Hi @vozalexander,
The reason we use 12 AWG power cable is quite simple. That’s the largest gauge cable we could stuff through our penetrators, we tested thoroughly, and it seemed to work fine. After years of sales and customer use, this amount of copper has never caused a problem.
The long answer is current rating is not so simple as single number, in fact its more of a religious question where you ask 10 different people and you may get 10 different answers, all valid in their own way. Going one amp over a current rating will not necessarily result in things exploding, it will just result in more heating and voltage drop in the cable than what the rater deemed acceptable. What is acceptable may be very different in different circumstances. Also of note is that most current ratings are constant, and flying an ROV is far from a constant load. If the ROV was capable of drawing 200 A in a spike, using 200 A constant rated components everywhere would be total overkill, as the actual sustained average is much lower. As long as everything can handle the actual average and the occasional high spike, all is well. Its also worth noting that most of the cable is exposed to water, which cools it far more effectively than air.
On a recent practical ROV dive, we logged current draw over time, and found the average was about 20 A. Sure, there were short spikes up into the 60-90 A range, but these last no more than a second or two. In synthetic (tank) testing with all thrusters at max throttle 100% gain, a BlueROV2 heavy will draw around 110-120 A in an instantaneous spike, but this will quickly drop. There are couple reasons for this:
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Under heavy load, the thruster will not see the 16.8 V of a max charged battery, but probably closer to 15 V due to voltage drop. Actual current draw will be closer to 18-20 A per thruster at full throttle, and this will of course decrease as the battery discharges and voltage drops more.
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At full throttle, Ardusub does not run does not run all thrusters at full, the rear horizontal thrusters will always run a bit lower to increase vehicle stability.
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All our thruster thrust and current draw numbers are all at static conditions, which means the absolute velocity of the water entering the thruster is zero before it passes through. As soon as this number is non zero, either because the ROV has nonzero velocity through the water or the water in a small test tank is circulating, both of these drop.
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As current is drawn through the power system, things will heat up a bit and resistance will increase, leading to greater voltage drop and thus lower current draw.
In your battery you should be using solid nickel strips at least 0.12 mm thick and 7 mm wide. Bigger is better, and between the battery halves where current flows you should be using at least 3 strips in parallel on each side.
-Adam