Surface or Onboard Battery

[CLYON]

I have noticed there are a lot of posts about where to put the battery, on the surface or in the ROV. Most of the questions about the battery on the surface, is the voltage drop. Most everyone is using CAT 5/6 cable. There is no way to safely run the current needed throught it. There is talk about upping the voltage so hopefully there is enough when it reaches the ROV. Has anyone ever tried to jump off a car with jumper cables? Has anyone ever tried to do it with those cheap small gauge cables? Usually what happens is it has to sit there and charge for a while. Sometimes they start to smoke. If you use the more expensive large gauge cables, it starts right away. One of my motors draws about 1/2 amp with no load. That is already more than a foot of CAT5 can safely handle. I have 2 5amp batteries in my 4 inch tube so I know you can get them on board. I know some may have reasons for putting them on the surface and thats fine as long as you have the wire to support it.

Anyway, not to make anyone mad, I just don't want anyone smoking wires when you are 100 feet down.

Chuck

[dna1990]

For us, the CAT5 is only for comms and control. Power is sent down separate 12AWG wiring. We have a smaller ROV that uses three pairs of 18AWG cable for power.

Agree, CAT5 is OK in some cases for powering electronics, camera, etc...not for motor loads.

[ROVER3D]

Power over Cable is not usable for "our" ROVs, because of cable-weight. Your ROV must be able to tug your complete cable (weight)!!!
If you use Power over cable with 10feet (VERY SHORT) your ROV has to tug the cable AND enough power left for control. Think about 100feet or like me 300feet cable! So i take Fiber-optic for control and Power onboard. If cable broken... the ROV can surface itself!
My ROV takes 80A@14,8V max. at "normal" use. How do you want to take this over (300feet) cable?

[Zaibach]

I had not considered the complete cable weight being an issue before I was planning on trying to make it as close to neutrally buoyant as possible in hopes that it would help somewhat with encumbrance tho overall drag was something that concerns me.

Well if Plan A pans out I will need larger motors and if it doesn't I will need to downsize my ROV
Which shouldn't be that hard since right now I've only partially hammered out the low power computational side of it.

[leeinmt]

Why not put the battery inside the ROV pressure vessel? I'm using two 12v gel cells in my project which can be moved fore or aft for balance. The tether only needs small gauge wires for control signal and video feed.

Lee

[rossrov]

Be aware that Lead-Acid batteries (includes gel and SLA etc) can produce an explosive gas mix if faulty or over-charged.

[leeinmt]

True, as the WWII submarine crews were well aware of.

I have had twin SLA batteries sealed in their weatherproof control box operating my electric 12 foot farm gate for 5 years w/o any problems. Gel cells are also enclosed in burglar alarm and security camera control cases as power failure backups.

The newer trickle chargers have smart sensor electronics that prevent over charging, even the cheap ones from Harbor Freight.

Lee

[ROVER3D]

12V Gel-Cells are not choice of battery. The cells are not made for strong current. Lead-Acid and 12V gel batteries capacity is usable in 20 hours rate! So a 12V 45A Battery can deliver the full 45A in 20 hours. not in 18,not in 10 or in 2!!! For use in ROV you need minimum 5A for one thruster. My last rov needs about ~12A in normal use. We've tested with 4 parallel gel cells 12V/12A and it didn't work well because of the voltage breaks down and the esc can't operate.
So i used Lipoly cells. A Lipoly-Pack 4S 5000mA delivers by 20C discarge rate 100A. These Cells are made for strong current! Think about using Lipo. For me its the only way.

In my current ROV-project i use 8x4S 5000mA Lipoly packs = 40A real Capacity usable in 2hours, 1hour or 10minutes!

[bikerbones1968]

I agree with ROVER3D. I am using Lipo's on board also. 4 x 3S 5000mah 50C paralleled. That's a total of 20000mah and 250 Amps constant current if required.

[leeinmt]

I'm familiar with LiPo's from my drone and FPV projects. They have revolutionized RC flight because of their weight / power ratios.

I'm uneasy about them in the ROV because they are so sensitive and touchy. 5000 ma 50c is a lot of current all at once. What would happen if they got shorted out by a water leak? Weight isn't so much a factor in an ROV as a quadcopter drone.

I want to keep an open mind about this and learn from other's experience.

Lee