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I want to use a small battery pack to power a strip of LEDs from Radioshack. They didn't have the power supply but list the requirements as 12VDC/1.5A. I know 8 AA batteries will provide 12 volts but I'm less certain how they will react to the 1.5 amp draw. We put 8 new alkaline batteries into a battery holder and did a quick non-solder test and the strip did light up nicely but the battery pack started to get warm even though we only had it connected for less than a minute or so. The plan is to solder a 9-volt type connector to the strip so it will be easy to connect/disconnect the battery pack instead of some type of switch. I know that I can add a current-limiting resistor inline with the wiring but I'm not sure what value to use. The strip looks like it has built in resistors so I'm at a loss as to what size resistor to try. The project is really just a short-term way to decorate a french horn for a school concert. My 15-year-old daughter can just connect the pack when the performance starts and then disconnect it when it is over. My concern is I don't want her to have the battery pack overheat during the performance. I don't care about how long the batteries last, this is a one-time use.

Maybe another way to ask this is, what is causing the battery pack to heat up, the LED strip drawing excessive current or some other reason?

I'm not trying to use rechargable batteries or build a charging circuit, just simply light up the instrument during a concert but safely.

[EDIT] We did a test using the battery pack and after 30 minutes we couldn't detect any noticable dimming and the battery pack was still around 90-92 degrees F, tested with an IR thermometer. I told her she can just leave the pack in her lap and if it gets too hot to unhook the connector or remove a battery. She plans on taking an extra set of batteries with her as replacements for when the first set run down. My guess is that we previously had shorted the battery pack accidently which caused the temp to spike but now that the wiring has been connected properly we are seeing more typical behavior heat-wise.

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  • \$\begingroup\$ Does your daughter play the French Horn because her family name is French? \$\endgroup\$ – Michael Karas Dec 13 '15 at 20:37
  • \$\begingroup\$ That has been the source amusement for us but is merely happenstance. \$\endgroup\$ – Kelly S. French Dec 13 '15 at 20:46
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I do not expect that LED strip to draw 1.5 A, more like 60 x 20 mA = 1.2 A. Still, that is a large current to draw from AA batteries. But good Alkaline type batteries will be able to handle that current.

Most Alkaline AAs are around 2000mAh so they should last about 1.5 hours. As the batteries deplete the brightness will be lower of course.

As long as the batteries are not too hot to hold in your hand then I would not worry. Make sure you do NOT isolate them thermally (put them in a small box for example) but make sure that the heat can escape.

You do not need a dropper resistor, there are resistors already on the LED strip and that can handle 12V which is exactly what you get from 8 x 1.5 V AA.

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  • \$\begingroup\$ I'm glad you mentioned not to thermally isolate them because that idea had occurred to me. \$\endgroup\$ – Kelly S. French Dec 13 '15 at 21:05
  • \$\begingroup\$ @FakeMoustache, yes true most alkaline are in the 2500, but internal resistance is high enough that at a discharge rate of only 500mA, they provide a wopping 600 mAh capacity according to greenbatteries.com/battery-myths-vs-battery-facts-1. This would also explain the massive heat build up. At 1.5 amps discharge rate, your alkaline AA won't last long at all. You will need a different battery chemistry or try C or D battery. Thermal isolation is certainly a good idea. \$\endgroup\$ – Filek Dec 14 '15 at 8:28
  • \$\begingroup\$ 600mAh, geez ! That is a lot less than the number I had in my head which I got from here: batteryshowdown.com/results-hi.html Which is with a 1A discharge current BTW. Unfortunately it does not mention if that is a continuous 1A or not (for example 1A for 1 minute and 1 minute 0A for the battery to recover). That could explain the difference. \$\endgroup\$ – Bimpelrekkie Dec 14 '15 at 9:46
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You could choose to use a larger sized battery. Try C or D sized cells. You will still get your 12V or so from the same number of cells. The larger batteries have lower internal resistance and thus generate less internal heat at the same amount of load current as compared to the AA cells. They are a bit heavier but at the same time will last for multiple concerts!

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  • \$\begingroup\$ +1 but if you go rechargeable check the capacity of the battery. Many cheap NiMH Cs and even Ds have the same mAh rating as AAs, and are largely empty. \$\endgroup\$ – Chris H Dec 14 '15 at 9:26
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1.5 Amps is too much for the type of AA cells which I use and they'd go dim before 1/4 of the 2000mAh are used up. That they get warm indicates that they are being somewhat overused. I'd prefer to go up a size to a Pb 'gel' battery or a scrap motorcycle battery for "12 V" Those are too heavy for a pocket at a concert so you should allow enough extra wire to place the battery on the floor. Another way would be to replicate your AA set so that two sets in parallel each making about 12V do about half the work.

I'm not greatly in favour of adding series resistors as the LED already have some resistance.

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  • \$\begingroup\$ LEDs don't have internal resistance in the normal sense of resistor. The LED strips do have additional resistance in series with the LEDs to limit / balance the current. The parallel battery packs is a good idea. \$\endgroup\$ – Transistor Dec 13 '15 at 20:53
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The strip is an array of groups of 3 led's and a ballast resistor. As long as you remain in the realm of 12VDC ie. 10.5 to 14.4VDC you won't need to add a ballast device.

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  • \$\begingroup\$ More like 10V low end. \$\endgroup\$ – Passerby Dec 13 '15 at 21:12
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You could forgo use of nickel based batteries, RC Lipo's are relatively cheap and have very high current sink capabilities. A 3S 2200mAH will provide you with a nominal 11.1V throughout most of its discharge cycle.

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