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Hybrid power puzzle


(Richard) #1

Hi All
I’m trying to figure out a way to supply some topside power (down the tether) to my BR2 to extend its run times. I would need a non-BR tether to do this, which is manageable, but the real question to my mind is how to combine the constant-voltage supply from an onboard DC-DC converter (to convert the high voltage power coming down the tether to a useable ~16 volts on the vehicle) with the variable-voltage output from the battery. I’m envisioning this as a sort of “trickle charge” setup, where the power from the tether supplies a continuous ~200 watts, constantly “topping up” the vehicle’s battery. Is what I’m looking for essentially an onboard battery charger that would constantly adjust the voltage of its output to match whatever the vehicle’s voltage is at any moment? Anybody know of such a thing?


#2

Hi Richard, I get what you’re saying but there are a number of issues which would make this a non-starter in a small ROV, let’s take a look…

  1. The system you describe (without the charging) is easily done with a system called Power over Ethernet (PoE), of which there are a number of different protocols. This is used mostly for security camera systems and line amplifiers, they can work because the current draw is relatively low. However, an ROV needs bags of current on demand due to the load demanded by the thrusters, lights, etc. and you cannot deliver 20 - 30 Amps down a PoE cable without significant loss through resistance and heat.

  2. The best way to get around the current load issue is drive a really high voltage down the tether and then convert it to low voltage/high current at the point of use. Unfortunately this requires a big DC/DC converter which will dissipate heat and would be too big to incorporate into a small ROV design. You would also need a BIG DC driver at the topside and this would also be impractical.

  3. You cannot charge a battery whilst it is in use - this would be akin to pushing water back up a tap whilst it’s open. Therefore you would need two batteries; one in use and one charging, and then a no-loss method of switching between the two. This is entirely possible but would require some pretty robust MOSFET switches and the inevitable cooling. Again, you have the current issue; you may be able to get enough current down the tether to charge the battery a little bit but this will not effectively compensate for the overall load on the system.

Then again, if you have two batteries, you have effectively doubled the usage time of your ROV anyway. In terms of price/cost efficacy, this is the best way to extend your swim time. Of course, lots of Lithium = more buoyancy so all these issues are intertwined on both the electrical and physical level.

SROV.


(Richard) #3

Thanks for offering your thoughts on the matter, @Sarawak_ROV. I think what I’m aiming for probably isn’t possible, but it would be really great if it worked so I’m going to keep trying. I probably could have expressed better what I’m aiming for in my original post… a term I’ve learned since then is “current sharing”. What I’m hoping can happen is that the DC converter (which is quite small, only a bit bigger than a matchbook) will contribute the 250 watts or so that it’s capable of providing, reducing the demand on the battery. When I’m cruising along at 50% gain, that’s about how much power the vehicle is using. At 75% or if I’m moving diagonally at 50%, that jumps to around 450 watts, so the battery would contribute more. On the other hand, if I’m sitting pretty much still looking at something or just moving slowly, the vehicle would be consuming less than 250 watts, so the excess could potentially go into the battery. The sensing and switching required to do all that is probably space and cost prohibitive, but who knows? If I had a buck/boost converter on the battery output to keep it at a constant 15v to match the DC converter, things would be easier, but one of those big enough to handle 50 or 60 amps is probably huge.


(Richard) #4

I just found this thing:


If I put in a battery charged to just under 16 volts, set this thing’s output to 16.5v to match the maximum output from the DC converter, then I’d have two 16.5v supplies. I ought to be able to cool them both if they were housed in an aluminum tube and could do away with that big heat sink. Interesting…


#5

Hi Richard, what you described in your second post it entirely do-able, something like this would solve the problem (apologies for the poor quality diagram)…

You can use one of these to sense the current, it will give you a ratiometric voltage output which can be read by the ADC of a small MCU. At a predetermined current level the MCU will switch on the battery.

The converter you have shown looks like you are still planning to run the thrusters with topside power, this is not a good idea; the load up current of the motors will easily be enough to sag the voltage causing your electronics to brown-out.

You should probably consider going the other way - run the electronics (and maybe a couple of Lumen lights) from the topside power and just use the batteries for the thrusters (that’s what they’re good at). You can then use one of these power regs instead of the DC/DC converter.

DC/DC converters are fine as long as you remember that you don’t get anything for free and you can lose as much as 20% in efficiency losses (i.e. Pout = (Pin - 20%)) especially if it’s working hard (or hot). As such, I would be inclined to put a much higher voltage down the tether (say 36 - 40VDC), therefore, if you get voltage drop down the tether, you will still have enough power headroom to work with at the ROV.

SROV.


(Richard) #6

Thanks again for the input here. I am indeed planning to put higher voltage down the tether-- 120vdc, to be exact. By the time it gets to the vehicle with the tether I’m thinking of using it should be down to something like 80 volts, depending on the length. The 72-to-15 volt DC Vicor converter can handle a wide input voltage range, something like 100 to 50 volts, so that will work fine. It’s output can be adjusted up to 16.5v, which is why I was thinking I might be able to get a pair of matching, reasonably stable voltage sources at 16.5 if I didn’t fully charge the battery and used that other DC-DC converter. Current limiting would then be the problem… the Vicor 72-to-15 converter is strictly limited to 250 watts, and the other converter is limited to 30A on the input side, providing around 450 watts. Seems like we’re getting into more-trouble-than-it’s-worth territory here, and just doubling the battery capacity would be way easier.