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Max depth for sending power over regular tether

Hi, I’m curious if anyone has tried to experiment with sending power over tether for shallow depths? I have an application that requires long survey missions (8 hours) but at a max depth of 20m. Doing a voltage drop calc suggests the voltage should only drop by 1V or so but I’m not sure if the ROV will still operate with a (slightly) lower voltage. Has anyone determined the tether length at which the battery stops getting charged?

I have recently returned from an archaeological survey in the rive Tiber in Rome, and I have rigged up a thru tether charging system that can keep the sub going all day and all night, if you need. Of course this depends on the % gain and the actual stick movements, but we were working in about 8m water with about 2 knots current, using a 50m tether. We were sending 50vdc down the tether using two of the spare twisted pairs, supplying a buck converter in a separate enclosure, which dropped the voltage down to 16.5vdc to supply a charge to the battery. It worked well, and the battery voltage shown on screen rarely dropped below 15v, and just a few minutes of reduced activity was enough to bring it back up to 16.5v.

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@AKABiggles Where do you tie in the incoming power after the buck converter?

Hi EspReSso,
I am using a 4-wire link cable between the main electronics enclosure and the buck converter enclosure, using a 4-way Subconn penetrator in the backplate. The 50VDC from the tether goes down two of the wires into the buck converter, and the 16.5v comes back up the other two wires, which are connected to the main power rails.
Hope this helps.

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@AKABiggles, I assume the reason for a buck enclosure is due to heat displacement? vs having 50VDC over 2 tether wires -> buck converter -> main power rails, within the electronics enclosure.
What did you use for an enclosure? I had debated just the 3.9"L 2" tube with plates but increased cost.

No room to fit the converter in the main tube, so I had a polycarbonate tube made with aluminium end caps. Had to absorb the cost, as this is for commercial use, and needs to be depth rated to 150msw.

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Typo, head, I had meant due to heat from the buck.

Very little heat from the buck, almost 90% efficient, so not much heat generated. Hardly feels warm to the touch after running for 6 hours.

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