Dumb RGB LED strips like you have are simple. Each segment consists of 3 channels. Each channel has 3 diodes and 1 resistor in series. Each segment is in parallel with each other. Cutting and extending the sections with wire does not change that. You will not experience an increase in current because of it (in fact, extending with wire will actually cause a decrease in current, due to the non-zero resistance of the wire. For your short runs, this is negligable).
That said, adding 100 segments, each with 3 ~20mA channels (so 20mA x 3 x 100 = 6 Amps!) To the dome circuit of your car will A, cause the fuse to blow, probably, and B, cause a fire as wiring for the dome light is not sized for multiple amps. A relay in line with the dome switched circuit, to a new fused circuit (or the cig circuit, that's normally fused for 10 Amps) is the safe approach.
Update: a 1.5 Amp controller will not work well with a full 300 led/5 meter length of rgb led strips... At most you can power 25 segments! Thats 500mA per channel/color
Interpreting specs such as these found on many marketplace sites can be a bit of an art form, especially when no datasheets are provided.
When in doubt, the best idea is to ask the seller. If they cannot provide the information, then you have two choices: take pot luck, or find another seller!
That being said, here's how I read the information you are being provided in this case:
- Input Voltage: Dc 12V – 3A
.. is I believe the spec of the power supply that is provided, not the strip itself. In other words, capable of delivering a total of 36W at 12V.
- Lamp power: 12V 4.8 Watts
.. is most likely per meter i.e. each 60 LEDs. That is a pure guess based typical power requirements and how specs for similar LED strips are presented. You can divide/multiply this out by the actual number of LEDs you plan to use.
If you are planning to power a strip with batteries, the main challenges will be battery life and how many LEDs you have the power to drive.
Trying to use 2x9V does not seem like a very good idea. While you could probably drive at least a few LEDs with 2x9V, you will be wasting a lot of their already-limited (~500mAh) capacity in the step-down to 12V. And a step-down with resistors is going to be tough to get right. If you cut the strip and can squeeze say 0.5A out of the batteries, that's still 3W to burn off. A voltage regulator would be better if you really must.
AA or even C, D or 6V lantern batteries will be able to drive more LEDs for longer, and you can avoid losses in voltage regulation.
Best Answer
IP68 indicates immersion resistance to at least 1m (3.2 Feet) or more, as described by the manufacturer. It should be just fine in a pool. Use adhesive heatshrink on the wire connections and coat the ends.
5V is not dangerous at all. You could put the leads in your mouth and not have any ill effects (except maybe over a week or two...). However, you should definitely have a fuse that will blow if excessive current is drawn. You can buy fuses and fuse holders on amazon. You should use a 2A fuse and a slightly-more-than-2A power supply, if you have one available. Your supply should have enough current to blow the fuse, or you can end up with the supply limiting instead of the fuse.