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2,000 Batteries Under the Sea

A conversation with WHOI engineer Daniel Gomez-Ibanez




Oceanus: Why are batteries so important for Nereus?








Gomez-Ibañez: That's not out of the question. Large cells are often used in military vehicles or satellites. But one advantage of using a small, mass-produced cell is that there's been a huge investment in the repeatability and quality control of the manufacturing process. A very large cell doesn’t have that track record of consistency, because it may be the first, second, or even a 100th cell that has been made, and it's hard to be sure that it's a good design. So we used a lot of small cells made in a large volume and plugged them into one big power cord to end up with a very much larger battery. It’s the size of the battery that brings unique challenges.

Oceanus: Such as?

Gomez-Ibañez: I think the biggest problem with large batteries is striking a balance between safety and reliability. There's a lot of energy stored in the batteries, so if they do for some reason heat up and catch on fire, a lot of energy is released. It would make a very big fire and probably destroy the vehicle and could endanger the safety of everyone on the ship. It's not good to have a fire when you're in the middle of the ocean. 

A lot of effort also went into making the battery able to be shipped safely and economically. There are rules on what you can ship and how you can ship it, and we've designed the battery in such a way that it can be shipped with minimal effort, minimal delay, and minimal expense—so that you can ship it without any special treatment and on a regular passenger plane.


Oceanus: How did you test them?

Gomez-Ibañez: We had a long list of possible failure modes of the batteries, and we tested each of the protection mechanisms we put in to prevent them. It doesn't do any good to build something in, if you don't know that it it's going to work. So, for example, I used a heat gun to put some heat into the battery pack, and just observed its response—which, by the way, is that it will stop, whether the battery is discharging or charging.

Oceanus: Were there any hitches during the process?


Oceanus: Any major problems?


We started testing them, and we found they were overheating. There was a problem in there. We didn't know what it was. So we started tearing battery packs apart and measuring temperatures and voltages  all over the place, and we traced it down to a single transistor that is supposed to be able to switch the current on and off.  But it had an unexpectedly high resistance.

It turned out that a “pirate” organization produced these parts, put some other manufacturer’s name on them, and sold them as legitimate parts. Once we had located this defective part, we sent it to the manufacturers that were supposed to have made it, and they said, "That's not ours. We didn't make it. It's a counterfeit." We had to redo those battery packs and essentially build them all over again with the actual transistor.

Oceanus: How much time did that cost you?




From the Series



Daniel Gomez-Ibañez is an engineer at Woods Hole Oceanographic Institution who specializes in developing battery systems to power deep-submergence vehicles, including the new "hybrid" deep-sea explorer, Nereus. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution)

The new deep-diving robot Nereus is tested at sea. (Photo by Robert Elder, Woods Hole Oceanographic Institution)

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