states the input voltage is 12V
and we all know leds have a forward voltage between 2V and 5V depending on the color.
8Ah/1A = 8 hours.
Set a timer on your cell phone for 7.5 hours and then shut it down to charge the battery.
If you turn it on at 5pm it should run till midnight.

Is there a way to see how much power it is actually drawing? I would not be using that whole strip. Very little of it actually.

I used some of these LED's on under cabinet lighting in the kitchen and when I use my P3 kill a watt device, it shows about .6 amps being pulled when lights are on (I have them limited to about 60% power) and it is also powering the arduino (just the LED's and a motion detection piece). I don't remember how many watts it showed.

Obviously I am not up to snuff on electronics. I know just enough to be dangerous. I am just trying to not be too dangerous. :slight_smile:

Well, as you can imagine, there are really only two ways to measure the current:
1: directly using a multimeter with the leads plugged into the 10A jack and the scale set to dc current.
2- Using a current shunt, (a very low resistance designed for current measurement. The one I have is
rated for 50mA @ 5A.
An alternate version of that is called a current sense resistor. Unlike a current shunt which is a brass
assembly built to last forever, a current sense resistor is a resistor with such a small resistance it does
not generate a very high voltage and therefore dissipates very little power.
This is the one I use.

At 0.1 ohm, at 500mA it would generate a voltage of V = IR = 0.5A0.1 ohm = 0.050V (50mV)
and would dissipate P = I * V = 0.5A * 0.050V = 0.55 W (550 mW, or just over 1/2W. It is rated for 1W.
If you put this in series between the battery and the led strip and measure the voltage with a multimeter
on Volts DC scale it would read 0.050V when the leds are drawing 1/2 A.

The major difference with this method is it frees the meter, which is only necessary long enough to read
the voltage across the sense resistor, and more importantly, if you place the resistor between the led
strip and GND then the voltage measured across can be sensed by an arduino with an analog input
and the 50mV would return 50/((5/1023)*5V)) = 50/ 4.88 mv = 10 analog counts.
This is useful because it is simple to code a While loop such that "While current<10 digitalWrite(relay_pin, HIGH), and if the current exceeds 0.5A the relay will turn off , turning off the leds.
If you put the sense resistor between the battery and the load then you can't do that because the current sense resistor is not connected to GND. You would need TWO analog inputs with voltage dividers and subtraction code whereas with the sense resistor between the leds and GND you only need one input and
you don't need voltage dividers because the voltage is only 50mW and not 12V-11.5V.
Make sense ?

After some research, I believe I understand. My only reason for asking was to see what it would actually draw but I don't know that it would be worth all of that. It was mostly to see how long the battery might last but I can get that from actual use. Thank you for that lesson though. I really appreciate the help. If I can figure out how to make it work, I will post a picture.

It's simple .
Google: "arduino"l (ALWAYS preface googl searches with this word) analogRead()"
Then repeat for "digitalWrite()"
and finally "relay driver"

If the LED strip is single color, or common-positive RGB (with a separate pin for negative side of each channel), tie the ground of the battery to ground of arduino (unless you're already powering the arduino from battery through Vin or external regulator, in which case they're already connected), instead of relays, I'd use a MOSFET for each channel (I'd pick an IRF3708 since they're in a nice through-hole package, have plenty of headroom too) - drain to negative side of load, source to ground, gate connected to arduino pin through 100 ohm resistor, and 10k resistor from arduino pin to ground so they lights don't go crazy when the arduino is in reset or being uploaded to.

There are two advantages - slightly lower power consumption (relays use power when turned on, mosfets don't), and you can use PWM to dim the lights (and get intermediate colors if it's RGB).

You can't PWM relays - they can't switch fast enough, and are rated for a limited number of switchings over the life of the relay, so even if they could switch fast enough, you'd burn through their lifetime really fast PWMing.

Follow Dr. Azzy's suggestion. I was trying to make it as simple as possible for you but he made some good points.

Two additional thoughts regarding measuring current:

  1. If you know the current spec for the original full LED strip you can estimate the current by the %age of LED you are planning on using.

  2. Each 2 or 3 LED's have a current limiting resistor. If you can read the value on the chip and measure the voltage you can calculate the current draw:

I = E/R = VoltageAcrossResistor / ResistorValue

I total = I x number of resistors on your LED strip.


That battery seems to physically too small
to have an 8Ah capacity.

After some research, I believe I understand. My only reason for asking was to see what it would actually draw but I don't know that it would be worth all of that. It was mostly to see how long the battery might last

The specs say 1A so I'd believe that.

At 8 Amp-Hours, that's 8 hours but of course the battery voltage drops over that period of time so the lights will probably be noticeably dimmer after 8 hours.

I'd plan on charging it every day or maybe every 2 days.

That's the same size battery used in wiress
speakers (what ?!, yeah I know)
I had 4 of them. (8Ahr)

Could I use that battery (12.4V when using a multi-meter - APCRBC110)

That looks like a lead acid battery, which is what I'd expect in a UPS. It might have been 12V4 when you measured it, but I bet it's not now. 6 cell lead acid batteries can have any voltage from about 11V (or less) when discharged to over 14V when fully charged. 12V LED strips are designed to be use on 12V, pretty much exactly. You need a buck / boost convertor between the battery and the LEDs to keep the voltage at 12V.

Don't leave the battery discharged for long, they don't last long discharged. Is it going outside? Does it get below freezing where you live? A charged lead acid battery has acid in it and a low freezing point. A discharged one as water in it, which can freeze and destroy the battery. If you are storing it for next year it needs to be kept charged.

Gel cell most likely.

Gel cell most likely.

Yes, I agree. Until someone tells me otherwise I am going to continue to believe the freezing point is a lot closer to 0 degrees C for a discharged battery than for a charged one.

Thanks for the help. I was able to come up with the attached pictures. It is going on a shelf in the house. Not outside. I hooked this up to a plug in to check the draw with the kill a watt P3. It was showing about 5 volts and .06 to .07 amps. I had it running for about 8 hours today and it was still pretty bright. I am charging the battery with the APC battery backup unit itself. Just plugging it in for the night. We will probably run it for about 2-4 hours a night so I think I will be able to get about 3 nights out of it before charging again.

Good to know on the charging of the battery though. I will maybe hook the battery backup to my alarm clock in the bedroom for the summer so it is not sitting for 8 months.

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