# Running three 10 W, 12 V COB LEDs on a 12 V, 7 Ah lead-acid battery

I am planning on buying this LED.

This is for my my project of doing a video light that I can carry to the field. Since these are 10 W, 12-14 V, my math (P/V = I) (10/14) equals to 710 mA (assuming 10 W is achieved at 14 V).

My plan is to buy three of them and wire them up in parallel. That means I will have a load of 0.71 mA × 3 = 2.13 A for the project.

If I computed this correctly (7 Ah/2.13 A), I will have around 3.2 hours of use before I run out of juice. The 7 Ah in the equation because I'm planning on using a 12 V, 7 Ah lead acid battery. It's a deep-cycle battery. The reason I am going to use this is because it is cheap and available, and though heavy, it's tolerable.

These computations are accurate if I'm going to use it at full capacity, but I can't drive the LEDs at full capacity because my supply is only 12 V. Also, I'm going also to use a step-down motor with a potentiometer to dim it when needed.

All-in all, I'm hoping if anyone can advise me on anything I have missed? Are my computations correct?

You stated your battery is 12V 7Ah battery.

LED (actually the bar light uses 4) is 12-14V (that's the drop), and they say can use up to 10W. Let's do the math: $$I = \frac{P}{E} = \frac{10W}{12V} = 833 mA \\$$

No sense in calculating at 14V, you're limited to 12V.

$$3 \cdot 0.833A = 2.499A$$

So, BEST case, 2.8 hours. But you really need to see the curve for your specific battery, and it's derating for thermal, etc, to see if it can realistically output 7Ah-- manufacturers (for marketing purposes) usually put the 'best case' information on the battery, but only the datasheet for it will provide the truth.

I'd expect 2 hours and change. If you put an ammeter on it when you get it, and see how much current it's actually drawing, that will let you get a better idea.

Not sure what "step-down motor with a potentiometer to dim it when needed" is. If in fact it does have a motor I would suggest you seriously look at a PWM module, the losses will be considerably less. Nice part about PWM is you dim the lights the longer the battery would ask. These dimmers can be purchased for a few dollars online. Be sure they are for DC as most AC units will latch on. I use these and they work very well.

A 7 Ah battery is "7 Ah if used for 20 hours" that is 7/20 = 0.35 A or 350 mA or less. This 7 Ah battery acts as 6.5 Ah if you discahrge in 10 hours, and only 5.2 Ah if you discharge in 3 hours.

You may want to see the chart below (capacity vs time). As stated above, you need the graph for your battery.