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I know this is going to be a completely stupid question, but since I'm working with a Li-Po I want to make sure I get this right.

What I am building is a modified Nerf blaster that has two motors connected in parallel drawing 20A each (40A in total) at peak, far less most of the time. Now, if I'm not mistaken, if I need to add an LED strip I would need to connect it in parallel to the motors, right? Is it a big problem if the voltage is far lower? Will the lights simply be dimmer (which is totally fine for my needs) or will they simply not work at all? Will I need any sort of short circuit protection when dealing with LED strips? Also, some people mentioned having to connect a capacitor as well but I'm not sure why.

Sorry for the dumb questions but I haven't done all that much work with anything other than very basic circuitry.

Thanks.

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  • \$\begingroup\$ This sounds like a case of badly selected components. At minimum, you need an LED strip with an operating voltage less than your loaded battery voltage, and a suitable mechanism for limiting its current. But likely, you should re-design around a pack with more cells and higher voltage, lower current motors - most systems get to cell counts higher than 2 long before they get to the currents you are contemplating. \$\endgroup\$ – Chris Stratton Dec 27 '18 at 17:24
  • \$\begingroup\$ Oh, it only gets to 40A at motor stall which is covered by the LiPo batteries I use. And as I checked, the lights do work but simply are dimmer, so what I want to know is whether that will damage any components in the long run. \$\endgroup\$ – Big Tendies Dec 27 '18 at 17:35
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if I need to add an LED strip I would need to connect it in parallel to the motors, right?

Yes.

Also, some people mentioned having to connect a capacitor as well but I'm not sure why.

That's for smart LEDs which are driven by a CPU to make colorful animations, not for lighting LEDs.

Will I need any sort of short circuit protection when dealing with LED strips?

Adding a fuse is never a bad idea...

Will the lights simply be dimmer (which is totally fine for my needs) or will they simply not work at all?

All 12V LED strips I've come across are built from series strings of 3 LEDs with a resistor. You can infer this from the documentation before buying when it says "strip can be cut every 3 LEDs".

Since Red LEDs will light up with about 1.6-1.8V, three of these in series means we add their voltages so three times 1.8V still makes less than 6V and you can light the strip with your 7.2V 2S LiPo. It will be less bright than when powered from 12V, but it will light. It should also work with orange and yellow LEDs which have voltages below 2.2V, green would be pushing it since it has a bit higher voltage, and white/blue won't work since these require 3V per LED at least, and you don't have 9V.

If you have a red/orange/yellow strip that lights on 7.2V, you can change the resistors to make it brighter. Remember I=V/R. If your red strip has 330R resistors you can put 100-120R instead. But after that don't connect it to 12V or it would exceed the LED max current.

If you really want to use a white or blue strip, you can desolder one LED in each string and replace it with a jumper.

You can also use a voltage booster, but I think it's a bit wasteful to boost 7.2V to 12V and then burn 6V off it in a resistor to light 3 red LEDs which only need 6V... better change the resistors so the red strip lights properly on 7.2V.

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  • \$\begingroup\$ Replacing the resistors may be a necessity, or may be more work than it is worth than using an inexpensive booster cable or it. I hate the idea of powering an inefficient strip with batteries. There are two unknowns here that could be relevant. The type of Li-ion polymer batteries (chemistry and capacity) of the and the current draw of the strip. 40 Amp is a heavy draw for a 2S Li-po. It should be a 2S2P as a single cell can source 20 Amp if the correct chemistry (e.g. Li-manganese and/or NMC) is used. If the strip draws significant amperage, it could exceed the power capacity of battery. \$\endgroup\$ – Misunderstood Dec 28 '18 at 7:37
  • \$\begingroup\$ The battery only reaches 40A at motor stall which is covered by the C rating. Normally it's only a few amperes. \$\endgroup\$ – Big Tendies Dec 28 '18 at 10:23
  • \$\begingroup\$ Note I suggested changing the resistors because you said you wanted to stick it in a Nerf blaster, which hints that the strip isn't going to be very long, so it should be easy... if it was 5 meter long that would be another story! \$\endgroup\$ – peufeu Dec 28 '18 at 11:10

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